Tandem queue weighted fair smooth scheduling

Abstract

Network devices supporting 100G links are in demand to meet the communication requirements of computing nodes in datacenters and warehouse computers. In this paper, we propose TQ and TQ-Smooth, two light-weight, fair schedulers that accommodate an arbitrarily large number of requestors and are suitable for ultra high-speed links. We show that our first algorithm, TQ, as well its predecessor, DRR, may result in bursty service even in the common case where flow weights are approximately equal, and we find that this can damage the performance of buffer-credit allocation schemes. Our second algorithm, TQ-Smooth, improves short-term fairness to deliver very smooth service when flow weights are approximately equal, while allocating bandwidth in a weighted fair manner. In many practical situations, a scheduler is asked to allocate resources in fixed-size chunks (e.g. buffer units), whose size may exceed that of (small) network packets. In such cases, byte-level fairness will typically be compromised when small-packet flows compete with large-packet ones. We describe and evaluate a scheme that dynamically adjusts the service rates of request/grant buffer reservation to achieve byte-level fairness based on received packet sizes.

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Notes

  1. 1.

    In the common case, flow weights are likely to be equal.

  2. 2.

    The two queues that we use bear a resemblance with the hot and cold queues that are used in RECN [16]. Note that we do not separate the flows in hot and cold as RECN does, but we use the two queues to priortize service and maintain fairness. In fact, any flow will spend some time in either queue.

  3. 3.

    Negative credits are also used in [17].

  4. 4.

    We ignore here the trivial case where \(f\) is the only active flow and thus receives all service.

  5. 5.

    Although any other DRR derivative would work as well, as explained in Sect. 2.2 and Fig. 2, the smooth service of TQ-Smooth is highly desirable when allocating buffers.

  6. 6.

    Note that instead of per-flow request counters we could maintain per-flow request queues to store the size of each individual request. However for large port numbers, this adds significant cost to the implementation.

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Correspondence to Nikolaos Chrysos.

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This is an extended version of our HiPEAC INA-OCMC paper [1], entitled “Arbitration of many thousand flows at 100G and beyond” by ACM, NY, USA ©2013, ISBN: 978-1-4503-1784-9, doi: 10.1145/2482759.2482761

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Chrysos, N., Neeser, F., Gusat, M. et al. Tandem queue weighted fair smooth scheduling. Des Autom Embed Syst 18, 183–197 (2014). https://doi.org/10.1007/s10617-014-9132-y

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Keywords

  • Packet Size
  • Active Flow
  • Flow Weight
  • Packet Scheduler
  • Service Credit