linux-kernel - Re: [PATCHSET] workqueue: concurrency managed workqueue, take#6

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Date:	Tue, 29 Jun 2010 09:05:25 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	torvalds@...ux-foundation.org, mingo@...e.hu,
	linux-kernel@...r.kernel.org, jeff@...zik.org,
	akpm@...ux-foundation.org, rusty@...tcorp.com.au,
	cl@...ux-foundation.org, dhowells@...hat.com,
	arjan@...ux.intel.com, oleg@...hat.com, axboe@...nel.dk,
	dwalker@...eaurora.org, stefanr@...6.in-berlin.de,
	florian@...kler.org, andi@...stfloor.org, mst@...hat.com,
	randy.dunlap@...cle.com
Subject: Re: [PATCHSET] workqueue: concurrency managed workqueue, take#6

Hello,

On 06/29/2010 01:18 AM, Frederic Weisbecker wrote:
> On Mon, Jun 28, 2010 at 11:03:48PM +0200, Tejun Heo wrote:
>> B. General documentation of Concurrency Managed Workqueue (cmwq)
>> ================================================================
> 
> 
> It would be nice to get this in Documentation/workqueue-design.txt,
> as the design is complicated enough to deserve this file :)

Yeah, I'm thinking about putting more technical description as the
head comment in workqueue.c and putting overview and information for
workqueue users under Documentation.

>> As multiple execution contexts are available for each wq, deadlocks
>> around execution contexts is much harder to create.  The default wq,
>> system_wq, has maximum concurrency level of 256 and unless there is a
>> scenario which can result in a dependency loop involving more than 254
>> workers, it won't deadlock.
> 
> Why this arbitrary limitation?

It's basically a safety mechanism to prevent a run away user from
saturating the system with workers.  256 seemed high enough for most
use cases yet low enough not to cause any major system failure.  So,
yeah, I pulled that number out of my ass.

Thanks.

-- 
tejun
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