linux-kernel - Re: [patch] cache-miss and cache-refs events on P6-mobile CPUs

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Date:	Tue, 11 Aug 2009 13:21:57 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Johannes Stezenbach <js@...21.net>, linux-kernel@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch] cache-miss and cache-refs events on P6-mobile CPUs

On Tue, 2009-08-11 at 13:06 +0200, Ingo Molnar wrote:
> * Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> 
> > On Tue, 2009-08-11 at 11:34 +0200, Ingo Molnar wrote:
> > 
> > > @@ -116,8 +116,8 @@ static const u64 p6_perfmon_event_map[] 
> > >  {
> > >    [PERF_COUNT_HW_CPU_CYCLES]		= 0x0079,
> > >    [PERF_COUNT_HW_INSTRUCTIONS]		= 0x00c0,
> > > -  [PERF_COUNT_HW_CACHE_REFERENCES]	= 0x0000,
> > > -  [PERF_COUNT_HW_CACHE_MISSES]		= 0x0000,
> > > +  [PERF_COUNT_HW_CACHE_REFERENCES]	= 0x0f2e,
> > > +  [PERF_COUNT_HW_CACHE_MISSES]		= 0x012e,
> > 
> > 2e is total numer of L2 events,
> > 
> > 0f is all mesi states
> > 01 is invalid states
> 
> here's Intel's own description:
> 
>  I_STATE 0x01 Counts how many times requests miss the cache.
>  MESI    0x0F Counts how many times cache lines in any state are accessed.
> 
> so it's pretty close in practice. The only counts that are a bit 
> inapplicable are fetches/prefetches it initiates on its own (they 
> are included here) - but those too are related to the workload in 
> general, so it's good as an approximation.
> 
> It's definitely better than 0x00 IMO. What do you think?

Well, if they say so. I was thinking that counting I states would count
invalidates due to remote S->{E,M} transitions and invpg ins' and such. 

And hitting an invalidated line is a whole different thing than plain
missing it due to it not being present.

Anyway, if this is the Intel recommended thing for cache misses, who am
I to argue.

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