linux-kernel - Re: [PATCH 3/3] pids: Make it possible to clone tasks with given pids

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Date:	Fri, 11 Nov 2011 20:49:50 +0400
From:	Pavel Emelyanov <xemul@...allels.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Glauber Costa <glommer@...allels.com>,
	Nathan Lynch <ntl@...ox.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Serge Hallyn <serue@...ibm.com>,
	Daniel Lezcano <dlezcano@...ibm.com>
Subject: Re: [PATCH 3/3] pids: Make it possible to clone tasks with given
 pids

On 11/11/2011 08:22 PM, Tejun Heo wrote:
> On Fri, Nov 11, 2011 at 08:18:19PM +0400, Pavel Emelyanov wrote:
>>> To add a bit, it's also one of the more *serialized* path. Doing it in
>>> parallel might actually hurt, 
>>
>> Can you elaborate on this, please?
> 
> It goes through a lot of locks - tasklist_lock, mmlist_lock, other
> subsys locks depending on CLONE_* options.  If you try to do that on
> multiple CPUs in parallel, depending on the level of concurrency,
> contention and extra cacheline bounces may dominate the overhead.

O_O Here are the times of forks on my box (4cores X 2threads, Xeon, RHEL6)

1 cpu 500k forks                                         - 37s
2 cpus on different cores 500k forks on each in parallel - 39s
4 cpus on different cores 500k forks on each in parallel - 41s

8 cpus 500k forks on each in parallel                    - 1m5s

So the fork() scaling seems quite good to me.

> Thanks.
> 

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