linux-kernel - Re: [PATCH -tip] x86: atomic64: inline atomic64

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Date:	Fri, 3 Jul 2009 23:40:38 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>, mingo@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -tip] x86: atomic64: inline atomic64_read()

* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> Do we actually _have_ any performance-critical 64-bit counters 
> that have monotonicity guarantees? I have no idea. I'm just 
> throwing out the notion.

Due to the 'counter value flipping' we do on perfcounters for 
certain workloads (attr.inherit_stat=1 counters - inherited counters 
that neverheless provide precise per thread readouts - not just 
precise summaries), the monotonicity assumption is not generally 
true anymore.

But it would be nice if Eric could measure your suggested primitive, 
just that we see how many cycles we are talking about. I suspect the 
biggest difference would be a many-readers testcase - but that wont 
normally happen on perfcounters as the counters typically get read 
only from a single thread of executin.

	Ingo
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