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SubjectRe: [RFC][PATCH] create workqueue threads only when needed
On Tue, 27 Jan 2009 01:17:11 +0100
Frederic Weisbecker <fweisbec@gmail.com> wrote:

> While looking at the statistics from the workqueue tracer, I've been
> suprised by the number of useless workqueues I had:
>
> CPU INSERTED EXECUTED NAME
> | | | |
>
> * 0 0 kpsmoused
> * 0 0 ata_aux
> * 0 0 cqueue
> * 0 0 kacpi_notify
> * 0 0 kacpid
> * 998 998 khelper
> * 0 0 cpuset
>
> 1 0 0 hda0/1
> 1 42 42 reiserfs/1
> 1 0 0 scsi_tgtd/1
> 1 0 0 aio/1
> 1 0 0 ata/1
> 1 193 193 kblockd/1
> 1 0 0 kintegrityd/1
> 1 4 4 work_on_cpu/1
> 1 1244 1244 events/1
>
> 0 0 0 hda0/0
> 0 63 63 reiserfs/0
> 0 0 0 scsi_tgtd/0
> 0 0 0 aio/0
> 0 0 0 ata/0
> 0 188 188 kblockd/0
> 0 0 0 kintegrityd/0
> 0 16 16 work_on_cpu/0
> 0 1360 1360 events/0
>
>
> All of the workqueues with 0 work inserted do nothing.
> For several reasons:
>
> _ Unneeded built drivers for my system that create workqueue(s) when
> they init _ Services which need their own workqueue, for several
> reasons, but who receive very rare jobs (often never)
> _ ...?
>
> And the result of git-grep create_singlethread_workqueue is even more
> surprising.
>
> So I've started a patch which creates the workqueues by default
> without thread except the kevents one.
> They will have their thread created and started only when these
> workqueues will receive a first work to do. This is performed by
> submitting a task's creation work to the kevent workqueues which are
> always there, and are the only one which have their thread started on
> creation.
>
> The result after this patch:
>
> # CPU INSERTED EXECUTED NAME
> # | | | |
>
> * 999 1000 khelper
>
> 1 5 6 reiserfs/1
> 1 0 2 work_on_cpu/1
> 1 86 87 kblockd/1
> 1 14 16 work_on_cpu/1
> 1 149 149 events/1
>
> 0 15 16 reiserfs/0
> 0 85 86 kblockd/0
> 0 146 146 events/0
>
>
> Dropping 16 useless kernel threads in my system.
> (Yes the inserted values are not synced with the executed one because
> the tracers looses the first events. I just rewrote some parts to
> make it work with this patch) .
> I guess I will update this tracer to display the "shadow workqueues"
> which have no threads too.
>
> I hadn't any problems until now with this patch but I think it needs
> more testing, like with cpu hotplug, and some renaming for its
> functions and structures... And I would like to receive some comments
> and feelings before continuing. So this is just an RFC :-)
>

one thing to look at for work queues that never get work is to see if
they are appropriate for the async function call interface
(the only requirement for that is that they need to cope with calling
inline in exceptional cases)


--
Arjan van de Ven Intel Open Source Technology Centre
For development, discussion and tips for power savings,
visit http://www.lesswatts.org


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