linux-kernel - Re: [PATCH] libata: use single threaded work queue

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Date:	Wed, 19 Aug 2009 17:15:10 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Tejun Heo <htejun@...il.com>
Cc:	Jeff Garzik <jeff@...zik.org>, Jens Axboe <jens.axboe@...cle.com>,
	Mark Lord <liml@....ca>, linux-ide@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH] libata: use single threaded work queue

> Are work threads per workqueue?  Combined with per-cpu binding,
> dynamic thread pool per workqueue can get quite messy.  All three
> factors end up getting multiplied - ie. #cpus * pool_size which can be
> enlarged by the same work hopping around * #workqueues.

Stop a moment. Exactly how many work queue users need per cpu binding
wizardry ?

> Another problem is that if we apply this to the existing default
> workqueue which is used by many different supposed-to-be-short works
> in essentially batch mode, we might end up enlarging cache footprint
> by scheduling unnecessarily many threads, which, in tight situations,
> might show up as small but noticeable performance regression.

Only if you make the default assumed max wait time for the work too low -
its a tunable behaviour in fact.

Alan
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