episode-13.xml

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  <title>Time Capsule</title>
  <guests>Jia Rao and Kun Suo from University of Texas at Arlington</guests>
  <description>
    <p>
      On Aug. 4 and 5, I attended <a
      href="http://www.cs.hku.hk/apsys2016/program.html">APSys 2016</a>, the
      Asia-Pacific Workshop on Systems.  I was impressed with how many of the
      papers presented there were relevant to Open vSwitch and virtualization
      in general.  This episode is an interview with Jia Rao and Kun (Tony) Suo
      of the University of Texas at Arlington, to talk about their APSys paper,
      <a href="http://i.cs.hku.hk/~fcmlau/papers/capsule.pdf">Time Capsule:
      Tracing Packet Latency across Different Layers in Virtualized
      Systems</a>, which received the conference's Best Paper award.
    </p>

    <p>
      The paper's abstract is:
    </p>

    <blockquote>
      Latency monitoring is important for improving user experience and
      guaranteeing quality-of-service (QoS).  Virtualized systems, which have
      complex I/O stacks spanning multiple layers and often with unpredictable
      performance, present more challenges in monitoring packet latency and
      diagnosing performance abnormalities compared to traditional systems.
      Existing tools either trace network latency at a coarse granularity, or
      incur considerable overhead, or lack the ability to trace across
      different boundaries in virtualized environments.  To address this issue,
      we propose Time Capsule (TC), an in-band profiler to trace packet level
      latency in virtualized systems with acceptable overhead.  TC timestamps
      packets at predefined tracepoints and embeds the timing information into
      packet payloads.  TC decomposes and attributes network latency to various
      layers in the virtualized network stack, which can help monitor network
      latency, identify bottlenecks, and locate performance problems.
    </blockquote>

    <p>
      The interview covers the basic idea behind Time Capsule, the mechanism
      that it uses, techniques for comparing clocks of different machines
      across a network, and how it helps users and administrators track down
      latency issues in a virtual network, with reference to a specific example
      in the paper that shows the advantage of the fine-grained latency
      monitoring available in Time Capsule.  ``You can find some interesting
      results that are totally different from the results you get from
      coarse-grained monitoring.''
    </p>

    <p>
      Other topics include comparison against whole-system profilers such as <a
      href="https://en.wikipedia.org/wiki/Perf_%28Linux%29">Perf</a> or <a
      href="http://xenoprof.sourceforge.net/">Xenoprof</a>, the overhead of
      using Time Capsule, how many tracepoints are usually needed, how to
      decide where to put them, and how to insert a tracepoint.
    </p>

    <p>
      There is a brief discussion of the relationship between Time Capsule and
      <a href="http://p4.org/p4/inband-network-telemetry/">In-Band Network
      Telemetry</a> (INT).  Time Capsule focuses on virtualization, timing, and
      network processing within computer systems, whereas INT tends to focus
      more on switching and properties of the network such as queue lengths.
    </p>

    <p>
      Time Capsule has not yet been released but it will be made available in
      the future.  For now, the best way to learn more is to read <a
      href="http://i.cs.hku.hk/~fcmlau/papers/capsule.pdf">the paper</a>.
      Readers who want to know more can contact the authors at the email
      addresses listed in the paper.
    </p>

    <p>
      The authors are using Time Capsule as the basis for continuing research
      into the performance of virtualized systems.
    </p>

    <p>
      Time Capsule has some limitations.  For example, it is limited to
      measurements of latency, and it cannot record packet drops.  It also,
      currently, requires tracepoints to be inserted manually, although <a
      href="http://www.brendangregg.com/blog/2015-05-15/ebpf-one-small-step.html">eBPF</a>
      might be usable in the future.
    </p>

    <p class="attribution">
      OVS Orbit is produced by <a href="mailto:blp@ovn.org">Ben Pfaff</a>.  The
      intro music in this episode is <a
      href="http://dig.ccmixter.org/files/AlexBeroza/43098">Drive</a>,
      featuring cdk and DarrylJ, copyright 2013, 2016 by Alex.  The bumper music is
      <a href="http://dig.ccmixter.org/files/speck/42100">Yeah Ant</a>
      featuring Wired Ant and Javolenus, copyright 2013 by Speck.  The outro
      music is <a href="http://dig.ccmixter.org/files/Kirkoid/43005">Space
      Bazooka</a> featuring Doxen Zsigmond, copyright 2013 by Kirkoid.  All content is licensed under a Creative Commons <a
      href="http://creativecommons.org/licenses/by/3.0/">Attribution 3.0
      Unported (CC BY 3.0)</a> license.
    </p>
  </description>
<pubDate>Sat, 20 Aug 2016 03:05:27 GMT</pubDate>
</item>