Re: Performance regression in v3.14

[Viresh Kumar]

Cc'ing Dirk who is taking care of intel-pstate driver.

On 6 May 2014 22:05, Johan Hovold <jhovold@xxxxxxxxx> wrote:
> After updating my main system from v3.13 to v3.14.2, I found that the
> git bash-completion was extremely sluggish. Completing a file name would
> take roughly six rather than one second on this Haswell machine
> (i7-4770). (Other things, such as git rebase, also felt slower, but
> the completion issue was much more obvious and easy to measure).
>
> I managed to reproduce the problem using the following minimal construct
>
>         cat dmesg.repeat | while read x; do true; done
>
> where dmesg.repeat is simply dmesg concatenated together to an
> equivalent number of lines as produced by git ls-files in the
> kernel-source tree root (45k), and where the actual processing of each
> line has been removed.
>
> Most of the time I get:
>
>         $ time cat dmesg.repeat | while read x; do true; done
>
>         real    0m6.091s
>         user    0m3.674s
>         sys     0m2.447s
>
> but sometimes it only takes one second.
>
>         $ time cat dmesg.repeat | while read x; do true; done
>
>         real    0m1.100s
>         user    0m0.544s
>         sys     0m0.570s
>
> I don't seem to be able to reproduce the problem on 3.13 where the pipe
> always takes about one second to finish.
>
> Taking all but one core offline seems to make the problem go away, and so
> does using the performance rather than powersave governor of the
> intel_pstate cpufreq driver (on at least one of two online cores).
>
> Moving the mouse cursor makes to loop finish faster, and so does
> switching to a another terminal to print cpufreq/cpuinfo_cur_freq which
> was around cpuinfo_min_freq several times (when tracing, see below).
>
> I could not reproduce the problem when using perf record, but I can get
> function-profile traces using ftrace (in which case the loop takes about
> 60 seconds instead of six seconds to finish).
>
> Comparing the traces I see a lot of functions taking ten times longer to
> finish, but I guess that's expected if this is indeed a cpufreq issue.
>
> Since this is my main machine (and only multi-core machine at the
> moment) I'm not able to bisect this myself. And for the same reason I
> have not verified that the problem persists in v3.15-rc.
>
> I don't see any cpufreq patches in the v3.14.3 stable queue nor anything
> obviously related and marked for stable in v3.15-rc.
>
> Any ideas about what might be going on?

I tried to take a look at the diff for cpufreq between 3.13 and 3.14.2 and
couldn't pin point on any change which might cause it. Don't have a clue
of what's going on. I don't know how to help you on this.

Normally I test my stuff on a ARM board and I don't remember facing
any such behavior there. There might be something wrong with intel-pstate
as well..

Also, can you try to use acpi-cpufreq instead? And see how that is behaving?

--
viresh
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