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

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Message-ID: <alpine.DEB.2.00.1112061604040.23414@cl320.eecs.utk.edu>
Date:	Tue, 6 Dec 2011 16:07:21 -0500
From:	Vince Weaver <vweaver1@...s.utk.edu>
To:	<eranian@...il.com>
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 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.  

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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