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

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Date:	Mon, 26 Apr 2010 13:09:21 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Tejun Heo <tj@...nel.org>
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

Hi,

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.

On 04/26/2010 09:22 AM, Pekka Enberg wrote:
>> OK, so does anyone know why hyper-threading would change things for
>> the per-CPU allocator?

On Mon, Apr 26, 2010 at 1:02 PM, Tejun Heo <tj@...nel.org> wrote:
> 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.

Yanmin, do we see a lot of remote frees for your hackbench run? IIRC,
it's the "deactivate_remote_frees" stat when CONFIG_SLAB_STATS is
enabled.

                        Pekka
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