On guts and I/O schedulers

Benchmarks and guts sometimes may contradict each other. Like, a benchmark tells that “performance difference is not big”, but guts do tell otherwise (something like “OH YEAH URGHH”). I was wondering why some servers are much faster than other, and apparently different kernels had different I/O schedulers. Setting ‘deadline’ (Ubuntu Server default) makes miracles over having ‘cfq’ (Fedora, and probably Ubuntu standard kernel default) on our traditional workload.

Now all we need is showing some numbers, to please gut-based thinking (though it is always pleased anyway):

Deadline:

avg-cpu:  %user   %nice    %sys %iowait   %idle
           4.72    0.00    7.95   18.18   69.15

Device:    rrqm/s wrqm/s   r/s   w/s  rsec/s  wsec/s
sda          0.00   0.10 91.30 31.30 3147.20 1796.00

    rkB/s    wkB/s avgrq-sz avgqu-sz   await  svctm  %util
  1573.60   898.00    40.32     0.98    7.98   3.65  44.80

CFQ:

avg-cpu:  %user   %nice    %sys %iowait   %idle
           4.65    0.00    7.62   38.26   49.48

Device:    rrqm/s wrqm/s   r/s   w/s  rsec/s  wsec/s
sda          0.00   0.10 141.26 38.86 4563.44 2571.03

    rkB/s    wkB/s avgrq-sz avgqu-sz   await  svctm  %util
  2281.72  1285.51    39.61     7.61   42.52   5.38  96.98

Though load slightly rises and drops, the await/svctime parameters are always better on deadline. The box does high-concurrency (multiple background InnoDB readers), high volume (>3000 SELECT/s) read-only (aka slave) workload on ~200gb dataset, on top of 6-disk RAID0 with write-behind cache. Whatever next benchmarks say, my guts will still fanatically believe that deadline rocks.

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3 Responses to On guts and I/O schedulers

  1. sapphirecat says:

    CFQ is meant to give a predictable, low latency to I/O requests, so that runnnig updatedb or starting Firefox doesn’t make your music skip. Deadline and anticipatory are oriented on the traditional measure of scheduler performance: raw, screaming throughput. Of the latter two, deadline is simpler, and the kernel config suggests it may be better for databases, but that’s something I’ve never bothered to benchmark.

  2. Hashar says:

    Have a look at “Enhancements to Linux I/O Scheduling”:

    http://www.linuxinsight.com/files/ols2005/seelam-reprint.pdf

  3. Pingback: domas mituzas: vaporware, inc. » Blog Archive » I/O schedulers seriously revisited

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