Date: Fri, 12 Jun 2009 14:12:06 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, linux-kernel@vger.kernel.org,
       Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 2/2] perf_counter: powerpc: Implement generalized cache
	events for POWER processors
Message-ID: <20090612121206.GB31845@elte.hu>
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* Paul Mackerras <paulus@samba.org> wrote:

> > I dont know whether we should do such combo counters in the 
> > kernel itself - i'm slightly against that notion. (seems 
> > complex)
> 
> Yeah.
> 
> When thinking about having "composite" events, i.e. a counter 
> whose value is computed from two or more hardware counters, I 
> couldn't see how to do sampling in the general case.  It's easy if 
> we're just adding multiple counters, but sampling when subtracting 
> counters is hard.  For example, if you want to sample every N 
> cache hits, and you're computing hits as accesses - misses, I 
> couldn't see a decent way to know when to take the sample, not 
> without having to take an interrupt on every access in some 
> circumstances.

We now have a period field - and that could be negative and be 
subtracted by the profiler automatically.

It's still statistical and a given instruction can go 'negative' 
sporadically, but in terms of total function averages and for any 
high-traffic place it's still pretty expressive IMO.

	Ingo
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