linux-kernel - Re: [PATCH 5/5] slub: Only IPI CPUs that have per cpu obj to flush

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Date:	Mon, 26 Sep 2011 09:36:05 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Gilad Ben-Yossef <gilad@...yossef.com>,
	linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Russell King <linux@....linux.org.uk>,
	Chris Metcalf <cmetcalf@...era.com>, linux-mm@...ck.org,
	Christoph Lameter <cl@...ux-foundation.org>,
	Matt Mackall <mpm@...enic.com>
Subject: Re: [PATCH 5/5] slub: Only IPI CPUs that have per cpu obj to flush

On Mon, 2011-09-26 at 09:54 +0300, Pekka Enberg wrote:
> 
> AFAICT, flush_all() isn't all that performance sensitive. Why do we
> want to reduce IPIs here? 

Because it can wake up otherwise idle CPUs, wasting power. Or for the
case I care more about, unnecessarily perturb a CPU that didn't actually
have anything to flush but was running something, introducing jitter.

on_each_cpu() things are bad when you have a ton of CPUs (which is
pretty normal these days). 
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