linux-kernel - Re: [patch 0/4] perf_counter tools: support annotation of live kernel modules

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Date:	Fri, 3 Jul 2009 09:27:17 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Mike Galbraith <efault@....de>
Cc:	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [patch 0/4] perf_counter tools: support annotation of live
	kernel modules


* Mike Galbraith <efault@....de> wrote:

> On Thu, 2009-07-02 at 09:42 +0200, Ingo Molnar wrote:
> 
> (squirrels suggestions away for later reference)
> 
> > Btw, another thing: we are thinking about making -F 1000 (1 KHz 
> > auto-freq sampling) the default for perf top and perf record. This 
> > way we'd always gather enough data (and never too much or too little 
> > data), regardless of the intensity of the workload. Have you played 
> > with -F before, what's your general experience about it? It's 
> > particularly useful for 'rare' and highly fluctuating events like 
> > cache-misses.
> > 
> > Maybe 1KHz is a bit too low - Oprofile defaults to 100000 cycles 
> > interval by default which is about 10 KHz on a 1GHz box and 30 
> > KHz on a 3GHz box. Perhaps 10 KHz is a better default?
> 
> My default usage is 1000Hz to keep overhead low.  Works fine for 
> me.

ah, so you use -F by default?

I still think 10 KHz would be better - especially for really short 
runs like 'perf record -f -g ./git gc'. Since Oprofile samples at 26 
KHz by default, we must not go to a too low frequency, otherwise 
people might get a 'hm, the perf profiles are worse than the 
Oprofile ones' first impression ...

We could perhaps add a freq=1000 switch to .perfconfig - we have 
Git's util/config.c facility - it's just unused right now ;-)

[ Look at the Git sources about how config.c is used. ]

	Ingo
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