linux-kernel - Re: [patch] CFS scheduler, v3

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Date:	Fri, 20 Apr 2007 12:38:56 -0700
From:	William Lee Irwin III <wli@...omorphy.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Con Kolivas <kernel@...ivas.org>,
	Nick Piggin <npiggin@...e.de>, Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Williams <pwil3058@...pond.net.au>,
	Thomas Gleixner <tglx@...utronix.de>, caglar@...dus.org.tr,
	Willy Tarreau <w@....eu>, Gene Heskett <gene.heskett@...il.com>
Subject: Re: [patch] CFS scheduler, v3

Fri, Apr 20, 2007 at 12:24:17PM -0700, Christoph Lameter wrote:
>>> False sharing for a per cpu data structure? Are we updating that 
>>> structure from other processors?

On Fri, 20 Apr 2007, William Lee Irwin III wrote:
>> Primarily in the load balancer, but also in wakeups.

On Fri, Apr 20, 2007 at 12:33:13PM -0700, Christoph Lameter wrote:
> That is fairly rare I think. What other variables that are also writtten 
> frequently would cause false sharing?

I wrote that backward, sorry. Cross-CPU wakeups' frequency depend
heavily on the workload. Probably the only other case I can think of
is io_schedule() but that's not really significant.

I'm not really convinced it's all that worthwhile of an optimization,
essentially for the same reasons as you, but presumably there's a
benchmark result somewhere that says it matters. I've just not seen it.


-- wli
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