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Uniprocessor scheduling of real-time synchronous dataflow tasks

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Abstract

The synchronous dataflow graph (SDFG) model is widely used today for modeling real-time applications in safety-critical application domains. Schedulability analysis techniques that are well understood within the real-time scheduling community are applied to the analysis of recurrent real-time workloads that are represented using this model. An enhancement to the standard SDFG model is proposed, which supports the specification of a real-time latency constraint between a specified input and a specified output of an SDFG. A polynomial-time algorithm is derived for representing the computational requirement of each such enhanced SDFG task in terms of the notion of the demand bound function (dbf), which is widely used in real-time scheduling theory for characterizing computational requirements of recurrent processes represented by, e.g., the sporadic task model. By so doing, the extensive dbf-centered machinery that has been developed in real-time scheduling theory for the hard-real-time schedulability analysis of systems of recurrent tasks may be applied to the analysis of systems represented using the SDFG model as well. The applicability of this approach is illustrated by applying prior results from real-time scheduling theory to construct an exact preemptive uniprocessor schedulability test for collections of independent recurrent processes that are each represented using the enhanced SDFG model.

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Notes

  1. 1.

    Most SDFG models allow for multigraphs, in which there may be multiple edges between the same pair of vertices. This feature is not particularly relevant to determining how they are scheduled and we, therefore, ignore them in this paper. For the same reason, we also ignore edges that are self-loops: lead from a vertex back to itself. We point out that our results are easily extended to deal with multiple edges and self-loops.

  2. 2.

    In addition to requiring that each buffer be of finite size, many SDFG scheduling algorithms seek to minimize the maximum number of tokens each buffer will need to hold. However the algorithms of Singh et al. (2017) do not consider buffer-size minimization while seeking to construct schedules in which real-time constraints are met, and we will do likewise here—require that buffers be of finite size, but leave as future work the problem of determining the minimum sizes needed.

  3. 3.

    An SDFG G is said to be connected if the undirected graph formed by taking the actors as vertices and channels as undirected edges is connected.

  4. 4.

    The rank of a matrix is the maximum number of linearly independent columns in it. Efficient polynomial-time algorithms are known for computing rank.

  5. 5.

    One may choose to think of src as a dummy actor that queues the external input tokens directed at \(v_{\mathrm {in}}\) until it has accumulated \(\mathbf {q}(v_{\mathrm {in}})\) tokens, at which instant it releases them all simultaneously to a; hence, a does not have to deal with the possibility of unbounded durations between the arrivals of the three tokens. (However, an unbounded duration may elapse before the next set of three tokens are released to it.) A similar interpretation may be made for dst.

  6. 6.

    We will show, in Lemma 2, that this skip vector is uniquely defined for a given G.

  7. 7.

    As we have stated in Sect. 2.3 in this paper we have assumed \(\delta =0\) in order to keep things simple. However, our algorithm may be extended to handle non-zero dependency distances: we briefly describe the extension in Sect. 4.3.

  8. 8.

    These are auxiliary variables in the sense that they will not appear in the actual code that implements the algorithm. They are defined solely to record information that is useful in proving properties (in our case, computational complexity) of the algorithm.

  9. 9.

    As with the \(\varvec{\pi }[\cdot ]\) auxiliary variables, the constraint graph is a concept used only in our proofs—no such graph is explicitly constructed by our algorithm.

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Acknowledgements

This research has been supported by NSF Grants CNS 1115284, CNS 1218693, CNS 1409175, and CPS 1446631, AFOSR Grant FA9550-14-1-0161, and ARO Grant W911NF14-1-0499.

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Correspondence to Sanjoy Baruah.

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Singh, A., Ekberg, P. & Baruah, S. Uniprocessor scheduling of real-time synchronous dataflow tasks. Real-Time Syst 55, 1–31 (2019). https://doi.org/10.1007/s11241-018-9310-2

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Keywords

  • Real-time systems
  • Synchronous dataflow (SDF)
  • Hard real-time streaming dataflow applications
  • Algorithms