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PIAS: Practical Information-Agnostic FlowScheduling for Commodity Data Centers
Wei Bai1 Li Chen1 Kai Chen1 Dongsu Han2 Chen Tian3 Hao Wang1
1SING Group @ HKUST 2KAIST 3Nanjing University
Abstract Many existing data center network (DCN) flowscheduling schemes that minimize flow completion times (FCT)assume prior knowledge of flows and custom switch functions,making them superior in performance but hard to implement inpractice. By contrast, we seek to minimize FCT with no priorknowledge and existing commodity switch hardware.
To this end, we present PIAS, a DCN flow scheduling mech-anism that aims to minimize FCT by mimicking Shortest JobFirst (SJF) on the premise that flow size is not known a priori.At its heart, PIAS leverages multiple priority queues availablein existing commodity switches to implement a Multiple LevelFeedback Queue (MLFQ), in which a PIAS flow is graduallydemoted from higher-priority queues to lower-priority queuesbased on the number of bytes it has sent. As a result, short flowsare likely to be finished in the first few high-priority queues andthus be prioritized over long flows in general, which enables PIASto emulate SJF without knowing flow sizes beforehand.
We have implemented a PIAS prototype and evaluated PIASthrough both testbed experiments and ns-2 simulations. We showthat PIAS is readily deployable with commodity switches andbackward compatible with legacy TCP/IP stacks. Our evalua-tion results show that PIAS significantly outperforms existinginformation-agnostic schemes, for example, it reduces FCT byup to 50% compared to DCTCP  and L2DCT ; and itonly has a 1.1% performance gap to an ideal information-awarescheme, pFabric , for short flows under a production DCNworkload.
Index Terms Data center networks, Flow scheduling
It has been a virtually unanimous consensus in the com-munity that one of the most important objectives for datacenter network (DCN) transport designs is to minimize theflow completion times (FCT) [13, 27, 33, 34]. This isbecause many of todays cloud applications, such as websearch, social networking, and retail recommendation, etc.,have very demanding latency requirements, and even a smalldelay can directly affect application performance and degradeuser experience [11, 34].
To minimize FCT, most previous proposals [13, 27, 33, 43]assume prior knowledge of accurate flow information, e.g.,flow sizes or deadlines, to achieve superior performance. Forexample, PDQ, pFabric and PASE [13, 27, 33] all assume flowsize is known a priori, and attempt to approximate ShortestJob First (SJF, preemptive), which is the optimal schedulingdiscipline for minimizing the average FCT over a single link.In this paper, we question the validity of this assumption,and point out that, for many applications, such information is
A subset of this work appears in USENIX NSDI15 .
difficult to obtain, and may even be unavailable (II). Existingtransport layer solutions with this assumption are thereforevery hard to implement in practice.
We take one step back and ask: without prior knowledge offlow size information, what is the best scheme that minimizesFCT with existing commodity switches?
Motivated by the above question, we set up our key designgoals as follows: Information-agnostic: Our design must not assume a
priori knowledge of flow size information being availablefrom the applications.
FCT minimization: The solution must be able to enforcea near-optimal information-agnostic flow schedule. Itshould minimize the average and tail FCT for latency-sensitive short flows, while not adversely affecting theFCT of long flows.
Readily-deployable: The solution must work with ex-isting commodity switches in DCNs and be backwardcompatible with legacy TCP/IP stacks.
When exploring possible solution spaces, we note thatsome existing approaches such as DCTCP, Timely, HULL,etc. [11, 12, 32, 34, 47] reduce FCT without relying on flowsize information. They generally improve FCT by maintain-ing low queue occupancy through mechanisms like adaptivecongestion control, ECN, pacing, etc. However, they do notprovide a full answer to our question, because they mainlyperform end-host based rate control which is ineffective forflow scheduling [13, 33].
In this paper, we answer the question with PIAS, a practi-cal information-agnostic flow scheduling that minimizes theFCT in DCNs. In contrast to previous FCT minimizationschemes [13, 27, 33] that emulate SJF by using prior knowl-edge of flow sizes, PIAS manages to mimic SJF with noprior information. At its heart, PIAS leverages multiple priorityqueues available in existing commodity switches to implementa Multiple Level Feedback Queue (MLFQ), in which a PIASflow is gradually demoted from higher-priority queues tolower-priority queues based on the bytes it has sent duringits lifetime. In this way, PIAS ensures in general that shortflows are prioritized over long flows, effectively emulating SJFwithout knowing flow sizes beforehand.
Generally speaking, there are three components in thePIAS framework. A central entity collects traffic informationreported from each end host periodically, calculates the demo-tion thresholds based on the aggregated traffic statistics fromthe entire network, and distributes the thresholds to all end
hosts. Then each host uses such thresholds to tag the traffic.And finally, the PIAS switches simply perform strict priorityqueueing based on the packet tags.
However, we face several concrete challenges before makingPIAS truly effective. First, how to determine the demotionthreshold for each queue of MLFQ? Second, as the trafficvaries across both time and space, how to keep PIASsperformance in such a dynamic environment? Third, how toensure PIASs compatibility with legacy TCP/IP stacks inproduction DCNs?
For the first challenge, we derive a set of optimal demotionthresholds for MLFQ through solving a FCT minimizationproblem. We further show that the derived threshold set isrobust to a reasonable range of traffic distributions. Thisenables PIAS to distribute the demotion thresholds to the endhosts for packet tagging while only performing strict priorityqueueing, a built-in function, in the PIAS switches.
For the second challenge, since one set of demotion thresh-olds only works the best for a certain range of traffic dis-tributions, PIAS adjusts the thresholds to keep up with thetraffic dynamics. However, the key problem is that a mismatchbetween thresholds and underlying traffic is inevitable. Oncehappens, short flows may be adversely affected by large onesin a queue, impacting on their latency. Inspired by ideas fromECN-based rate control , PIAS employs ECN to mitigatethe mismatch problem. Our insight is that, by maintaining lowqueue occupancy, short flows always see small queues and thuswill not be seriously delayed even if they are mistakenly placedin a queue with a long flow due to mismatched thresholds.
For the third challenge, we employ DCTCP-like transport atthe PIAS end hosts and find that PIAS interacts favorably withDCTCP or other legacy TCP protocols with ECN enabled. Apotential problem is that many concurrent short flows maystarve a coexisting long flow, triggering TCP timeouts anddegrading application performance. We measure the extent ofstarvation on our testbed with a realistic workload and analyzepossible solutions. We ensure that all mechanisms in PIAS canbe implemented by a shim layer over NIC without touchingthe TCP stack.
We have implemented a PIAS prototype (IV). On the endhost, we implement PIAS as a kernel module in Linux, whichresides between the Network Interface Card (NIC) driver andthe TCP/IP stack as a shim layer. It does not touch any TCP/IPimplementation that natively supports various OS versions.In virtualized environments, PIAS can also support virtualmachines well by residing in hypervisor (or Dom 0). At theswitch, PIAS only needs to enable priority queues and ECNwhich are both built-in functions readily supported by existingcommodity switch hardware.
We evaluate PIAS on a small-scale testbed with 16 Dellservers and a commodity Pronto-3295 Gigabit Ethernet switch(Broadcom BCM#56538). In our experiments, we find thatPIAS reduces the average FCT for short flows by 29-49%and 18-34% compared to DCTCP under two realistic DCNtraffic patterns. It also improves the query performance by28-30% in a Memcached  application (V-A). We furtherdig into different design components of PIAS such as queues,optimal demotion threshold setting, ECN, and demonstrate
the effectiveness of each of their contributions to PIASsperformance (V-B).
To complement our small-scale testbed experiments, wefurther conduct large-scale simulations in a simulated 10/40Gnetwork using ns-2 . In our simulations, we show thatPIAS outperforms all existing information-agnostic solutionsunder realistic DCN workloads, reducing the average FCTfor short flows by up to 50% compared to DCTCP andL2DCT. In addition, our results show that PIAS, as a readily-deployable information-agonistic scheme, also delivers a com-parable performance to a clean-slate information-aware design,pFabric , in certain scenarios. For example, there is onlya 1.1% gap to pFabric for short flows in a data miningworkload  (V-C).
To make our work easy to reproduce, we made ourimplementation and evaluation scripts available online at:http://sing.cse.ust.hk/projects/PIAS.
To motivate our design, we introduce a few cases in whichthe accurate flow size information is hard to obtain, or simplynot available.
HTTP chunked transfer: Chunked transfer has been sup-ported since HTTP 1.1 , where dynamically generatedcontent is transferred during the generation process. This modeis widely used by datacenter appli