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Date: Fri, 20 Apr 2007 12:38:56 -0700
From: William Lee Irwin III <wli@holomorphy.com>
To: Christoph Lameter <clameter@sgi.com>
Cc: Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Con Kolivas <kernel@kolivas.org>, Nick Piggin <npiggin@suse.de>,
       Mike Galbraith <efault@gmx.de>, Arjan van de Ven <arjan@infradead.org>,
       Peter Williams <pwil3058@bigpond.net.au>,
       Thomas Gleixner <tglx@linutronix.de>, caglar@pardus.org.tr,
       Willy Tarreau <w@1wt.eu>, Gene Heskett <gene.heskett@gmail.com>
Subject: Re: [patch] CFS scheduler, v3
Message-ID: <20070420193856.GC2986@holomorphy.com>
References: <20070418175017.GA5250@elte.hu> <20070418212645.GU2986@holomorphy.com> <Pine.LNX.4.64.0704201223280.21417@schroedinger.engr.sgi.com> <20070420192906.GB2986@holomorphy.com> <Pine.LNX.4.64.0704201231250.21514@schroedinger.engr.sgi.com>
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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