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    SubjectRe: [RFC][PATCH] create workqueue threads only when needed
    On Mon, Feb 02, 2009 at 09:42:45AM +0100, Stefan Richter wrote:
    > Benjamin Herrenschmidt wrote:
    > > On Mon, 2009-02-02 at 03:24 +0100, Frederic Weisbecker wrote:
    > >> On Mon, Feb 02, 2009 at 08:37:41AM +1100, Benjamin Herrenschmidt wrote:
    > >> >
    > >> > > I don't know, most of those I've looked on are not documented about the reason
    > >> > > for a private workqueue. I guess most of them can use the usual kevent.
    >
    > I rather suspect that the majority of private workqueues are there for
    > good reasons.
    >
    > >> > The main problem with kevent is that it gets clogged up.
    > >>
    > >> I don't think so. Here is a snapshot of the workqueue tracer in my
    > >> box currently:
    > >
    > > That's not quite what I meant ...
    > >
    > > The main problem with keventd I'd say is that it's used in all sort of
    > > exeptional code path (ie, driver reset path, error handling, etc...) for
    > > things that will msleep happily for tenth milliseconds, that sort of
    > > thing.
    > >
    > > IE. It will be pretty responsive -in general- but can suffer form
    > > horrible latencies every now and then.
    >
    > Actually it /should/ be the other way around:
    >
    > The shared workqueue should only be used for work that sleeps only
    > briefly (perhaps with the exception of very unlikely longer sleeps e.g.
    > for allocations that cause paging).
    >
    > Work which /may/ sleep longer, for example performs SCSI transactions,
    > needs to go into a private workqueue or other kind of context.


    Right. But most of the time, these workqueues receive few events.
    That's why async looks a good alternative for such cases. The threads
    from async core which perform the jobs are created and destroyed on the fly,
    depending on the number of jobs queued.


    > OTOH you are right too; work which must not be deferred too long by work
    > from another uncooperative/ unfair subsystem is probably also better off
    > in an own workqueue...



    Or callsites which use kevent and may sleep for too long could be identified
    and fixed...


    > Stefan Richter
    > -=====-==--= --=- ---=-
    > http://arcgraph.de/sr/



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