Re: Memstore performance improvements v0.90 vs v0.87

[Blair Bethwaite <blair.bethwaite@gmail.com>]

Hi James,

Interesting results, but did you do any tests with a NUMA system? IIUC
the original report was from a dual socket setup, and that'd
presumably be the standard setup for most folks (both OSD server and
client side).

Cheers,

On 20 February 2015 at 20:07, James Page <james.page@ubuntu.com> wrote:
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> Hi All
>
> The Ubuntu Kernel team have spent the last few weeks investigating the
> apparent performance disparity between RHEL 7 and Ubuntu 14.04; we've
> focussed efforts in a few ways (see below).
>
> All testing has been done using the latest Firefly release.
>
> 1) Base network latency
>
> Jay Vosburgh looked at the base network latencies between RHEL 7 and
> Ubuntu 14.04; under default install, RHEL actually had slightly worse
> latency than Ubuntu due to the default enablement of a firewall;
> disabling this brought latency back inline between the two distributions:
>
> OS                      rtt min/avg/max/mdev
> Ubuntu 14.04 (3.13)     0.013/0.016/0.018/0.005 ms
> RHEL7 (3.10)            0.010/0.018/0.025/0.005 ms
>
> ...base network latency is pretty much the same.
>
> This testing was performed on a matched pair of Dell Poweredge R610's,
> configured with a single 4 core CPU and 8G of RAM.
>
> 2) Latency and performance in Ceph using Rados bench
>
> Colin King spent a number of days testing and analysing results using
> rados bench against a single node ceph deployment, configured with a
> single memory backed OSD, to see if we could reproduce the disparities
> reported.
>
> He ran 120 second OSD benchmarks on RHEL 7 as well as Ubuntu 14.04 LTS
> with a selection of kernels including 3.10 vanilla, 3.13.0-44 (release
> kernel), 3.16.0-30 (utopic HWE kernel), 3.18.0-12 (vivid HWE kernel)
> and 3.19-rc6 with 1, 16 and 128 client threads.  The data collected is
> available at [0].
>
> Each round of tests consisted of 15 runs, from which we computed
> average latency, latency deviation and latency distribution:
>
>> 120 second x 1 thread
>
> Results all seem to cluster around 0.04->0.05ms, with RHEL 7 averaging
> at 0.044 and recent Ubuntu kernels at 0.036-0.037ms.  The older 3.10
> kernel in RHEL 7 does have some slightly higher average latency.
>
>> 120 second x 16 threads
>
> Results all seem to cluster around 0.6-0.7ms.  3.19.0-rc6 had a couple
> of 1.4ms outliers which pushed it out to be worse than RHEL 7. On the
> whole Ubuntu 3.10-3.18 kernels are better than RHEL 7 by ~0.1ms.  RHEL
> shows a far higher standard deviation, due to the bimodal latency
> distribution, which from the casual observer may appear to be more
> "jittery".
>
>> 120 second x 128 threads
>
> Later kernels show up to have less standard deviation than RHEL 7, so
> that shows perhaps less jitter in the stats than RHEL 7's 3.10 kernel.
> With this many threads pounding the test, we get a wider spread of
> latencies and it is hard to tell any kind of latency distribution
> patterns with just 15 rounds because of the large amount of latency
> jitter.  All systems show a latency of ~ 5ms.  Taking into
> consideration the amount of jitter, we think these results do not make
> much sense unless we repeat these tests with say 100 samples.
>
> 3) Conclusion
>
> We’ve have not been able to show any major anomalies in Ceph on Ubuntu
> compared to RHEL 7 when using memstore.  Our current hypothesis is that
> one needs to run the OSD bench stressor many times to get a fair capture
> of system latency stats.  The reason for this is:
>
> * Latencies are very low with memstore, so any small jitter in
> scheduling etc will show up as a large distortion (as shown by the large
> standard deviations in the samples).
>
> * When memstore is heavily utilized, memory pressure causes the system
> to page heavily and so we are subject to the nature of perhaps delays on
> paging that cause some latency jitters.  Latency differences may be just
> down to where a random page is in memory or in swap, and with memstore
> these may cause the large perturbations we see when running just a
> single test.
>
> * We needed to make *many* tens of measurements to get a typical idea of
> average latency and the latency distributions. Don't trust the results
> from just one test
>
> * We ran the tests with a pool configured to 100 pgs and 100 pgps [1].
> One can get different results with different placement group configs.
>
> I've CC'ed both Colin and Jay on this mail - so if anyone has any
> specific questions about the testing they can chime in with responses.
>
> Regards
>
> James
>
> [0] http://kernel.ubuntu.com/~cking/.ceph/ceph-benchmarks.ods
> [1] http://ceph.com/docs/master/rados/configuration/pool-pg-config-ref/
>
> - --
> James Page
> Ubuntu and Debian Developer
> james.page@ubuntu.com
> jamespage@debian.org
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-- 
Cheers,
~Blairo
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