linux-kernel - Re: [Bug #15713] hackbench regression due to commit 9dfc6e68bfe6e

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Message-ID: <4BD564BE.6020700@kernel.org>
Date:	Mon, 26 Apr 2010 12:02:38 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Pekka Enberg <penberg@...helsinki.fi>
CC:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Christoph Lameter <cl@...ux.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Maciej Rutecki <maciej.rutecki@...il.com>,
	Alex Shi <alex.shi@...el.com>, tim.c.chen@...el.com
Subject: Re: [Bug #15713] hackbench regression due to commit 9dfc6e68bfe6e

Hello,

On 04/26/2010 09:22 AM, Pekka Enberg wrote:
> On Mon, Apr 26, 2010 at 9:59 AM, Zhang, Yanmin
> <yanmin_zhang@...ux.intel.com> wrote:
>>> I haven't been able to reproduce this either on my Core 2 machine.
>> Mostly, the regression exists on Nehalem machines. I suspect it's related to
>> hyper-threading machine.
> 
> OK, so does anyone know why hyper-threading would change things for
> the per-CPU allocator?

My wild speculation is that previously the cpu_slub structures of two
neighboring threads ended up on the same cacheline by accident thanks
to the back to back allocation.  W/ the percpu allocator, this no
longer would happen as the allocator groups percpu data together
per-cpu.

Thanks.

-- 
tejun
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