linux-kernel - Re: [generalized cache events] Re: [PATCH 1/1] perf tools: Add missing user space support for config1/config2

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Date:	Mon, 25 Apr 2011 21:55:54 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Stephane Eranian <eranian@...gle.com>, arun@...rma-home.net,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	linux-kernel@...r.kernel.org, Lin Ming <ming.m.lin@...el.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, eranian@...il.com,
	Arun Sharma <asharma@...com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [generalized cache events] Re: [PATCH 1/1] perf tools: Add
 missing user space support for config1/config2

* Andi Kleen <ak@...ux.intel.com> wrote:

> One example of this we had recently in the kernel: 
> 
> function accesses three global objects. Scalability tanks when the test is 
> run with more CPUs.  Now the hit is near the three memory accesses. Which one
> is the one that is actually bouncing cache lines?

that's not an example - you are still only giving vague, untestable, 
unverifiable references. You need to give us something specific and 
reproducible - preferably a testcase.

Peter and me are doing lots of scalability work in the core kernel and for most 
problems i've met it was enough if we knew the function name - the scalability 
problem is typically very obvious from that point on - and an annotated profile 
makes it even more obvious.

I've never met a situation what you describe, that it was not possible to 
disambiguate a real SMP bounce - and i've been fixing SMP bounces in the kernel 
for over ten years.

So you really will have to back up your point with an accurate, reproducible 
testcase - vague statements like the ones you are making i do not accept at 
face value, sorry.

Thanks,

	Ingo
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