linux-kernel - Re: [PATCH 17/21] workqueue: simple reimplementation of SINGLE

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Date:	Tue, 17 Nov 2009 07:05:25 -0500
From:	Andy Walls <awalls@...ix.net>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org, linux-media@...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, torvalds@...ux-foundation.org,
	avi@...hat.com, peterz@...radead.org, andi@...stfloor.org,
	fweisbec@...il.com
Subject: Re: [PATCH 17/21] workqueue: simple reimplementation of
 SINGLE_THREAD workqueue

On Tue, 2009-11-17 at 14:23 +0900, Tejun Heo wrote:
> Hello,
> 
> 11/17/2009 09:47 AM, Andy Walls wrote:
> > An important property of the single threaded workqueue, upon which the
> > cx18 driver relies, is that work objects will be processed strictly in
> > the order in which they were queued.  The cx18 driver has a pool of
> > "work orders" and multiple active work orders can be queued up on the
> > workqueue especially if multiple streams are active.  If these work
> > orders were to be processed out of order, video artifacts would result
> > in video display applications.
> 
> That's an interesting use of single thread workqueue.  Most of single
> thread workqueues seem to be made single thread just to save number of
> threads.  Some seem to depend on single thread of execution but I
> never knew there are ones which depend on the exact execution order.
> Do you think that usage is wide-spread?

I doubt it.

Most that I have seen use the singlethreaded workqueue object with a
queue depth of essentially 1 for syncronization - as you have noted.


>   Implementing strict ordering
> shouldn't be too difficult but I can't help but feeling that such
> assumption is abuse of implementation detail.

Hmmm, does not the "queue" in workqueue mean "FIFO"?

If not for strict ordering, why else would a driver absolutely need a
singlethreaded workqueue object?  It seems to me the strict ording is
the driving requirement for a singlethreaded workqueue at all.  Your
patch series indicates to me that the performance and synchronization
use cases are not driving requirements for a singlethreaded workqueue.

Thanks for your consideration.

Regards,
Andy

> Thanks.


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