linux-kernel - Re: workqueue thing

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Date:	Wed, 23 Dec 2009 09:31:08 +0100
From:	Stijn Devriendt <highguy@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	torvalds@...ux-foundation.org, awalls@...ix.net,
	linux-kernel@...r.kernel.org, jeff@...zik.org, mingo@...e.hu,
	akpm@...ux-foundation.org, jens.axboe@...cle.com,
	rusty@...tcorp.com.au, cl@...ux-foundation.org,
	dhowells@...hat.com, arjan@...ux.intel.com, a@...per.es
Subject: Re: workqueue thing

On Tue, Dec 22, 2009 at 12:50 AM, Tejun Heo <tj@...nel.org> wrote:
>>  2) doesn't deal with cpu heavy tasks/wakeup parallelism
>
> workqueue was never suited for this.  MT workqueues have strong CPU
> affinity which doesn't make sense for CPU-heavy workloads.

It does, really. Have a look at TBB and others. You always want to keep
workqueue items as close to the scheduling thread as possible as the
chance of having a hot cache and TLBs and such are far greater.
The end result is CPU-affine threads fetching work from CPU-affine
queues with workitems scheduled by a thread on that CPU.
To improve parallellism, workqueue threads with empty workqueues
start stealing work away from non-empty workqueues.
The added warmup doesn't weigh in against the added parallellism
in those cases.

Stijn
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