linux-kernel - Re: perf_event self-monitoring overhead regression

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Message-ID: <CAMsRxfL3NVuz0dL-=iTJUsPMBt1frcfQ43fyc8pNoHEM9NXiOg@mail.gmail.com>
Date:	Wed, 7 Dec 2011 16:44:19 -0800
From:	stephane eranian <eranian@...glemail.com>
To:	Vince Weaver <vweaver1@...s.utk.edu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: perf_event self-monitoring overhead regression

On Tue, Dec 6, 2011 at 1:07 PM, Vince Weaver <vweaver1@...s.utk.edu> wrote:
> On Thu, 1 Dec 2011, stephane eranian wrote:
>
>> I take it your test is all about self-monitoring, single event, single thread.
>> Did you try breaking down the cost using  TSC and rdtsc() to pinpoint where the
>> regression is coming from in the 3 perf_event syscalls you're using?
>
> I've started gathering results using rdtsc() and the results are even more
> puzzling.
>
Are those the results for surrounding the ioctl() with rdtsc()?

What do the axis actually represent?

> See:
>   http://web.eecs.utk.edu/~vweaver1/projects/perf-events/benchmarks/rdtsc_overhead/core2_raw_null_kernel_rdtsc.png
> for example.
>
> Those results are from a core2 machine, with the CPU scaling governor set
> to "performance", the test bound to CPU0, and the test run 1000 times.
>
> Part of the issue is that a few of those kernels are Debian unstable
> distro kernels and not hand-compiled stock kernels.  I'll rebuild a full
> set of kernels myself and see if I can reproduce the results.
>
> Vince
>
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