Fixed priority scheduling with pre-emption thresholds and cache-related pre-emption delays: integrated analysis and evaluation
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Abstract
Commercial off-the-shelf programmable platforms for real-time systems typically contain a cache to bridge the gap between the processor speed and main memory speed. Because cache-related pre-emption delays (CRPD) can have a significant influence on the computation times of tasks, CRPD have been integrated in the response time analysis for fixed-priority pre-emptive scheduling (FPPS). This paper presents CRPD aware response-time analysis of sporadic tasks with arbitrary deadlines for fixed-priority pre-emption threshold scheduling (FPTS), generalizing earlier work. The analysis is complemented by an optimal (pre-emption) threshold assignment algorithm, assuming the priorities of tasks are given. We further improve upon these results by presenting an algorithm that searches for a layout of tasks in memory that makes a task set schedulable. The paper includes an extensive comparative evaluation of the schedulability ratios of FPPS and FPTS, taking CRPD into account. The practical relevance of our work stems from FPTS support in AUTOSAR, a standardized development model for the automotive industry. [(This paper forms an extended version of Bril et al. (in Proceedings of 35th IEEE real-time systems symposium (RTSS), 2014). The main extensions are described in Sect. 1.2.]
Keywords
Fixed-priority pre-emptive scheduling Fixed-priority scheduling with pre-emption thresholds Cache-related pre-emption delay Response-time analysis1 Introduction
1.1 Background and motivation
For cost-effectiveness reasons, it is preferred to use commercial off-the-shelf (COTS) programmable platforms for real-time embedded systems rather than dedicated, application-domain specific platforms. These COTS platforms typically contain a cache to bridge the gap between the processor speed and main memory speed and to reduce the number of conflicts with other devices on the system bus. Unfortunately, caches give rise to additional delays upon pre-emptions, because pre-emptions may lead to cache flushes and reloads of blocks that are replaced. These cache-related pre-emption delays (CRPDs) can significantly increase the computation times of tasks, i.e., literature has reported inflated computation times of up to 50% (Pellizzoni and Caccamo 2007. In order to account for the impact of the CRPD on the timeliness of a task system, CRPD has therefore been integrated into the schedulability analysis of tasks (Busquets-Mataix et al. 1996; Lee et al. 1998; Staschulat et al. 2005; Ramaprasad and Mueller 2006; Altmeyer et al. 2012).
In real-time embedded systems, such as embedded vehicle control, fixed-priority pre-emptive scheduling (FPPS) is widely used. The majority of the commercial real-time operating systems (RTOSes) supports FPPS and makes use of corresponding timing-analysis tools. FPPS is inherently fully pre-emptive, which causes at least two types of pre-emption costs when using COTS hardware: spatial costs for saving and restoring the context of all tasks in memory and contention delays such as CRPD when cache blocks need to be reloaded. With FPPS these run-time overheads cannot be resolved analytically. An important disadvantage of FPPS therefore remains that arbitrary pre-emptions during execution may lead to inefficient memory use and high run-time overheads (Gai et al. 2001; Ghattas and Dean 2007).
In order to overcome these inefficiencies, some RTOS manufacturers were inclined to use two static priorities per task (Carbone 2013; Wang and Saksena 1999): one base priority is applied at task dispatching (sometimes also referred to as a task’s dispatching priority) and a second priority is applied once a task is selected for execution until its completion (referred to as a task’s pre-emption threshold). This scheme of fixed-priority scheduling with pre-emptions thresholds (FPTS) has been shown to greatly reduce the memory footprint of concurrent task systems (Gai et al. 2001) and reduce the average case response times of tasks (Ghattas and Dean 2007). Currently, FPTS is therefore already adopted by industry.
An important reason for the success of FPTS in industry is that pre-emption thresholds can be applied to task systems even without making any changes to the tasks’ code. Pre-emption thresholds can be easily assigned to tasks at integration time. Such support is specified by both the OSEK (OSE 2005) and AUTOSAR (AUT 2010) operating-system standards in the form of internal resources. Strictly speaking, the restriction in OSEK and AUTOSAR to assign at most one internal resource to each task must be lifted in order to fully implement and deploy FPTS. Many standards-compliant RTOSes therefore go beyond the standard by implementing internal resources more liberally than prescribed by their standard.
To the best of our knowledge, however, the integration of CRPD in the schedulability analysis of FPTS has not been considered. The limited pre-emptive nature of FPTS gives rise to specific challenges when integrating CRPD in the analysis, in particular to prevent over-estimations of CRPD. For example, not all tasks contributing to the worst-case response time of a task can actually pre-empt the execution of a job of that task, unlike with FPPS, as illustrated by a non-pre-emptive task. Next, there is no optimal (pre-emption) threshold assignment (OTA) algorithm available for FPTS taking CRPD into account, not to mention an algorithm that minimizes CRPD. Finally, existing comparisons between FPPS and FPTS, e.g. Buttazzo et al. (2013), do not consider CRPD.
1.2 Contributions
This paper presents four main contributions. Firstly, it provides worst-case response-time analysis of sporadic tasks with arbitrary deadlines for FPTS with CRPD, generalizing the work in Altmeyer et al. (2012) from FPPS to FPTS and from constrained deadlines to arbitrary deadlines. Secondly, it provides and proves an OTA algorithm for FPTS with CRPD. Thirdly, it presents a schedulable task-layout search (STLS) algorithm that searches for a layout of tasks in memory that makes a task set schedulable. The algorithm generalizes the one in Lunniss et al. (2012) from FPPS to FPTS by exploring memory layouts and applying the OTA algorithm to them. In this way, reloads of memory blocks into the cache result in minimal CRPD for the considered memory layout. Finally, this paper presents an extensive comparative evaluation of the schedulability ratios of FPPS and FPTS with and without CRPD. The evaluation is based on three orthogonal dimensions, i.e. (i) the CRPD approach applied in the analysis, (ii) the deadline type, being constrained, implicit, and arbitrary deadlines, and (iii) the memory layout, and seven main experiments in which task-set parameters and cache related parameters are varied. In addition, the effectiveness of the STLS algorithm is evaluated.
1.2.1 Extended version
Compared to Bril et al. (2014), this extended version has the following two major contributions. Firstly, it presents a generalized algorithm to improve the layout of tasks in memory (Sect. 10). Secondly, it presents a major extension of the comparative evaluation (Sect. 11). In particular, we added two orthogonal dimensions, i.e. the CRPD approach and the deadline type, and two experiments, i.e. the evaluation of the STLS algorithm (Sect. 11.2.2) and cache reuse (Sect. 11.4.3).
1.3 Outline
The remainder of this paper is organized as follows. Section 2 presents related work. Section 3 presents our scheduling model for FPTS and CRPD. Section 4 recapitulates analysis for FPTS without CRPD and analysis for FPPS with CRPD. Sections 5–8 present our response-time analysis for FPTS with CRPD [which revisits our analysis in Bril et al. (2014)]. The analysis is split into the following sections: Sect. 5 addresses the main challenges, Sect. 6 focusses on pre-empting tasks, Sect. 7 on the pre-empted tasks and Sect. 8 combines pre-empting and pre-empted tasks.
Next, Sect. 9 presents our Optimal Threshold Assignment (OTA) algorithm. Section 10 presents our STLS algorithm which aims at further decreasing the CRPD by improving the layout of the memory blocks of tasks. Section 11 evaluates the performance of FPPS and FPTS in the presence of CRPD. Finally, Sect. 12 concludes this paper. A complementary appendix contains all graphs of the comparative evaluation.
2 Related work
In this section, we first present an overview of scheduling schemes (including FPTS) that may reduce the number of pre-emptions and their related costs in concurrent real-time task systems. Secondly, we look at related works that investigated techniques for dealing with CRPDs in pre-emptive systems.
2.1 Limited pre-emptive scheduling
Limited pre-emptive scheduling schemes received a lot of attention from academia in the last decade. In particular, fixed-priority scheduling with deferred pre-emption (FPDS) (Burns 1994; Bril et al. 2009; Davis and Bertogna 2012), also called co-operative scheduling, and fixed-priority scheduling with pre-emption thresholds (FPTS) (Wang and Saksena 1999; Saksena and Wang 2000; Regehr 2002; Keskin et al. 2010) are considered viable alternatives between the extremes of fully pre-emptive and non-pre-emptive scheduling. Compared to fully pre-emptive scheduling, limited pre-emptive schemes can (i) reduce memory requirements (Saksena and Wang 2000; Gai et al. 2001; Davis et al. 2000) and (ii) reduce the cost of arbitrary pre-emptions (Burns 1994; Bril et al. 2009; Bertogna et al. 2011b). In addition, compared to both FPPS and non-pre-emptive scheduling, these schemes may significantly improve the schedulability of a task set (Bril et al. 2009; Saksena and Wang 2000; Bertogna et al. 2011a; Davis and Bertogna 2012).
Assuming strictly periodic tasks with known phasing, a single non-pre-emptive region (NPR) can significantly reduce the pre-emptions that can feasibly occur (Ramaprasad and Mueller 2008). NPRs may be placed statically in the code of a task (as they are with FPDS) or they may be floating. Baruah (2005) proposed the use of sporadic tasks with floating NPRs. Floating NPRs were designed for earliest-deadline-first (EDF) scheduling of tasks in order to retain schedulability with limited pre-emptions. However, floating NPRs require specific operating-system support, as investigated by Baldovin et al. (2013), and they could lead to pre-emptions by all higher priority tasks at arbitrary points in the code (Yao et al. 2009). These pre-emptions may incur highly fluctuating CRPDs, which are non-monotonic in the length of the NPR (Marinho et al. 2012), and CRPDs are therefore hard to analyze. With fixed-priority scheduling, FPDS shows more schedulability improvements with its statically placed NPRs compared to task models with floating NPRs, even when pre-emption costs are ignored (Buttazzo et al. 2013).
Although FPDS also outperforms FPTS from a theoretical perspective (Buttazzo et al. 2013), applying FPDS in practice is still a challenge, because pre-emption points have to be explicitly added in the code. Bertogna et al. (2011b) presented a model based on constant pre-emption costs in order to place pre-emption points in the tasks’ code appropriately. Recently, Cavicchio et al. (2015) have further extended this work by placing pre-emption points after computing and optimizing the CRPDs of a task. However, these works assume a linear flow of the code blocks of tasks. In our current work on FPTS we refrain from any assumption on the structure of the tasks’ code.
2.2 Cache-related pre-emption delays (CRPDs)
There are different techniques to deal with CRPDs. If the total number of memory blocks of the tasks in a system exceeds the cache size, then this may obviously lead to CRPDs due to reloads of blocks from memory to the cache. However, even if all memory blocks fit in the cache simultaneously, there are scenarios in which some memory blocks that are occupied by the tasks may be mapped to the same cache block. Since the mapping of memory to cache is often statically prescribed by the hardware (Patterson and Hennessy 2014), a proper memory layout of the tasks is important even when the total number of occupied memory blocks fits into the cache. Gebhard and Altmeyer (2007) and Lunniss et al. (2012) therefore tried to optimize the CRPDs by changing the layout of tasks in memory, subject to a static mapping of memory blocks to cache blocks. In our paper, we build upon the earlier work for FPPS by Lunniss et al. (2012) and we generalize their approach to FPTS.
The resulting optimization procedures have complex underlying models for the mapping of memory to cache and their usage by the tasks. These models are unnecessary if one could avoid the eviction of cache blocks by other tasks. For this purpose, cache locking and cache partitioning techniques have been devised. Using cache locking, the eviction of cache blocks is restricted once a cache block has been loaded. This restriction can either be for the duration of the system, resulting in a static locking scheme (Campoy et al. 2001, 2005; Puaut and Decotigny 2002; Liu et al. 2012), or for specific intervals of time, such as the duration of a code-fragment or until a pre-emption occurs, resulting in a dynamic locking scheme (Campoy et al. 2002; Arnaud and Puaut 2006; Liu et al. 2012). Moreover, cache-locking can either be global, where each task “owns” a specific part of the cache, or local, where each task can use the entire cache, but the cache is reloaded each time a pre-emption occurs. Although static and dynamic cache locking schemes are incomparable in general, the dynamic scheme typically performs better than the static scheme, in particular when the cache is relatively small compared to the size of the code (Campoy et al. 2003; Liu et al. 2012). The reloading costs for dynamic schemes give rise to pessimistic results, however. Using cache partitioning, each tasks “owns” a specific part of the cache, like global cache-locking. Unlike cache locking, self-evictions of cache blocks by tasks are not restricted or prevented. Cache partitioning (or cache locking) may be implemented by means of hardware support (Kirk 1989) or by means of software support (Puaut and Decotigny 2002). Altmeyer et al. (2014) showed that cache partitioning may slightly improve the performance of simple, short control tasks of which the pre-emption costs are relatively high compared to the computation times. However, they observed that the advantage of cache partitioning is often negligible when the memory layout of tasks is improved, so that memory blocks are loaded in the cache with less overlap. Moreover, cache partitioning is not very suitable for tasks with lower locality of memory accesses and higher amounts of computation, i.e. when the pre-emption costs are small compared to the computation times.
Wang et al. (2015) extended the applicability of cache partitioning to larger task sets with the help of FPTS. They created mutual non-pre-emptive task groups, so that tasks of the same group can together use a larger cache partition. However, we expect that the scalability of their approach is limited, because for large task sets, with lower locality of memory accesses and higher amounts of computation, FPTS will suffer from the same drawbacks as FPPS. The elimination of CRPDs between tasks may then not compensate for the performance degradation in the computation times of tasks. In the current paper, we therefore follow the line of reasoning by Altmeyer et al. (2014) and we complement our assignment of pre-emption thresholds with an algorithm for improving the memory layout of tasks.
The CRPDs of tasks can be analysed based on the concepts of evicting cache blocks (ECBs) and useful cache blocks (UCBs) (Lee et al. 1998; Altmeyer and Maiza 2011). A cache block that may be accessed by a task is termed an ECB, as it may overwrite the content of that cache block. A cache block that may be (re-) used at multiple program points without being evicted by the task itself is termed a UCB. The set of UCBs and ECBs of tasks can be analyzed with, for example, a prototype version of AbsInt’s aiT Timing Analyzer for ARM (Ferdinand and Heckmann 2004). This type of analysis using ECBs and UCBs applies to direct-mapped caches with a write-through policy and to set-associative caches with a least-recently used (LRU) replacement policy and a write-through policy (Altmeyer et al. 2012). The concepts of ECBs and UCBs cannot be applied to set-associative caches with a first-in-first-out (FIFO) or a pseudo-LRU (PLRU) replacement policy, as shown in Burguière et al. (2009).
3 Models and notation
This section presents the models and notation that we use throughout this paper. We start with a basic, continuous scheduling model for FPPS, i.e., we assume time to be taken from the real domain (\(\mathbb {R}\)), similar to, e.g., Koymans (1990), Bril et al. (2009) and Bertogna et al. (2011a). We subsequently refine this basic model for FPTS (Wang and Saksena 1999). Next, we introduce a basic memory model and a model for cache-related pre-emption costs. The section is concluded with remarks.
3.1 Basic model for FPPS
We assume a single processor and a set \(\mathcal{T}\) of n independent sporadic tasks \(\tau _1, \tau _2, \ldots , \tau _n\), with unique priorities \({\pi }_1,{\pi }_2,\ldots ,{\pi }_n\). At any moment in time, the processor is used to execute the highest priority task that has work pending. For notational convenience, we assume that (i) tasks are given in order of decreasing priorities, i.e. \(\tau _1\) has the highest and \(\tau _n\) the lowest priority, and (ii) a higher priority is represented by a higher value, i.e. \({\pi }_1> {\pi }_2> \ldots > {\pi }_n\). We use \({\text{ hp }}({\pi })\) (and \({\text{ lp }}({\pi })\)) to denote the set of tasks with priorities higher than (lower than) \({\pi }\). Similarly, we use \({\text{ hep }}({\pi })\) (and \({\text{ lep }}({\pi })\)) to denote the set of tasks with priorities higher (lower) than or equal to \({\pi }\).
Each task \(\tau _i\) is characterized by a minimum inter-activation time\(T_i~{\in }~\mathbb {R}^+\), a worst-case computation time\(C_i~{\in }~\mathbb {R}^+\), and a (relative) deadline\(D_i~{\in }~\mathbb {R}^+\). We assume that the constant pre-emption costs, such as context switches and pipeline flushes, are subsumed into the worst-case computation times. We feature arbitrary deadlines, i.e. the deadline \(D_i\) may be smaller than, equal to, or larger than the period \(T_i\). The utilization\(U_i\) of task \(\tau _i\) is given by \(C_i / T_i\), and the utilizationU of the set of tasks \(\mathcal{T}\) by \(\sum _{1 \le i \le n} U_i\). An activation of a task is also termed a job. The first job arrives at an arbitrary time.
We also adopt standard basic assumptions (Liu and Layland 1973), i.e. tasks do not suspend themselves and a job of a task does not start before its previous job is completed.
For notational convenience, we introduce \(E_j(t) = \left\lceil t/T_j \right\rceil \) and \(E_j^\mathrm{*}(t) = \left( 1 + \left\lfloor t/T_j \right\rfloor \right) \) to represent the maximum number of activations of \(\tau _j\) in an interval \([x, x+t)\) and \([x, x+t]\), respectively, where both intervals have a length t.
3.2 Refined model for FPTS
In FPTS, each task \(\tau _i\) has a pre-emption threshold\({\theta }_i\), where \({\pi }_1 \ge {\theta }_i \ge {\pi }_i\). When \(\tau _i\) is executing, it can only be pre-empted by tasks with a priority higher than \({\theta }_i\). Note that we have FPPS and FPNS as special cases when \(\forall _{1 \le i \le n} {\theta }_i = {\pi }_i\) and \(\forall _{1 \le i \le n} {\theta }_i = {\pi }_1\), respectively.
Notations for various sets of indices of tasks
Classic notations for FPPS | Additional notations for FPTS |
---|---|
\({\text{ hep }}({\pi }) \mathop {=}\limits ^{\mathrm {def}}\{h| {\pi }_h \ge {\pi }\}\) | \({\text{ het }}({\pi }) \mathop {=}\limits ^{\mathrm {def}}\{h| {\theta }_h \ge {\pi }\}\) |
\({\text{ lp }}({\pi }) \mathop {=}\limits ^{\mathrm {def}}\{\ell | {\pi }> {\pi }_{\ell }\}\) | \({\mathrm{lt}}({\pi }) \mathop {=}\limits ^{\mathrm {def}}\{\ell | {\pi }> {\theta }_{\ell }\}\) |
\({\text{ hp }}({\pi }) \mathop {=}\limits ^{\mathrm {def}}\{h| {\pi }_h > {\pi }\}\) | \(\mathrm{b}(i) \mathop {=}\limits ^{\mathrm {def}}{\text{ lp }}({\pi }_i) \setminus {\mathrm{lt}}({\pi }_i)\) |
\({\text{ lep }}({\pi }) \mathop {=}\limits ^{\mathrm {def}}\{\ell | {\pi }\ge {\pi }_{\ell }\}\) |
3.3 A memory model
The cache utilization of a task \(\tau _i\) is given by \(U_i^\mathrm{C} = \vert {\text{ MB }}_i \vert /N^\mathrm{C}\), where \(\vert {\text{ MB }}_i \vert \) denotes the cardinality of the set \({\text{ MB }}_i\). The cache utilization of an individual task can therefore be larger than one, i.e. when \(\vert {\text{ MB }}_i \vert > N^\mathrm{C}\). The cache utilization\(U^\mathrm{C}\) of the set of tasks \(\mathcal T\) is given by \(U^\mathrm{C} = \sum _{1 \le i \le n} U_i^\mathrm{C}\).
The set of cache blocks of task \(\tau _i\) is determined by \({\text{ MB }}_i\) and \({ MapM2C}\).
3.4 A model for cache-related pre-emption costs
Example 1
Whether or not all memory blocks of a task \(\tau _i\) can be mapped on different cache blocks depends on the memory size \(\vert {\text{ MB }}_i \vert \) of \(\tau _i\) and the size \(N^\mathrm{C}\) of the cache. As described in Altmeyer et al. (2014) and Wang et al. (2015), the worst-case computation time of a task depends on the size of the cache. Whereas the worst-case computation \(C_i\) of task \(\tau _i\) is fixed when \(\vert {\text{ MB }}_i \vert \le N^\mathrm{C}\), it may increase when \(\vert {\text{ MB }}_i \vert \) becomes larger than \(N^\mathrm{C}\) due to self-eviction, i.e. \(\tau _i\) may evict some of its own cache blocks. In the remainder, we will assume that the costs of self-evictions, which are also referred to as intra-task CRPDs, are subsumed into the worst-case computation times.
3.5 Concluding remarks
The schedulability analyses presented in this paper (Sect. 5–8) assumes direct-mapped caches with a write-through policy and applies to instruction, data, and unified caches. The analysis only operate on the sets of UCBs and ECBs and are thus (i) independent of the mapping \({ MapM2C}\) from memory blocks to cache blocks and (ii) applicable for every cache size. Primarily for ease of evaluation, we will make simplifying assumptions for \({ MapM2C}\), e.g. assume the typical mapping scheme as given by (1).
4 Recap of response time analysis for FPPS and FPTS
This section starts with a recapitulation of the exact schedulability analysis for FPTS, as presented in Keskin et al. (2010). Next, that analysis is specialized for FPPS with constrained deadlines, i.e. for cases with \(D_i \le T_i\), and extended with CRPD (Altmeyer et al. 2012).
4.1 FPTS with arbitrary deadlines (without CRPD)
4.2 FPPS with constrained deadlines and CRPD
As we observed before (see Sect. 2), the integration of CRPD in the schedulability analysis of tasks has been addressed for FPPS with a focus on the pre-empting tasks (Busquets-Mataix et al. 1996; 2000, the pre-empted tasks (Lee et al. 1998), and by considering both the pre-empting and pre-empted tasks (Staschulat et al. 2005; 2007; Altmeyer et al. 2012). These techniques use different ways to bound the contribution of the CRPD, \(\gamma _{i,j}(R_i)\), in the response-time analysis of a task \(\tau _i\). Below, we briefly recapitulate representative approaches that we will use to illustrate our analysis for FPTS including CRPD in subsequent chapters; see Altmeyer et al. (2012) for further explanations of these approaches.
4.2.1 Pre-empting tasks
4.2.2 Pre-empted tasks
Applying the UCB-Only Multiset approach to Example 1 would yield a CRPD of \({\text{ BRT }}\cdot \vert {\text{ UCB }}_2 \vert = {\text{ BRT }}\cdot 3\) rather than \({\text{ BRT }}\cdot 2\) for a pre-emption of task \(\tau _2\) by task \(\tau _1\), i.e. a pessimistic result.
4.2.3 Pre-empting and pre-empted tasks
Applying the ECB-Union Multiset approach to Example 1 would yield a CRPD of \({\text{ BRT }}\cdot \vert {\text{ UCB }}_2 \cap {\text{ ECB }}_1 \vert = {\text{ BRT }}\cdot 2\) for every pre-emption of task \(\tau _2\) by task \(\tau _1\).
In the remainder of this paper, we follow a similar structure for extending FPTS with CRPD. Before looking at specific approaches, we consider challenges for FPTS with CRPD (Sect. 5). We subsequently focus on pre-empting tasks (Sect. 6), pre-empted tasks (Sect. 7), and the combination of pre-empting and pre-empted tasks (Sect. 8).
5 FPTS with CRPD: Preliminaries and challenges
To extend the schedulability analysis of FPTS with CRPD, we must extend the corresponding formulas. For this purpose, we extend the worst-case length \(L_i\) of the level-i active period in (3), the worst-case start-time \(S_{i,k}\) in (5) and the worst-case finalization time \(F_{i,k}\) in (6) of job k of task \(\tau _i\) with a new term \(\gamma _{i,j}(t)\) in a similar way as the worst-case response time \(R_i\) in (9) has been extended for FPPS with constrained deadlines. However, due to (i) the generalization towards arbitrary deadlines and (ii) the limited-pre-emptive nature of FPTS, it is not possible to simply extend these equations for FPTS with a term \(\gamma _{i,j}(t)\) by reusing the existing approaches to determine CRPD. This section addresses preliminaries and challenges for FPTS with CRPD.
5.1 Distinguishing executing and affected tasks
Overview of tasks that can execute and affect the execution of task \(\tau _i\) in a level-i active period starting at time \(t = 0\) for both FPPS with constrained deadlines and FPTS with arbitrary deadlines, assuming a task \(\tau _b\) that blocks \(\tau _i\) for FPTS, i.e. \(b \in \mathrm{b}(i)\)
Interval | Execute | Affected by \(\tau _j\) | #-jobs | |
---|---|---|---|---|
FPPS | \([0, R_i)\) | \({\text{ hep }}({\pi }_i)\) | \({\text{ hep }}({\pi }_i) \cap {\text{ lp }}({\pi }_j)\) | \( \left\{ \begin{array}{ll} E_h(R_i) &{}\quad \mathrm{if}\; h \in {\text{ hep }}({\pi }_i) \\ 0 &{}\quad \mathrm{otherwise} \\ \end{array} \right. \) |
FPTS | \([0, H_i)\) | \(\{i\} \cup {\text{ hp }}({\theta }_i)\) | \((\{i\} \cup {\text{ hp }}({\theta }_i)) \cap {\mathrm{lt}}({\pi }_j)\) | \( \left\{ \begin{array}{ll} E_h(H_i) &{}\quad \mathrm{if}\; h \in {\text{ hp }}({\theta }_i) \\ 1 &{}\quad \mathrm{if}\; i \\ 0 &{}\quad \mathrm{otherwise} \\ \end{array} \right. \) |
\([0, L_i)\) | \(\{b\} \cup {\text{ hep }}({\pi }_i)\) | \((\{b\} \cup {\text{ hep }}({\pi }_i)) \cap {\mathrm{lt}}({\pi }_j)\) | \( \left\{ \begin{array}{ll} E_h(L_i) &{}\quad \mathrm{if}\; h \in {\text{ hep }}({\pi }_i) \\ 1 &{}\quad \mathrm{if}\; b \\ 0 &{} \quad \mathrm{otherwise} \\ \end{array} \right. \) | |
\([0, S_{i,k})\) | \(\begin{array}{c} \text{ As } \text{ above } \text{ for }\\ {[0, L_i)}\\ \end{array}\) | \(\begin{array}{c} \text{ As } \text{ above } \text{ for }\\ {[0, L_i)}\\ \end{array}\) | \( \left\{ \begin{array}{ll} E_h(S_{i,k}) &{}\quad \mathrm{if}\; h \in {\text{ hp }}({\pi }_i) \\ k &{}\quad \mathrm{if}\; i \\ 1 &{}\quad \mathrm{if}\; b \\ 0 &{}\quad \mathrm{otherwise} \\ \end{array} \right. \) | |
\([0, F_{i,k})\) | \(\begin{array}{c} \text{ As } \text{ above } \text{ for }\\ {[0, L_i)}\\ \end{array}\) | \(\begin{array}{c} \text{ As } \text{ above } \text{ for }\\ {[0, L_i)}\\ \end{array}\) | \( \left\{ \begin{array}{ll} E_h(F_{i,k}) &{}\quad \mathrm{if}\;h \in {\text{ hp }}({\theta }_i) \\ E_h(S_{i,k}) &{}\quad \mathrm{if}\; h \in {\text{ hp }}({\pi }_i) \setminus {\text{ hp }}({\theta }_i) \\ k+1 &{}\quad \mathrm{if}\; i \\ 1 &{}\quad \mathrm{if}\; b \\ 0 &{}\quad \mathrm{otherwise} \\ \end{array} \right. \) |
Interval: A description of an interval under consideration, being \([0, R_i)\);
Execute: The tasks that can execute jobs in the interval, being tasks with a priority higher than or equal to the priority of \(\tau _i\), i.e. \({\text{ hep }}({\pi }_i)\);
Affected by\(\tau _j\): The set of tasks that (i) can execute jobs in the interval and (ii) can be pre-empted by task \(\tau _j\), i.e. \({\text{ hep }}({\pi }_i) \cap {\text{ lp }}({\pi }_j)\);
\(\#\)-jobs: The number of job activations of a task that can execute in the interval, i.e. \(E_h(R_i)\) for each task \(\tau _h \in {\text{ hep }}({\pi }_i)\).
When we focus only on the pre-empting tasks, e.g. when using the ECB-Only approach, we only need the information of the row affected by\(\tau _j\) in Table 2; see (10). When we consider the pre-empted tasks, e.g. when using the UCB-Only Multiset approach, the \(\#\)-jobs also play a role. To be more specific, the multiset \(M^{{\text {ucb-o}}}_{i,j}(t)\) in (12) contains \(E_j(R_h)\) copies of the size of \({\text{ UCB }}_h\) for each of the \(E_h(t)\) jobs of task \(\tau _h\), with \(h \in {\text{ aff }}({\pi }_i,{\pi }_j)\), affecting \(\tau _i\) and affected by \(\tau _j\).
In the remainder of this section, we first show how the number of pre-emptions \(E_j(R_h)\) of a job of a task \(\tau _h\) by a task \(\tau _j\) can be tightened for FPTS. Next, we determine the information in Table 2 for FPTS. We subsequently address specific topics related to FPTS, such as blocking and termination of the iterative procedure for \(L_i\). We conclude with a brief description of how the information presented in this section can be applied to the extensions for FPTS with CRPD, which is addressed in the next sections.
5.2 Bounding the number of pre-emptions using hold times
For FPPS with constrained deadlines, all pre-emptions during the response time of a job of a task may actually evict \({\text{ UCB }}\)s of that job. For FPTS, however, some pre-emptions can only take place between the activation and the start of a job, and therefore do not evict \({\text{ UCB }}\)s of that job. An obvious example is a non-pre-emptive task, where no pre-emption can take place during the actual execution of its jobs.
Being the worst-case hold time \(H_i\) of a task \(\tau _i,H_i\) is an upper bound for the hold time for every job of \(\tau _i\) in general and for every job in the level-i active period with a worst-case length \(L_i\) in particular. The former is an immediate consequence of the fact that the tasks that can influence the hold time of an individual job k of \(\tau _i\) are identical to those that can influence \(H_i\), i.e. \({\text{ hp }}({\theta }_i)\). The latter follows from the observation that a critical instant to determine the worst-case response time \(R_i\) is not necessarily a critical instant for the worst-case hold time \(H_i\), hence \(\forall _{0 \le k < E_i(L_i)} H_{i,k} \le H_i\). The worst-case hold time \(H_i\) is therefore a proper value to determine an upper bound on the number of pre-emptions of a job of task \(\tau _i\).
The worst-case hold time \(H_i\) of a task \(\tau _i\) is at most equal to the worst-case response time \(R_i\) of \(\tau _i\), i.e. \(H_i \le R_i\). This result immediately follows from the fact that the set of tasks that influences the worst-case hold time \(H_i\) of task \(\tau _i\) is a subset of the set of tasks that influences the worst-case response time \(R_i\) of \(\tau _i\). The worst-case hold time \(H_i\) of a task \(\tau _i\) may be smaller than the worst-case response time \(R_i\). This is because (i) the potential delay of the execution of a job by a previous job (Bril et al. 2008), (ii) the blocking by a task \(\tau _b\) with \(b \in \mathrm{b}(i)\), and (iii) the interference of tasks \(\tau _j\) with \(j \in {\text{ hp }}({\pi }_i) \cap {\text{ lep }}({\theta }_i)\) are included in \(R_i\) but not in \(H_i\). Example 2 below illustrates (i) and Example 3 illustrates (ii) and (iii).
Example 2
The characteristics of a set \(\mathcal{T}_2\) of periodic tasks is given in Table 3. The timeline shown in Fig. 3 illustrates both the worst-case hold time \(H_2 = 8.2\) and the worst-case response time \(R_2 = 8.6\) for the job activated at time \(t = 14\). \(R_2\) is larger than \(H_2\), because \(R_2\) includes a delay of 0.4 of the job activated at time \(t = 7\). This illustrates (i).
Task characteristics of \(\mathcal{T}_2\) and worst-case response times and hold times of periodic tasks with non-constrained deadlines under FPPS without CRPD
T | D | C | \({\pi }= {\theta }\) | R | H | |
---|---|---|---|---|---|---|
\(\tau _1\) | 5 | 5 | 2 | 2 | 2 | 2 |
\(\tau _2\) | 7 | 9 | 4.2 | 1 | 8.6 | 8.2 |
Task characteristics of \(\mathcal{T}_2\) and worst-case response times and hold times of periodic tasks under FPTS without CRPD
\(T = D\) | C | \({\pi }\) | \({\theta }\) | R | H | |
---|---|---|---|---|---|---|
\(\tau _1\) | 6 | 1 | 4 | 4 | 3 | 1 |
\(\tau _2\) | 7 | 2 | 3 | 4 | 5 | 2 |
\(\tau _3\) | 9 | 2 | 2 | 3 | 8 | 3 |
\(\tau _4\) | 11 | 2 | 1 | 3 | 8 | 3 |
Example 3
The characteristics of a set \(\mathcal{T}_3\) of periodic tasks are given in Table 4. The worst-case hold times of all tasks are smaller than their worst-case response times. Task \(\tau _1\) is an example of (ii), task \(\tau _4\) is an example of (iii), and tasks \(\tau _2\) and \(\tau _3\) are examples of both (ii) and (iii).
5.3 Determining the tasks that can execute and are affected by \(\tau _j\)
Having introduced the worst-case hold time \(H_i\) of task \(\tau _i\), we now determine for each of the intervals \([0, H_i),[0, L_i)\), \([0, S_{i,k})\), and \([0, F_{i,k})\) the tasks that can execute in the interval (“execute”) and from these tasks those that are affected by task \(\tau _j\) (“affected by \(\tau _i\)”) for FPTS in Table 2.
The tasks that can execute in \([0, H_i)\) can immediately be derived from (18), i.e. task \(\tau _i\) and all tasks with a priority higher than the pre-emption threshold \({\theta }_i\) of task \(\tau _i\). This set of tasks is therefore characterized by the set of indices \(\{i\} \cup {\text{ hp }}({\theta }_i)\). Similarly, the set of tasks that can execute in \([0, L_i),[0, S_{i,k})\), and \([0, F_{i,k})\) can immediately be derived from (3), (5), and (6), respectively. Assuming a task \(\tau _b\) that blocks \(\tau _i\), i.e. \(b {\in \mathrm b}(i)\), all these three sets are characterized by the set of indices \(\{b\} \cup {\text{ hep }}({\pi }_i)\).
To determine the “affected by \(\tau _j\)” for each of these intervals, we simply take the intersection of the set of indices for “execute” with \({\mathrm{lt}}({\pi }_j)\), similar to FPPS.
5.4 Determining the number of job activations “ \(\#\)-jobs”
We now show that we can derive the “ \(\#\)-jobs” for FPTS in Table 2 from the equations corresponding to the intervals, similar to FPPS. We start with the interval \([0, H_i)\). The intervals \([0, L_i),[0, S_{i,k})\) and \([0,F_{i,k})\) are subsequently addressed for \(B_i \ne 0\) and \(B_i = 0\).
5.4.1 \(\#\)-jobs for \([0,H_i)\)
The “ \(\#\)-jobs” for the interval \([0, H_i)\) follows immediately from (18). Exactly 1 activation of \(\tau _i\) is taken into account. To prevent pessimism when \(T_i\) is smaller than \(H_i\), Table 2 contains a dedicated clause for identifying the appropriate number of job activations of task \(\tau _i\) itself.
Example 4
We reconsider \(\mathcal{T}_2\) of Example 2. For that example, \(E_2(H_2) = 2\) rather than 1. To prevent this pessimism, we take exactly one activation of \(\tau _i\) into account.
5.4.2 \(\#\)-jobs for \([0,L_i),[0,S_{i,k})\), and \([0,F_{i,k})\) when \(B_i \ne 0\)
Given a task \(\tau _b\) that blocks \(\tau _i\) under FPTS, i.e. \(b \in \mathrm{b}(i)\), the number of activations \(\#\)-jobs in the intervals \([0, L_i),[0, S_{i,k})\) and \([0, F_{i,k})\) in Table 2 can be immediately derived from (3) for \(L_i\), (5) for \(S_{i,k}\) and (6) for \(F_{i,k}\). To prevent pessimism, exactly one activation of \(\tau _b\) is taken into account. Similarly, exactly k and \(k+1\) jobs of \(\tau _i\) are taken into account when determining \(S_{i,k}\) and \(F_{i,k}\), respectively.
Example 5
We reconsider \(\mathcal{T}_2\) of Example 2. The worst-case finalization time \(F_{2,0}\) of the first job of \(\tau _2\) is equal to 8.2. Because \(E_2(8.2) = 2\), (12) would include 2 jobs of \(\tau _2\) in \(M_{2,1}^{{\text {ucb-o}}}(8.2)\) rather than 1. To prevent this pessimism, we explicitly take the number of jobs of \(\tau _i\) into account.
5.4.3 \(\#\)-jobs for \([0,L_i),[0,S_{i,k})\), and \([0,F_{i,k})\) when \(B_i = 0\)
Lemma shows that \(E_j^\mathrm{*}(S_{i,k})\) can be replaced by \(E_j(S_{i,k})\) for the case \(B_i = 0\) in (6) for \(F_{i,k}\).
Lemma 1
Proof
The term \(E^\mathrm{*}_j(S_{i,k})\) represents the maximum number of activations of \(\tau _j\) in the interval \([0, S_{i,k}]\). When \(\exists _{m \in \mathbb {N}} S_{i,k} = m \cdot T_j\), task \(\tau _j\) is activated at time \(S_{i,k}\). This would imply that \(\tau _i\) cannot start at \(S_{i,k}\), which contradicts the definition of \(S_{i,k}\). We therefore conclude that \(\not \exists _{m \in \mathbb {N}} S_{i,k} = m \cdot T_j\). As a result, \(E^\mathrm{*}_j(S_{i,k}) = E_j(S_{i,k})\), which proves the lemma. \(\square \)
Corollary 1
Similarly, Lemma 2 shows that \(\gamma _{i,j}(t)\) can be defined in terms of \(E_j(S_{i,k})\) rather than \(E_j^*(S_{i,k})\) for the case \(B_i = 0\) in (5) when determining \(S_{i,k}\).
Lemma 2
When \(S_{i,k}\) is extended with a term \(\gamma _{i,k}(t)\) for the case \(B_i = 0,\gamma _{i,k}(t)\) can be based on \(E_j(t)\) rather than \(E^\mathrm{*}_j(t)\).
Proof
A solution for the recurrent relation for \(S_{i,k}\) is found when \(S_{i,k}^{(\ell )} = S_{i,k}^{(\ell +1)}\) for two subsequent iterations. For \(S_{i,k}^{(\ell )}\) there are two cases, either \(E_j(S_{i,k}^{(\ell )}) = E^\mathrm{*}_j(S_{i,k}^{(\ell )})\) or \(E_j(S_{i,k}^{(\ell )}) \ne E^\mathrm{*}_j(S_{i,k}^{(\ell )})\).
Let \(E_j(S_{i,k}^{(\ell )}) = E^\mathrm{*}_j(S_{i,k}^{(\ell )})\), i.e. \(\not \exists _{m \in \mathbb {N}} S_{i,k}^{(\ell )} = m \cdot T_j\). As a result, it doesn’t matter whether \(E_j(t)\) or \(E^\mathrm{*}_j(t)\) is used in \(\gamma _{i,k}(t)\).
Now let \(E_j(S_{i,k}^{(\ell )}) \ne E^\mathrm{*}_j(S_{i,k}^{(\ell )})\), i.e. \(\exists _{m \in \mathbb {N}} S_{i,k}^{(\ell )} = m \cdot T_j\). As a result, an additional activation of \(\tau _j\) will be taken into account when determining \(S_{i,k}^{(\ell +1)}\), irrespective of using either \(E_j(t)\) or \(E^\mathrm{*}_j(t)\) in \(\gamma _{i,k}(t)\). Together, these two cases prove the lemma. \(\square \)
We therefore conclude that, apart from the number of job activations of \(\tau _b\), the information in Table 2 also holds for \(\tau _i\) when \(B_i = 0\).
5.5 Identifying the task causing the largest blocking delay
A nice property of FPTS is that just one job of lower priority is able to cause blocking delays. In the presence of CRPD, however, the largest computation time among the blocking tasks does not necessarily result in the largest worst-case response time.
Example 6
We reconsider \(\mathcal{T}_3\) of Example 3. Without CRPD, the blocking of \(\tau _2\) due to \(\tau _3\) and \(\tau _4\) is the same because \(C_3 = C_4\), i.e. \(B_2 = \max (0, \max \{C_3, C_4\}) = 1\). The blocking including CRPD may be different, however, due to different UCBs of \(\tau _3\) and \(\tau _4\) and the ECBs of \(\tau _1\). Even a smaller computation time of a blocking task may result in a larger overall blocking effect when CRPD is included.
For the case with blocking (\(B_i \ne 0\)), we therefore need a more complex procedure to compute response times. Our new procedure determines the values for \(L_i,S_{i,k},F_{i,k}\), and \(R_i\) with CRPD by taking the maximum value over all tasks that may block \(\tau _i\).
5.6 Termination of the iterative procedure for \(L_i\)
Termination of the iterative procedure to determine \(L_i\) is no longer guaranteed when \(U < 1\), because the CRPD is not taken into account in the utilization U. To address this problem, we first observe that by definition every level-i active period, with \(1 \le i < n\), is contained in a level-n active period (Bril et al. 2009). Hence, termination of the iterative procedure to determine \(L_n\) guarantees termination for \(L_i\) for all \(1 \le i < n\). Next, the lowest priority task \(\tau _n\) cannot be blocked. As a result, when \(L_n\) exceeds the least common multiple (LCM) of the periods of the task set \(\mathcal{T}\), the iterative procedure will not terminate. This is because at the LCM the activation pattern is repeated and if the iterative procedure for \(L_n\) did not terminate at the LCM then there is pending load pushed across the LCM boundary. By integrating CRPD into the analysis, the effective utilization with CRPD is apparently larger than 1. The set is therefore considered unschedulable when \(L_n\) exceeds the LCM.
5.7 Applying the results
apply the notion of worst-case hold time by using \(E_j(H_h)\) rather than \(E_j(R_h)\) to tighten the number of times that \(\tau _j\) may pre-empt a job of \(\tau _h\) for approaches considering pre-empted tasks. This influences the definition of the multiset \(M_{i,j}\) for the UCB-Only Multiset approach, the ECB-Union Multiset approach, and the UCB-Union Multiset approach.
apply the derived “affected by\(\tau _j\)” information in the definitions of \(\gamma _{i,j}\) and \(M_{i,j}\) for the various approaches. This requires an extension of the subscripts of \(S_{i,k}\), \(F_{i,k},\gamma _{i,j}\) and \(M_{i,j}\) with b for those cases where a task \(\tau _b\) may block a task \(\tau _i\).
apply the derived “\(\#\)-jobs” information for approaches considering pre-empted tasks. This requires a case distinction following the information in Table 2 in the definition of the multiset \(M_{i,j}\). Moreover, it requires a further extension of the subscripts of \(\gamma _{i,j}\) and \(M_{i,j}\) with k, and the introduction of an additional parameter for both \(\gamma _{i,j}\) and \(M_{i,j}\) to cater for the pre-emptions in the intervals corresponding to the worst-case start-time and the worst-case finalization time.
take the maximum value over all tasks that may block \(\tau _i\) to determine \(L_i\) and \(F_{i,k}\), when \(\tau _i\) can be blocked.
6 FPTS with CRPD: pre-empting tasks
To determine the worst-case response time \(R_i\) of task \(\tau _i\), we can then reuse (7). In the subsections below, we consider the cases for tasks without and with blocking separately.
6.1 Worst-case length \(L_i\)
6.2 Worst-case start time \(S_{i,k}\)
6.3 Worst-case finalization time \(F_{i,k}\)
7 FPTS with CRPD: pre-empted tasks
In this section, we consider the UCB-Only Multiset approach, i.e. we focus on the pre-empted tasks. In this case, the worst-case hold time \(H_i\) and the row \(\#\)-jobs in Table 2 also play a role. As shown in Table 2, a case distinction is needed to capture the tasks that are being pre-empted, and these cases differ for \([0,H_i),[0, L_i),[0, S_{i,k})\) and \([0, F_{i,k})\). As a consequence, this section presents dedicated adaptations of \(\gamma ^{{\text {ucb-o}}}_{i,j}(t)\) and \(M^{{\text {ucb-o}}}_{i,j}(t)\), for each interval. For ease of presentation, we only consider the case where tasks may experience blocking. The other case is similar.
7.1 Worst-case hold time \(H_i\)
7.2 Worst-case length \(L_i\)
7.3 Worst-case start time \(S_{i,k}\)
7.4 Worst-case finishing time \(F_{i,k}\)
We may subsequently determine \(F_{i,k}\) by (29) and can derive \(R_i\) through (7) as before.
8 FPTS with CRPD: pre-empting and pre-empted tasks
In this section, we consider the ECB-Union and UCB-Union Multiset approaches, i.e. we consider both the pre-empting and the pre-empted tasks. As described in Sect. 4.2 for FPPS with CRPD, the definitions of the multisets for the ECB-Union and UCB-Union Multiset approaches can be derived from the definition of the multiset for the UCB-Only Multiset approach. A similar derivation applies for FPTS with CRPD. We therefore only consider the definition of the multisets \(M^{{\text {ecb-u}}}_{i,j,k,b}(t_s,t_f)\) and \(M^{{\text {ucb-u}}}_{i,j,k,b}(t_s,t_f)\) for the worst-case finalization time \(F_{i,k}\) for the case with blocking. The derivation of the definitions for the case without blocking and for the worst-case hold time \(H_i\), worst-case length \(L_i\) and worst-case start time \(S_{i,k}\) are similar.
8.1 ECB-Union Multiset approach
8.2 UCB-Union Multiset approach
8.3 Composite approach
9 An optimal threshold assignment algorithm
This section presents an OTA algorithm for FPTS with CRPD, yielding the maximum pre-emption thresholds of tasks when the set is schedulable. The algorithm also assumes that priorities of tasks are given and traverses the tasks in descending priority order. It exploits the property that once a task \(\tau _i\) is schedulable, it remains schedulable when the pre-emption threshold \({\theta }_\ell \) of a task \(\tau _\ell \) with a priority lower than task \(\tau _i\) is reduced and the pre-emption threshold \({\theta }_\ell \) either was or becomes lower than priority \({\pi }_i\).
9.1 Algorithm description
Our OTA algorithm (see Algorithm 1) uses an auxiliary set \(\widehat{\Theta } = \{\widehat{{\theta }_1}, \widehat{{\theta }_2}, \ldots , \widehat{{\theta }_n}\}\) of maximum pre-emption thresholds next to a set \(\Theta = \{{\theta }_1, {\theta }_2, \ldots , {\theta }_n\}\) of assigned pre-emption thresholds. Upon initialization, all values in \(\widehat{\Theta }\) are set to the highest priority \({\pi }_1\) (line 2), i.e. tasks are non-pre-emptive and therefore experience minimal CRPD. The algorithm traverses the tasks in descending priority order (lines 5–23). When it considers a task \(\tau _i\), it first assigns its maximum pre-emption threshold \(\widehat{{\theta }_i}\) to \({\theta }_i\) (line 7). Next, it tests schedulability of \(\tau _i\)without any blocking and returns unschedulable when the test fails (line 9). Otherwise, it tests schedulability of \(\tau _i\) with blocking by considering each lower priority task \(\tau _\ell \) in isolation (lines 11–22). It decreases the maximum pre-emption threshold \(\widehat{{\theta }_\ell }\) of \(\tau _\ell \) if-and-only-if \(\tau _i\) is unschedulable due to blocking by task \(\tau _\ell \) (lines 17–19). In that case, \(\widehat{{\theta }_\ell }\) is decreased to the highest priority of all tasks with a priority lower than \(\tau _i\), i.e. \({\pi }_{i+1}\) of \(\tau _{i+1}\). This may increase the CRPD of tasks with a priority lower than \(\tau _i\) but does not affect the schedulability of tasks with a priority higher than \({\pi }_i\). Hence, when the algorithm returns schedulable, i.e. the task set is schedulable, it has assigned the maximum pre-emption threshold to each task. A proof of correctness and detailed explanation of our OTA algorithm using invariants are given in the next subsection.
9.2 Correctness and proof of OTA algorithm
Our algorithm is based on two invariants, which use \(\Pi = \{{\pi }_1, {\pi }_2, \ldots , {\pi }_n\}\) to denote the set of priorities and \(\mathcal{T}^\mathrm{H}_m\) to denote the subset of m highest priority tasks with \(0 \le m \le n\), i.e. \(\mathcal{T}^\mathrm{H}_0 = \emptyset \), \(\mathcal{T}^\mathrm{H}_i = \{\tau _h \vert h \in {\text{ hep }}({\pi }_i)\}\) for \(1 \le i \le n\), and \(\mathcal{T}^\mathrm{H}_n = \mathcal{T}\).
If the following main invariant holds for \(\mathcal{T}\), then \(\Theta \) contains the maximum pre-emption thresholds for which all tasks in \(\mathcal{T}\) are schedulable, where \(\Theta = \widehat{\Theta } \subseteq \Pi \).
Invariant 1
- 1.
the set \(\widehat{\Theta }\) contains the maximum pre-emption threshold of each task such that all tasks in \(\mathcal{T}^\mathrm{H}_m\) meet their deadlines, i.e. \(\forall _{\tau _i \in \mathcal{T}^\mathrm{H}_m} R_i \le D_i\), where \(\widehat{\Theta } \subseteq \Pi \).
- 2.
the set \(\Theta \) contains the assigned pre-emption threshold of \(\tau _j\) if \(\tau _j \in \mathcal{T}^\mathrm{H}_m\), i.e. \({\theta }_j = \widehat{{\theta }_j}\), and it contains the priority of \(\tau _j\) if \(\tau _j \notin \mathcal{T}^\mathrm{H}_m\), i.e. \({\theta }_j = {\pi }_j\).
The variables in \(\widehat{\Theta }\) and \(\Theta \) are initialized to the highest (non-pre-emptive) priority \({\pi }_1\) (line 2) and the (fully pre-emptive) priority of the corresponding task (line 3), respectively. As a result, Invariant 1 holds for the empty set \(\mathcal{T}^\mathrm{H}_0\).
Next, the algorithm traverses the tasks in descending priority order (lines 5–23). When a task \(\tau _i\) is considered (line 5), Invariant 1 holds for \(\mathcal{T}^\mathrm{H}_{i-1}\). First the pre-emption threshold of \(\tau _i\) is assigned its maximum value, i.e. \({\theta }_i\) is set to \(\widehat{{\theta }_i}\) (line 7), and the schedulability of \(\tau _i\)without blocking is determined. If \(\tau _i\) is not schedulable, then the algorithm returns unschedulable (line 9), i.e. there does not exist a pre-emption threshold assignment making the set of tasks \(\mathcal{T}^\mathrm{H}_i\) schedulable. Otherwise 2) has been established for \(\mathcal{T}^\mathrm{H}_i\) and the inner-loop is entered.
The inner-loop (lines 11–22) considers each task \(\tau _\ell \) with a priority lower than \(\tau _i\) separately. The aim is to establish 1) for \(\mathcal{T}^\mathrm{H}_i\), based on the following invariant.
Invariant 2
- 1.
all tasks in \(\mathcal{T}^\mathrm{H}_{i-1}\) are schedulable, and
- 2.
\(\tau _i\) is schedulable when only the set \(\mathcal{T}^\mathrm{H}_{\ell }\) is considered, i.e. when all tasks in \(\mathcal{T} \setminus \mathcal{T}^\mathrm{H}_{\ell }\) are ignored.
If this invariant holds for \(\tau _i\) and \(\mathcal{T}\) then \(\widehat{\Theta }\) contains the maximum pre-emption thresholds for which all tasks in \(\mathcal{T}^\mathrm{H}_i\) are schedulable, where \(\widehat{\Theta } \subseteq \Pi \), i.e. Invariant 1 holds for \(\mathcal{T}^\mathrm{H}_i\).
Before the inner-loop, Invariant 2 holds for \(\tau _i\) and \(\mathcal{T}^\mathrm{H}_i\), and when a task \(\tau _\ell \) is considered (line 11), it holds for \(\tau _i\) and \(\mathcal{T}^\mathrm{H}_{\ell -1}\). When \(\tau _i\) remains schedulable when blocked by \(\tau _\ell ,\widehat{{\theta }_\ell }\) remains unchanged. Otherwise \(\widehat{{\theta }_\ell }\) is set to the priority \({\pi }_{i+1}\) of task \(\tau _{i+1}\), i.e. the highest priority in \(\Pi \) for which \(\tau _i\) is not blocked by \(\tau _\ell \). This may increase the CRPD of tasks with a priority lower than \(\tau _i\), but does not affect the schedulability of tasks with a priority higher than \(\tau _i\). Note that it doesn’t make sense to decrease the threshold of \(\tau _\ell \) to a priority higher than or equal to the priority of \(\tau _i\), because the CRPD experienced by \(\tau _i\) remains at best the same and may even increase due to additional pre-emptions during the execution of a job of \(\tau _\ell \). Invariant 2 has therefore been established for \(\mathcal{T}^\mathrm{H}_{\ell }\).
Theorem 1
Given a set of tasks \(\mathcal{T}\) and a priority assignment \(\Pi \), the OTA algorithm (Algorithm 1) assigns the maximum pre-emption thresholds \(\Theta \subseteq \Pi \) to tasks achieving schedulability, if such an assignment exists.
Proof
At each iteration of the outer-loop, the set \(\mathcal{T}^\mathrm{H}_m\) of Invariant 1 is increased by one task. Similarly, at each iteration of the inner-loop, the set \(\mathcal{T}^\mathrm{H}_\ell \) of Invariant 2 is increased by one task. Hence, the algorithm terminates with either schedulable and a set of maximum pre-emption thresholds that deem the task set schedulable with the least possible CRPD or unschedulable, in which case no assignment of pre-emption thresholds achieving schedulability exists under the given priority assignment. \(\square \)
9.3 Algorithmic complexity
Algorithm 1 traverses the set of tasks (of size n) in descending priority order and it may then consider any lower-priority task (at most \(n-1\) tasks). Hence, just like the algorithm in Wang and Saksena (1999), our algorithm has \(\mathcal{O}(n^2)\) iterations. In each iteration, the response time analysis is applied, which has a pseudo-polynomial time complexity.
10 Layout of tasks in memory
The analysis presented in the previous sections integrates CRPD into the analysis of FPTS based on ECBs and UCBs of tasks, i.e. the analysis is independent of the memory blocks of tasks and the mapping from memory blocks to cache blocks. In this section, we take a closer look at how the layout of tasks in memory influences the schedulability of task sets.
10.1 Influence of task layout on CRPD
Given a mapping \({ MapM2C}\) from memory blocks to cache blocks, the layout of a task \(\tau _i\) in memory, as described by \({\text{ MB }}_i\), determines \(\tau _i\)’s set of evicting cache blocks \({\text{ ECB }}_i\), see (2). The layout of tasks in memory therefore impacts the pre-emption delays, as illustrated by the following example.
Example 7
Figure 5 illustrates the impact of a task layout for FPTS. The cache contains 8 cache blocks. The task set contains 4 tasks, each with 4 ECBs and 4 UCBs. Task \(\tau _1\) and \(\tau _2\) as well as \(\tau _3\) and \(\tau _4\) are mutually non-pre-emptive due to pre-emption thresholds. An initial task layout resulting in \({\text{ ECB }}_1 = {\text{ ECB }}_3\) and \({\text{ ECB }}_2 = {\text{ ECB }}_4\) produces pre-emption related cache eviction in all cache blocks, whereas an optimal layout resulting in \({\text{ ECB }}_1 = {\text{ ECB }}_2\) and \({\text{ ECB }}_3 = {\text{ ECB }}_4\) eliminates CRPD completely under FPTS. Unlike FPTS, both layouts produce CRPD in all cache blocks under FPPS for this task set.
The pre-emption costs can thus be reduced and the schedulability improved by determining an appropriate memory layout. An intuitive task layout positions the memory blocks of all tasks consecutively in memory without leaving gaps, i.e. without leaving unused memory blocks between tasks’ blocks. This means that the memory blocks of the first task \(\tau _1\) are positioned at initial memory block \(M_{\text {init}}\), the blocks of the second task \(\tau _2\) at \(M_{\text {init}} + \vert {\text{ MB }}_1 \vert \), and of task \(\tau _i\) at \(M_{\text {init}} + \sum _{j < i} \vert {\text{ MB }}_j \vert \). Lunniss et al. (2012) have observed that gaps within a task layout, i.e. memory blocks that are left empty between the tasks, only improves the schedulability slightly for FPPS, at the cost of wasting memory. We therefore focus on sequential layouts in this paper and only vary the order in which tasks are positioned in memory.
10.2 Determining ECBs and UCBs for a given task layout
In the following, we will use \(\mathcal{T}^\mathrm{N}\) to denote a task set with ECBs and UCBs in normalized form. Moreover, we assume a function \({ ShiftCBs}(\mathcal{T}^\mathrm{N},\mathfrak {P}, N^\mathrm{C})\) which takes a task set \(\mathcal{T}^\mathrm{N}\) with ECBs and UCBs in normalized form and yields the same task set but with ECBs and UCBs determined for permutation \(\mathfrak {P}\) and cache size \(N^\mathrm{C}\).
10.3 An algorithm to search for a schedulable task layout
For a task set consisting of n tasks, there exists n! permutations. Given the size of this space, we search for a schedulable task layout using simulated annealing (SA), similar to Lunniss et al. (2012). When we encounter a schedulable task layout, we stop immediately. In order to compare an unschedulable task layout with a new, unschedulable, candidate layout, we need a metric. For this purpose, we use the breakdown utilization \(U^*\) (Lehoczky et al. 1989) based on scaling the computation times of tasks with a factor \(\Delta \). For an unschedulable task layout of a task set \(\mathcal{T}\), the breakdown utilization \(U^*\) is smaller than the utilization U of \(\mathcal{T}\), i.e. the largest possible scaling factor \(\Delta ^*\) for which \(\mathcal{T}\) is schedulable for that layout will satisfy \(0< \Delta ^* < 1\).
The STLS algorithm (Algorithm 2) starts with an initial task permutation \(\mathfrak {P}_\mathrm{init}\) (line 1). Next, it tests schedulability of the task set for the initial permutation and returns schedulable when the test succeeds (line 4). When the test fails, the initializations required for simulated annealing are performed (lines 7–8). The algorithm subsequently repeatedly selects new layout candidates until either a schedulable layout is found (line 19) or the bound on the maximum number of permutations considered is reached (line 9). This bound can be expressed in terms of an initial temperature \(\mathfrak {T}_\mathrm{init}\) (line 7) with \(0 < \mathfrak {T}_\mathrm{init}\), a target temperature \(\mathfrak {T}_\mathrm{target}\) (line 9) with \(0 < \mathfrak {T}_\mathrm{target} \le \mathfrak {T}_\mathrm{init}\), and a cooling factor \(f_\mathrm{cooling}\) (line 27) with \(0< f_\mathrm{cooling} < 1\). A candidate layout is randomly chosen by swapping the position of two tasks in the current permutation (lines 11–15). With equal probability, the algorithm swaps two neighboring tasks, or two tasks at random irrespective of the position in the current layout. When the candidate is schedulable, we are done (lines 17–19). Otherwise, we determine whether or not to select the new candidate (lines 22–26).
Although the SA algorithm will not always find a schedulable layout whenever one exists, i.e. Algorithm 2 is not an optimal algorithm, it performs close to a brute-force algorithm (Lunniss et al. 2012) in terms of precision when appropriate parameters are used.
10.4 Algorithmic complexity
The STLS algorithm (Algorithm 2) tries at most \(\left\lceil \frac{\log \mathfrak {T}_\mathrm{target} - \log \mathfrak {T}_\mathrm{init}}{\log f_\mathrm{cooling}} + 1 \right\rceil \) out of n! permutations of a task set \(\mathcal{T}\) of size n. For each permutation \(\mathfrak {P}\), the response time analysis is applied to determine schedulability of \(\mathcal{T}\) using \(\textit{IsSchedulable}(\mathcal{T})\), which has a pseudo-polynomial time complexity. The algorithm BreakdownUtil\((\mathcal{T})\) determines the breakdown utilization of an unschedulable task layout. The breakdown utilization can be approximated with a binary search on the scaling factor \(0< \Delta < 1\) and the schedulability test. With a fixed number of m steps, an approximation \(\Delta '\) on the scaling factor \(\Delta \) is derived with a precision of \(\frac{1}{2^{m+1}}\), i.e. \(\Delta ' - \frac{1}{2^{m+1}} \le \Delta < \Delta ' + \frac{1}{2^{m+1}}\).
10.5 Instantiating the algorithm
Algorithm 2 is applicable to both FPPS and FPTS, i.e. the specific schedulability tests to be executed are invoked within the functions IsSchedulable\((\mathcal{T})\) and BreakdownUtil\((\mathcal{T})\). Our optimal threshold assignment algorithm (Algorithm 1) is executed as part of the schedulability test for FPTS.
11 Evaluation
- 1.
CRPD approach: To compute the schedulability of a task set under CRPD, we compare the most effective approaches, i.e. the composite approach combining the UCB-Union Multiset and the ECB-Union Multiset, both for FPPS (see Altmeyer et al. 2012) and FPTS (developed in this paper). In addition, we compare the various approaches presented in this paper, i.e. the composite approach, the UCB-Union Multiset, the ECB-Union Multiset, the UCB-Only Multiset, and the ECB-Only approach.
- 2.
Deadline type: We consider constrained deadlines, where tasks’ relative deadlines are at most equal to their periods (i.e. \(D_i \le T_i\)), implicit deadlines, where relative deadlines are equal to periods (i.e. \(D_i =T_i\)), and arbitrary deadlines, where no relationship exists between relative deadlines and periods of tasks.
- 3.
Memory layout: Next to the initial (sequential) layout of tasks in memory we also consider permutations of the sequential layout using our schedulable task-layout search (STLS) algorithm (Algorithm 2). These evaluations are only performed for the composite approach, however.
In the remainder of this section, we first present our basic system configuration. Next, we present the results of a series of experiments. In the first series of experiments, we show the ratio of schedulable task sets as a function of task-set utilization and evaluate our STLS algorithm for the composite approach. In the next two series of experiments we vary task-set parameters and cache-related parameters.
11.1 Experimental setup
As described in Sect. 3.5, we assume the typical mapping scheme from memory blocks to cache blocks as given in (1).
In our basic system configuration, we assume a cache with \(N^\mathrm{C} = 512\) cache blocks and a total cache utilization of \(U^\mathrm{C} = 4\), i.e. the total number of ECBs of all tasks is \(N^\mathrm{C} \times U^\mathrm{C} = 2048\). We then select the cache utilization \(U_i^\mathrm{C}\) of each task (the number of MBs of a task, \(\vert {\text{ MB }}_i \vert \)) using UUnifast (Bini and Buttazzo 2005), and derive the number of ECBs of a task, \(\vert {\text{ ECB }}_i \vert \) using (2). \(40\%\) of a task’s ECBs are also UCBs, i.e. \(\vert {\text{ UCB }}_i \vert = 0.4 \cdot \vert {\text{ ECB }}_i\vert \). We assume a block reload time (\({\text{ BRT }}\)) of 8 \(\upmu \)s. For each experiment and for each parameter configuration, we generate a new set of 1000 systems.
For each system, we generate \(n = 10\) tasks which are assigned deadline monotonic priorities. For constrained deadlines and arbitrary deadlines, the deadlines \(D_i\) are selected from \([(C_i+T_i)/2, T_i]\) and \([(C_i + T_i)/2, 4 T_i]\), respectively. The task periods \(T_i\) are randomly drawn from the interval [10, 1000] ms. The individual task utilizations \(U_i\) (with \(C_i = U_i\times T_i\)) are generated using the UUnifast algorithm (Bini and Buttazzo 2005). The pre-emption thresholds of tasks are selected by our OTA algorithm (see Sect. 9).
The parameters used for simulated annealing in the algorithm searching for a schedulable layout of tasks in memory (see Sect. 10) match those in Lunniss et al. (2012). The breakdown utilization is calculated in \(m = 10\) steps, yielding a scaling factor \(\Delta \) with a precision of \(\frac{1}{2^{m+1}} \approx 0.5 \times 10^{-3}\). The initial temperature is set to \(\mathfrak {T}_\mathrm{init} = 1\), the cooling factor is given by \(f_\mathrm{cooling} = 0.98\), and the target temperature by \(\mathfrak {T}_\mathrm{target} = 0.05\). Hence, the task-layout search algorithm tries at most \(\left\lceil \frac{\log \mathfrak {T}_\mathrm{target} - \log \mathfrak {T}_\mathrm{init}}{\log f_\mathrm{cooling}} + 1 \right\rceil = 150\) out of \(n! = 3,628,800\) permutations. The evaluation for FPPS in Lunniss et al. (2012) has shown that even though the number of evaluated layouts is only a fraction of the total number of layouts, the layout search is likely to find a schedulable layout, if one exists. We perform a similar evaluation for FPTS in the next section.
11.2 Task-sets’ utilization
11.2.1 CRPD approaches and deadline types
The CRPD approaches and deadline types are evaluated by varying the task-set utilization in four experiments. In the first three experiments, we evaluate the CRPD approaches for implicit deadlines, constrained deadlines, and arbitrary deadlines. The results of these experiments are presented by six graphs on two facing pages. The even pages show 3 graphs for the composite approach for constrained (top), implicit (middle), and arbitrary (bottom) deadlines using both the initial layout and the layout search. The odd pages show the 3 additional graphs for the various CRPD approaches presented in this paper for constrained (top), implicit (middle), and arbitrary (bottom) deadlines using the initial layout. The graphs have been aligned both vertically (on one page) as well as horizontally (on the even and odd page) to ease comparison. Furthermore, the lines on the graphs appear in the same order as they are described in the legend. The graphs are best viewed online in color. In the fourth experiment, we evaluate the CRPD approaches by varying the deadline factor, i.e. by determining the weighted schedulability ratio for different values of a deadline factor x, where the relative deadline of each task \(\tau _i\) is given by \(D_i = x \cdot T_i\).
Figure 6 (middle) shows the ratio of task sets deemed schedulable for implicit deadlines, where the composite approach is used when CRPD is taken into account. The relative performance improvement of FPTS compared to FPPS is strongly amplified when including the CRPD. In contrast, FPTS and FPPS ignoring inter-task CRPD, which is denoted by means of “without CRPD” in the figures, only differ in case of high task utilization (starting at \(U=0.85\)) and at most by \(20\%\). In the presence of CRPD, however, FPPS is only able to schedule half of all generated task sets at a utilization of \(U=0.8\) for the initial permutation, while FPTS is able to schedule more than \(90\%\). FPTS only experiences a similar performance degradation at a considerably higher utilization, i.e. approximately at \(U = 0.88\). With the task-layout search algorithm, the performance of FPPS with CRPD can be improved, but remains well below the performance of FPTS with CRPD for the initial permutation. The task-layout search algorithm allows to improve the performance of FPTS with CRPD even further, e.g. with approximately \(20\%\) for a utilization \(U = 0.9\). The evaluation indicates that even though FPTS with layout-search cannot completely hide the effects of CRPD, it can mitigate the impact significantly.
Figure 7 (middle) shows the ratio of task sets deemed schedulable for implicit deadlines and the initial memory layout using various approaches when CRPD is taken into account. We have put Figures 6 and 7 on facing pages to ease comparison. Note that the lines in Figures 6 and 7 for FPTS and FPPS without CRPD, and FPNS are the same. Moreover, the line for FPTS with CRPD (initial layout) in Fig. 6 is the same as the line for FPTS - Composite Approach in Fig. 7. For this experiment, the composite approach and the UCB-Union Multiset approach give comparable results, i.e. the ECB-Union Multiset approach provides hardly any advantage over the UCB-Union Multiset approach for the settings of this experiment. The UCB-Only Multiset and ECB-Only approach are outperformed by the UCB-Union Multiset and ECB-Union Multiset approaches, as expected. For FPTS with CRPD, the UCB-Only Multiset and ECB-Only approach (shown if Fig. 7) are even outperformed by FPPS with CRPD and the combined approach (shown in Fig. 6), clearly showing the superiority of the composite approach over other approaches.
Our second and third experiments consider the ratio of task sets deemed schedulable versus the task set utilization for constrained and arbitrary deadlines. From constrained towards arbitrary deadlines, the performance of all algorithms improve; see Fig. 6. The relative performance improvement of FPTS compared to FPPS when including CRPD is remarkable; FPPS with CRPD and layout search can hardly schedule any task sets for arbitrary deadlines and a utilization of 0.975, while FPTS can still schedule approximately 45% for the initial layout and almost 70% with layout search. Moreover, the advantage of layout search over the initial layout for FPTS only increases for increasing utilizations, whereas the advantage reduces again after an initial increase for FPPS.
Our fourth experiment concerns the weighted schedulability ratio for a varying deadline factor, using the composite approach when CRPD is taken into account; see Fig. 8. For any deadline factor, a deadline monotonic priority assignment is identical to a rate monotonic priority assignment. For FPPS, the worst-case response times of tasks are therefore independent of the deadline factor. For FPTS, where pre-emption thresholds can still be selected, worst-case response times are not necessarily fixed, however. As an example, with an increasing deadline factor, a task can tolerate more blocking from lower priority tasks, potentially allowing more lower tasks to raise their preemption threshold. As a result, the ability to increase worst-case response times of higher priority tasks for an increasing deadline factor, allows lower priority tasks to reduce their worst-case response times, and therefore meet their deadlines at lower deadline factors. Although this potential advantage of FPTS over FPPS is hardly noticeable without CRPD, it explains (i) why FPTS with CRPD performs close to FPPS and FPTS without CRPD, in particular for larger deadline factors, and (ii) why FPPS with CRPD experiences a clear performance loss compared to FPTS with CRPD, in particular for larger deadline factors. As expected, the weighted schedulability ratio is increasing as a function of the deadline factor, although the lines for FPPS with CRPD converge to a value well below 1. Figure 9 complements Fig. 8 by also showing the weighted schedulability ratio for the various CRPD approaches for the initial memory layout. Similar to Fig. 8, the weighted schedulability ratio is increasing for an increasing deadline factor for all approaches. Although the UCB-Only Multiset and the ECB-Only approaches are considerably less effective in bounding the CRPD than the UCB-Union Multiset and the ECB-Union Multiset approaches, their performance remain increasing for FPTS whereas the combined approach converged for FPPS in Fig. 8. The relative performance improvement of FPTS compared to FPPS is highest around a deadline factor equal to one (i.e. for implicit deadlines) and gradually decreases for both a decreasing as well as an increasing deadline factor. For an increasing deadline factor, both FPTS and FPPS can achieve a weighted schedulability ratio of 1. In the presence of CRPD, however, FPPS is only able to achieve a weighted schedulability ratio of 80% of the task sets (with layout search), while FPTS is able to achieve close to 100% for an increasing deadline factor. The evaluation therefore indicates that FPTS can almost completely hide the effects of CRPD when the deadline factor is increased.
11.2.2 Schedulable task-layout search (STLS) algorithm
To evaluate the effectiveness of the STLS algorithm, we compare the ratio of schedulable task sets with \(n = 7\) tasks of a brute force algorithm, with the STLS algorithm using different values for the cooling factor \(f_{\textit{cooling}}\) and the initial (sequential) layout of tasks in memory. The brute-force algorithm, potentially trying every permutation of task ordering, determines the schedulability of at most \(7! = 5040\) different layouts. Figure 10 (middle) shows the results for implicit deadlines for an initial temperature \(\mathfrak {T}_\mathrm{init} = 100\) and cooling factors 0.98, 0.95, 0.9, and 0.8, resulting in at most 378, 150, 74, and 36 configurations to be examined, respectively.
Weighted schedulability ratio for the various scheduling algorithms
Scheduling algorithm | Weighted schedulability ratio | Notation | ||
---|---|---|---|---|
Constrained | Implicit | Arbitrary | ||
FPTS with CRPD (layout search) | 0.762431 | 0.857890 | 0.974838 | \(W_{\mathrm{FPTS}_\mathrm{LS}}\) |
FPTS with CRPD (initial layout) | 0.711737 | 0.818222 | 0.948140 | \(W_{\mathrm{FPTS}_\mathrm{IL}}\) |
FPPS with CRPD (layout search) | 0.647439 | 0.724327 | 0.792571 | \(W_{\mathrm{FPPS}_\mathrm{LS}}\) |
FPPS with CRPD (initial layout) | 0.593637 | 0.644919 | 0.722304 | \(W_{\mathrm{FPPS}_\mathrm{IL}}\) |
- 1.
layout search with FPPS: \((W_{\mathrm{FPPS}_\mathrm{LS}} - W_{\mathrm{FPPS}_\mathrm{IL}})/W_{\mathrm{FPPS}_\mathrm{IL}}\);
- 2.
layout search with FPTS: \((W_{\mathrm{FPTS}_\mathrm{LS}} - W_{\mathrm{FPTS}_\mathrm{IL}})/W_{\mathrm{FPTS}_\mathrm{IL}}\);
- 3.
FPTS instead of FPPS with initial layout: \((W_{\mathrm{FPTS}_\mathrm{IL}} - W_{\mathrm{FPPS}_\mathrm{IL}})/W_{\mathrm{FPPS}_\mathrm{IL}}\);
- 4.
FPTS instead of FPPS with layout search: \((W_{\mathrm{FPTS}_\mathrm{LS}} - W_{\mathrm{FPPS}_\mathrm{LS}})/W_{\mathrm{FPPS}_\mathrm{LS}}\);
- 5.
both FPTS and layout search over FPPS: \((W_{\mathrm{FPTS}_\mathrm{LS}} - W_{\mathrm{FPPS}_\mathrm{IL}})/W_{\mathrm{FPPS}_\mathrm{IL}}\).
Relative improvements achieved for the weighted schedulability ratio
# | Metric | Weighted schedulability ratio | ||
---|---|---|---|---|
Constrained | Implicit | Arbitrary | ||
1 | Layout search with FPPS | 0.09 | 0.12 | 0.10 |
2 | Layout search with FPTS | 0.07 | 0.05 | 0.03 |
3 | FPTS instead of FPPS with initial layout | 0.20 | 0.27 | 0.31 |
4 | FPTS instead of FPPS with layout search | 0.18 | 0.18 | 0.23 |
5 | Both FPTS and layout search over FPPS | 0.28 | 0.33 | 0.35 |
Metrics 1 and 2 illustrate that the layout search for FPPS is more effective than for FPTS; whereas a 12% improvement can be achieved for FPPS with implicit deadlines, only 5% can be achieved for FPTS; see Table 6. The improvement that can be achieved by the layout search for FPTS decreases from constrained towards arbitrary deadlines. This is an immediate consequence of the improved performance for FPTS with CRPD, decreasing the relative advantage of the layout search over the initial layout; see Fig. 6. Metrics 3 and 4 show the amount of improvement we get employing pre-emption thresholds, e.g. 27% for the initial layout and implicit deadlines and 18% with the layout search and implicit deadlines. Because the improvement of FPTS compared to FPPS when CRPD is included increases from constrained towards arbitrary deadlines (see Fig. 6) both metric 3 and 4 increase from constrained towards arbitrary deadlines as well. Finally, metric 5 shows the merit of applying both FPTS and layout search, i.e. the recommended solution, over what might be considered the default option of FPPS and initial layout. The amount of improvement is almost 33% for implicit deadlines.
11.3 Varying task-set parameters
In this first series of experiments, we vary task-set parameters, i.e. the range of the task period and the number of tasks. For each of these experiments, we use the weighted schedulability ratio as metric.
11.3.1 Period range
Figure 11 (top and bottom) also shows the results for constrained and arbitrary deadlines (respectively). The graphs clearly illustrate that the weighted schedulability ratio increases from constrained to arbitrary deadlines for all algorithms. The graphs also illustrate that the performance loss for FPTS due to CRPD gradually decreases from constrained to arbitrary deadlines, whereas the performance loss for FPPS due to CRPD remains roughly the same. As before, we attribute this relative strength of FPTS to its ability to increase the worst-case response time of higher priority tasks allowing a decrease of response times of lower priority tasks. This strength becomes amplified for increasing deadlines.
11.3.2 Number of tasks
In the second experiment we vary the number of tasks from 2 to 20 in steps of 2. Figure 13 (middle) shows the results for implicit deadlines. An increasing number of tasks leads to an improved performance of FPTS with CRPD relative to FPPS with CRPD. There are two reasons for this: (i) as the cache utilization remains constant, the ECBs per task decrease and (ii) by increasing the number of tasks, the individual task utilizations and execution times decrease, thus decreasing the potential blocking times. This gives the OTA algorithm more freedom to set pre-emption thresholds such that most tasks cannot pre-empt each other, again greatly reducing CRPD. For a low number of tasks, the task-layout search algorithm has only a minor impact on the performance of FPPS and FPTS. The number of task layouts is limited, and thus also the potential gain. The difference between the initial and the improved layout becomes noticeable at a task-set size of 6, and has its peak at 10 and 12 tasks. Although the task-layout search remains effective in case of large task sets, the performance benefits drop slightly. The larger the task set, the more potential task permutations exist. Consequently, the search algorithm is only able to explore a smaller fraction of the complete search-space making it less likely to find an optimal or near-optimal task layout.
Figure 13 (top and bottom) also shows the results for constrained and arbitrary deadlines (respectively). The performance of FPTS with CRPD converges to FPTS without CRPD for an increasing number of tasks. For arbitrary deadlines, the performance of FPTS with CRPD and FPTS without CRPD are almost the same. For FPPS, however, a relative performance improvement of FPPS with CRPD compared to FPPS without CRPD is not noticeable from constrained deadlines towards arbitrary deadlines.
Figure 14 shows the results for the various CRPD approaches for constrained, implicit, and arbitrary deadlines. Similar to the earlier experiments, the UCB-Union Multiset approach and the composite approach have overlapping lines in the graphs. For an increasing number of tasks, the performance of the UCB-Only Multiset approach degrades faster than that of the ECB-Only approach. The rationale for this behavior is that as the number of tasks gets larger, so the affected sets tend to become bigger and hence the change that the number of UCBs of the tasks affected by a task \(\tau _j\) is larger than the ECBs of task \(\tau _j\) increases.
11.4 Varying cache-related parameters
In the second series of experiments, we vary cache-related parameters, i.e. the block-reload time, the cache utilization, the cache reuse, and the number of cache blocks. For each of these experiments, we use the weighted schedulability ratio as a metric. Because we assume that intra-task CRPD is subsumed in the worst-case response times of tasks and we generate a new set of 1000 systems for each parameter configuration the weighted schedulability ratios for FPTS and FPPS without CRPD as well as FPNS are independent of the parameter configuration. Stated differently, FPTS and FPPS without CRPD as well as FPNS are represented in the graphs by means of horizontal lines.
11.4.1 Block reload time
In the first experiment, we vary the block reload time (BRT) from 0 to 640 \(\upmu \)s. Figure 15 (middle) shows the results for implicit deadlines. By increasing the BRT, we increase the CRPD and therefore penalise pre-emption. Consequently, the number of task sets deemed schedulable with FPPS with CRPD quickly drops to zero, while the performance of FPTS with CRPD converges to the performance of FPNS (as expected). The impact of the task-layout is naturally limited on the two extremes, i.e. when the overall impact of the pre-emption delay is either negligible or dominating. Consequently, the layout-search is most efficient in the middle range. Nevertheless, the absolute difference between the initial layout and the improved layout remains largely constant for most values of the BRT and hence, the relative benefits of the task-layout search increase with the pre-emption overhead.
It is interesting to see that FPTS with CRPD is able to deem more task sets schedulable than FPNS, even for an infinite BRT. The reason is as follows. If the sets of UCBs and ECBs of two tasks are completely disjoint (which may happen for randomly generated UCBs and ECBs of tasks), the CRPD of these two tasks pre-empting each other will remain zero. It is therefore possible that FPTS with CRPD outperforms FPNS, because not every pre-emption will be penalised.
Figure 16 (middle) shows the results for various CRPD approaches and implicit deadlines. Similar to the earlier experiments, the UCB-Union Multiset approach and the composite approach have overlapping lines in the graphs.
Figures 15 and 16 also show the results for constrained and arbitrary deadlines. Again, FPTS with CRPD can take advantage of increasing deadlines, as illustrated by (i) the reducing performance gap between FPTS without CRPD and FPTS with CRPD and (ii) the increasing performance gap between FPTS with CRPD and FPPS with CRPD from constrained deadlines to arbitrary deadlines.
11.4.2 Cache utilization
In the second experiment, we vary the total cache utilization (\(U^\mathrm{C}\)) from 0 to 160 and we reset the BRT to 8 \(\upmu \)s. Since the number of cache blocks (\(N^\mathrm{C}\)) remains the same, increasing \(U^\mathrm{C}\) means increasing the number of ECBs of tasks. Figure 17 (middle) shows again a weighted schedulability ratio for implicit deadlines. FPPS and FPTS with CRPD are both able to schedule considerably more task sets than FPNS. This is due to the fixed number of cache blocks, which restricts the maximum possible pre-emption cost. At a total cache utilization of 40, each pre-emption evicts most of the cache contents which then need to be reloaded, hence further increases in cache utilization have little effect on schedulability. The performance of the task-layout search follows the same scheme as in Fig. 15: The task layout has no impact when there is no CRPD at all, and also, when each task evicts the complete cache content on pre-emption.
Because our earlier experiments assume a relatively low cache utilization, i.e. \(U^\mathrm{C} = 4\), the lines in the graphs for the UCB-Union Multi-set approach and the composite approach coincide. From Fig. 10 in Altmeyer et al. (2012) and Fig. 18 we observe that the point at which the lines for the UCB-Union Multi-set approach and the ECB-Union Multiset approach cross differ. In the case of FPPS, they cross at \(U^\mathrm{C} = 9\), while for FPTS they cross at \(U^\mathrm{C} = 20\).
11.4.3 Cache reuse
In the third experiment, we vary the cache reuse, i.e. the percentage of ECBs that are also UCBs. Figure 19 (middle) shows the weighted schedulability ratio for implicit deadlines. As the UCB percentage increases, the performance of FPTS and FPPS with CRPD decreases. Figure 19 also shows the results for constrained and arbitrary deadlines. Similar to earlier experiments, e.g. where the block reload time is varied, FPTS with CRPD can take more advantage of increasing deadlines than FPPS with CRPD. Considering the graphs from constrained deadlines to arbitrary deadlines, this is illustrated by (i) the reducing performance gap between FPTS without CRPD and FPTS with CRPD and (ii) the increasing performance gap between FPTS with CRPD and FPPS with CRPD.
Figure 20 shows the results for constrained (top), implicit (middle) and arbitrary (bottom) deadlines. In general, the graphs have the same trends as those of earlier experiments, with the exception of the ECB-Only approach. Because the number of ECBs remains the same, Fig. 20 contains horizontal lines for the ECB-Only approach. This figure nicely illustrates the difference between the ECB-Only approach and the UCB-Only Multiset approach. When including a contribution for a task \(\tau _j\), the ECB-Only approach includes the ECBs of task \(\tau _j\) itself, whereas the UCB-Union Multiset approach uses the ECBs of the tasks affected by task \(\tau _j\). Which method performs best depends on the comparison between these two factors. When the UCB percentage is high, the number of UCBs of affected tasks is larger than the number of ECBs of task \(\tau _j\), and the ECB-Only approach outperforms the UCB-Only Multiset approach. In contrast, when the UCB percentage is small, the opposite is true and the UCB-Only Multiset approach outperforms the ECB-Only approach.
11.4.4 Number of cache blocks
In the last experiment of this series, we vary the number of cache blocks (\(N^\mathrm{C}\)). Figure 21 (middle) shows the weighted schedulability ratio for implicit deadlines. As \(N^\mathrm{C}\) increases, the total number of ECBs being used by tasks also increases and, contrary to the second experiment, more of these ECBs fit into the cache. Hence, the pre-emption costs increase when more blocks need to be reloaded. The schedulability ratios of FPPS and FPTS with CRPD therefore decrease. FPPS will eventually be unable to schedule any tasks. The performance of FPTS, however, converges to the performance of FPNS, i.e. with FPNS task sets are unaffected by the increased pre-emption costs. We recall that FPTS with CRPD still outperforms FPNS, because, after assigning the highest possible pre-emption thresholds to tasks using our OTA, some of the remaining pre-emptions in the system may effectively come for free due to the limited overlap between the UCBs of some tasks and the ECBs of others. While the schedulability ratios for FPPS and FPTS decrease with the number of cache blocks, the impact of the task-layout search increases. More cache blocks means that the difference between different layouts increases. Nevertheless, the overall trend remains: increasing the cache size decreases the schedulability ratios.
Figure 21 again shows that FPTS with CRPD can take more advantage of increasing deadlines than FPPS with CRPD.
Figure 22 shows the results for various CRPD approaches for constrained (top), implicit (middle), and arbitrary (bottom) deadlines. These figures have the same trends as those of earlier experiments.
12 Conclusions
In this paper, we integrated analysis of CRPD into response time analysis for fixed priority scheduling of tasks with pre-emption thresholds (FPTS) and arbitrary deadlines. Moreover, we introduced an OTA algorithm that minimizes the effects of CRPD given an initial set of task priorities. The analysis we provided generalizes existing analysis for FPPS with constrained deadlines and CRPD described in Altmeyer et al. (2012), and covers the most effective approaches presented in that paper, in particular the ECB-Union and UCB-Union Multiset approaches. Finally, building on the work in Lunniss et al. (2012), we presented a Schedulable Task-Layout Search (STLS) algorithm to improve the layout of tasks in memory in order to make the task set schedulable.
We presented an extensive comparative evaluation of the performance of the schedulability tests for FPTS and FPPS with and without CRPD based on 3 orthogonal dimensions and seven main experiments. Interestingly, we found that the theoretical performance advantage that FPTS has over FPPS when there are no CRPDs is magnified when CRPDs are taken into account. Further, even when the overheads (block reload times) affecting CRPD are increased to very high levels, FPTS still retains a performance advantage over FPNS (which it also dominates). This is due to the limited overlap between the UCBs of some tasks and the ECBs of others, meaning that some pre-emptions effectively come for free (i.e. no CRPD).
Regarding the three orthogonal dimensions on which the comparative evaluation is based, i.e. CRPD approach, deadline type, and task layout, we can draw the following conclusions. In most of our experiments, the UCB-Union Multiset approach outperforms the ECB-Union Multiset approach for FPTS with CRPD. In particular, the UCB-Union Multiset approach has the same performance as the composite approach that combines the UCB-Union Multiset and ECB-Union Multiset approaches. This differs from the results in Altmeyer et al. (2012) for FPPS and CRPD. The reason for this can be found in the experiment in which the cache utilization is varied, which shows that the UCB-Union Multiset approach out performs the ECB-Union Multiset approach until a cache utilization of 20 is reached (compared to 9 for a similar transition with FPPS), showing that the two methods are incomparable. In our evaluation, we considered constrained, implicit, and arbitrary deadlines. We observed that in all major experiments the performance of FPTS with CRPD improved significantly from constrained towards arbitrary deadlines, unlike FPPS with CRPD, which showed only marginal improvements. We attribute this strength of FPTS to its ability to decrease the worst-case response time of lower priority tasks by means of preemption thresholds at the expense of an increase of the worst-case response time of higher priority tasks whenever higher priority tasks tolerate the additional blocking incurred. Finally, our evaluation shows the merit of applying both FPTS and layout search, i.e. the recommended solution, over what might be considered the default option of FPPS and initial layout. The amount of improvement in the weighted schedulability range is 33% for implicit deadlines.
Our results indicate that FPTS can rightly be viewed as a potential successor to FPPS as a defacto standard in industry, where it is already supported by both OSEK (2005) and AUTOSAR (AUT 2010) compliant operating systems.
There are a number of ways in which this work can be extended. Firstly, our STLS-algorithm is based on simulated annealing and considers sequential layouts of tasks in memory. A more comprehensive search based on genetic algorithms, including variations in layout including gaps between tasks, is a direction for future work. Secondly, OSEK and AUTOSAR only specify/require a restricted version of FPTS. Although the consequences of this restriction on the schedulability ratio of task sets without CRPD is shown to be limited (Hatvani and Bril 2015), the consequences with CRPD are to be investigated. Thirdly, our OTA algorithm assumes that task priorities are provided. The problem of optimally assigning both priorities and thresholds using a computationally tractable method remains open.
Footnotes
- 1.
Strictly speaking, the condition \({\text{ aff }}({\pi }_i,{\pi }_j) \ne \emptyset \) in (10) can be removed, because \(\gamma ^{{\mathrm{ecb}\hbox {-}\mathrm{o}}}_{i,j}(t)\) is only applied in a context where \(i \in {\text{ lp }}({\pi }_j)\). We inserted the condition to ease the comparison of FPPS (this section) and FPTS (later on).
- 2.
Compared to (10) in Bril et al. (2014), Eq. (13) has been simplified. Because \(M^{{\text {ucb-o}}}_{i,j}(t)\) contains the sizes of sets of UCBs, i.e. non-negative values rather than arbitrary values or the sets themselves, applying the closed operator “\(\vert \cdot \vert \)” to sort\(\left( M^{{\text {ucb-o}}}_{i,j}(t)\right) [\ell ]\) is either redundant, i.e. when the operator is interpreted as absolute value, or wrong, i.e. when interpreted as set-cardinality. The operator is therefore absent in (13). This simplification also applies to equations that have been derived from (13), in particular (32), (34), and (38). We observe that Eq. (13) for \(\gamma _{i,j}^{{\text {ecb-u}}}(t)\) in Altmeyer et al. (2012) contains the same redundancy or problem as (10) in Bril et al. (2014).
- 3.
This approach to reduce pessimism, i.e. taking the sum of a finite number of largest values from a multiset rather than multiplying that number with the largest value, has also been applied for blocking in the context of synchronization protocols in Behnam et al. (2010).
- 4.
The notion of hold time is inspired by the term resource hold times in Bertogna et al. (2007) and the observation in Davis et al. (2000) and Gai et al. (2001) that it is possible to make two tasks mutually non-pre-emptive by letting them share a so-called pseudo-resource. Our hold time is the same as the resource hold time of the pseudo-resource.
- 5.
In Bril et al. (2014), only the simple composite approach is described and used in the evaluation.
Notes
Acknowledgements
We thank Leo Hatvani for pointing us at anomalies in the results of the ECB-Union Multiset approach and the UCB-Union Multiset approach for FPTS and CRPD, caused by flaws in the implementation. Due to these flaws, the results of the ECB-Union Multiset, the UCB-Union Multiset, and the composite approach presented in the evaluation in Bril et al. (2014) are pessimistic. We also thank the anonymous referees of the Real-Time Systems journal for there comments on an earlier version of this paper.
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