linux-kernel - Re: workqueue thing

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Date:	Mon, 21 Dec 2009 12:17:00 +0100
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Jens Axboe <jens.axboe@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Tejun Heo <tj@...nel.org>, torvalds@...ux-foundation.org,
	awalls@...ix.net, linux-kernel@...r.kernel.org, jeff@...zik.org,
	mingo@...e.hu, akpm@...ux-foundation.org, rusty@...tcorp.com.au,
	cl@...ux-foundation.org, dhowells@...hat.com, avi@...hat.com,
	johannes@...solutions.net
Subject: Re: workqueue thing

On 12/21/2009 12:09, Andi Kleen wrote:
> On Mon, Dec 21, 2009 at 10:17:54AM +0100, Jens Axboe wrote:
>> On Fri, Dec 18 2009, Arjan van de Ven wrote:
>>> in addition, threads are cheap. Linux has no technical problem with
>>> running 100's of kernel threads (if not 1000s); they cost basically a
>>> task struct and a stack (2 pages) each and that's about it.  making an
>>> elaborate-and-thus-fragile design to save a few kernel threads is
>>> likely a bad design direction...
>>
>> One would hope not, since that is by no means outside of what you see on
>> boxes today... Thousands. The fact that they are cheap, is not an
>> argument against doing it right. Conceptually, I think the concurrency
>> managed work queue pool is a much cleaner (and efficient) design.
>
> Agreed. Even if possible thousands of threads waste precious cache.

only used ones waste cache ;-)

> And they look ugly in ps.

that we could solve by making them properly threads of each other; ps and co
already (at least by default) fold threads of the same program into one.

>
> Also the nice thing about dynamically sizing the thread pool
> is that if something bad (error condition that takes long) happens
> in one work queue for a specific subsystem there's still a chance
> to make process with other operations in the same subsystem.

yup
same is true for hitting some form of contention; just make an extra thread
so that the rest can continue.
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