linux-kernel - Re: [PATCH 0/5] perf events finer grained context instrumentation / context exclusion

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Message-ID: <20100610101637.GA10406@elte.hu>
Date:	Thu, 10 Jun 2010 12:16:37 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Zhang Yanmin <yanmin_zhang@...ux.intel.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 0/5] perf events finer grained context instrumentation /
 context exclusion

* Frederic Weisbecker <fweisbec@...il.com> wrote:

>  Performance counter stats for './hackbench 5' (10 runs):
> 
>          1313640764  instructions             #      0,241 IPC     ( +-   1,393% )  (scaled from 100,05%)
>           214737441  branches                   ( +-   0,948% )
> 
>          1293802776  instructions             #      0,245 IPC     ( +-   0,343% )
>           209495435  branches                   ( +-   0,392% )

Indeed it's about 4 times less noise, not bad.

Cycles is fundamentally random.

> So yeah, the results look a bit better. Still not perfects:
> 
> - we are still instrumenting the tiny parts between the true interrupt
>   and irq_enter() (same for irq_exit() and the end). Same for softirqs.
> 
> - random randomnesses...

Random randomness shouldnt occur for something like instructions or branches.

Could you try some 'must not be variable' workload, like:

    taskset 1 ./hackbench 1

If the workload is pinned to a single CPU then it ought to not be variable at 
all. (modulo things like hash chain lengths and slab caching details, but 
those should not cause 0.4% kind of noise IMO)

Btw., we could try to record all branches of an execution (using BTS, of a 
relatively short but static-length run), and see where the variance comes 
from. I doubt the current BTS code is ready for that, but it would be 'the' 
magic trace-from-hell that includes all execution of the task, recorded at the 
hardware level.

	Ingo
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