linux-kernel - Re: [patch] Performance Counters for Linux, v2

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Message-ID: <20081209130045.GA32479@elte.hu>
Date:	Tue, 9 Dec 2008 14:00:45 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Stephane Eranian <eranian@...glemail.com>,
	Eric Dumazet <dada1@...mosbay.com>,
	Robert Richter <robert.richter@....com>,
	Arjan van de Veen <arjan@...radead.org>,
	Peter Anvin <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [patch] Performance Counters for Linux, v2


* Paul Mackerras <paulus@...ba.org> wrote:

> > Things like: "kerneltop would not be as accurate with: ..., to the 
> > level of adding 5% of extra noise.". Would that work for you?
> 
> OK, here's an example.  I have an application whose execution has 
> several different phases, and I want to measure the L1 Icache hit rate 
> and the L1 Dcache hit rate as a function of time and make a graph.  So 
> I need counters for L1 Icache accesses, L1 Icache misses, L1 Dcache 
> accesses, and L1 Dcache misses.  I want to sample at 1ms intervals. The 
> CPU I'm running on has two counters.
> 
> With your current proposal, I don't see any way to make sure that the 
> counter scheduler counts L1 Dcache accesses and L1 Dcache misses at the 
> same time, then schedules L1 Icache accesses and L1 Icache misses.  I 
> could end up with L1 Dcache accesses and L1 Icache accesses, then L1 
> Dcache misses and L1 Icache misses - and get a nonsensical situation 
> like the misses being greater than the accesses.

yes, agreed, this is a valid special case of simple counter readout - 
we'll add support to couple counters like that.

Note that this issue does not impact use of multiple counters in 
profilers. (i.e. anything that is not a pure readout of the counter, 
along linear time, as your example above suggests).

Once we start sampling the context, grouping of counters becomes 
irrelevant (and a hindrance) and static frequency sampling becomes an 
inferior method of sampling.

( The highest quality statistical approach is the kind of multi-counter
  sampling model you can see implemented in KernelTop for example, where
  the counters are independently sampled. Can go on in great detail about
  this if you are interested - this is the far more interesting usecase
  in practice. )

	Ingo
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