linux-kernel - Re: [PATCHSET] workqueue: concurrency managed workqueue, take#5

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Message-ID: <4C17C353.80708@s5r6.in-berlin.de>
Date:	Tue, 15 Jun 2010 20:15:47 +0200
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Tejun Heo <tj@...nel.org>, mingo@...e.hu, awalls@...ix.net,
	linux-kernel@...r.kernel.org, jeff@...zik.org,
	rusty@...tcorp.com.au, cl@...ux-foundation.org,
	dhowells@...hat.com, arjan@...ux.intel.com,
	johannes@...solutions.net, oleg@...hat.com, axboe@...nel.dk
Subject: Re: [PATCHSET] workqueue: concurrency managed workqueue, take#5

Andrew Morton wrote:
> On Mon, 14 Jun 2010 23:37:17 +0200
> Tejun Heo <tj@...nel.org> wrote:
> 
>> This is the fifth take of cmwq (concurrency managed workqueue)
>> patchset.
> 
> What is a concurrency managed workqueue and why do we want one?

>From what I understood, this is about the following:

  - Right now, a workqueue is backed by either 1 or by #_of_CPUs
    kernel threads.  There is no other option.

  - To avoid creating half a million of kernel threads, driver authors
    resort to either
       - using the globally shared workqueue even if they might queue
         high-latency work in corner cases,
    or
       - creating a single-threaded workqueue even if they put unrelated
         jobs into that queue that should better be executed in
         parallel, not serially.
    (I for one have both cases in drivers/firewire/, and I have similar
    issues in the old drivers/ieee1394/.)

The cmwq patch series reforms workqueues to be backed by a global thread
pool.  Hence:

  + Driver authors can and should simply register one queue for any one
    purpose now.  They don't need to worry anymore about having too many
    or too few backing threads.

  + [A side effect:  In some cases, a driver that currently uses a
    thread pool can be simplified by migrating to the workqueue API.]

Tejun, please correct me if I misunderstood.
-- 
Stefan Richter
-=====-==-=- -==- -====
http://arcgraph.de/sr/
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