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	 <1254384558-1018-20-git-send-email-tj@kernel.org>
Date: Fri, 2 Oct 2009 16:28:54 +0200
Message-ID: <c62985530910020728tf2cb581t5d7b7ef99f35395c@mail.gmail.com>
Subject: Re: [PATCH 19/19] workqueue: implement concurrency managed workqueue
From: =?ISO-8859-1?Q?Fr=E9d=E9ric_Weisbecker?= <fweisbec@gmail.com>
To: Tejun Heo <tj@kernel.org>
Cc: jeff@garzik.org, mingo@elte.hu, linux-kernel@vger.kernel.org,
       akpm@linux-foundation.org, jens.axboe@oracle.com, rusty@rustcorp.com.au,
       cl@linux-foundation.org, dhowells@redhat.com, arjan@linux.intel.com
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2009/10/1 Tejun Heo <tj@kernel.org>:
> Currently each workqueue has its own dedicated worker pool. ?This
> causes the following problems.
>
> * Works which are dependent on each other can cause a deadlock by
> ?depending on the same execution resource. ?This is bad because this
> ?type of dependency is quite difficult to find.
>
> * Works which may sleep and take long time to finish need to have
> ?separate workqueues so that it doesn't block other works. ?Similarly
> ?works which want to be executed in timely manner often need to
> ?create it custom workqueue too to avoid being blocked by long
> ?running ones. ?This leads to large number of workqueues and thus
> ?many workers.
>
> * The static one-per-cpu worker isn't good enough for jobs which
> ?require higher level of concurrency necessiating other worker pool
> ?mechanism. ?slow-work and async are good examples and there are also
> ?some custom implementations buried in subsystems.
>
> * Combined, the above factors lead to many workqueues with large
> ?number of dedicated and mostly unused workers. ?This also makes work
> ?processing less optimal as the dedicated workers end up switching
> ?among themselves costing scheduleing overhead and wasting cache
> ?footprint for their stacks and as the system gets busy, these
> ?workers end up competing with each other.
>
> To solve the above issues, this patch implements concurrency-managed
> workqueue.
>
> There is single global cpu workqueue (gcwq) for each cpu which serves
> all the workqueues. ?gcwq maintains single pool of workers which is
> shared by all cwqs on the cpu.
>
> gcwq keeps the number of concurrent active workers to minimum but no
> less. ?As long as there's one or more running workers on the cpu, no
> new worker is scheduled so that works can be processed in batch as
> much as possible but when the last running worker blocks, gcwq
> immediately schedules new worker so that the cpu doesn't sit idle
> while there are works to be processed.


That's really a cool thing.
So once such new workers are created, what's the state/event that triggers their
destruction?

Is it the following, propagated recursively?

    Worker A blocks.
    B is created.
    B has just finished a worklet and A has been woken up
    Then destroy B
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