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    SubjectRe: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority
    On Thu, Oct 02, 2008 at 05:32:04PM +0200, Bodo Eggert wrote:
    > Jens Axboe <> wrote:
    > > On Thu, Oct 02 2008, Dave Chinner wrote:
    > >> On Thu, Oct 02, 2008 at 09:55:11AM +0200, Jens Axboe wrote:
    > >> > Good point. I think we should mark the IO as sync, and maintain the same
    > >> > priority level. Any IO that ends up being waited on is sync by
    > >> > definition, we just need to expand the coverage a bit.
    > >>
    > >> That's what XFS has always done - mark the journal I/O as sync.
    > >> Still, once you load up the elevator, the sync I/O can still get
    > >> delayed for hundreds of milliseconds before dispatch, which was
    > >> why I started looking at boosting the priority of the log I/O.
    > >> It proved to be much more effective at getting the log I/O
    > >> dispatched than the existing "mark it sync" technique....
    > >
    > > Sure, just marking it as sync is not a magic bullet. It'll be in the
    > > first priority for that class, but it'll share bandwidth with other
    > > processes. So if you have lots of IO going on, it can take hundreds of
    > > miliseconds before being dispatched.
    > Sounds like you need a priority class besides sync and async.

    There's BIO_META now as well, which I was testing at the same time
    as RT priority. Marking all the metadata I/O as BIO_META did help,
    but once again I never got to determining if that was a result of
    the different tagging or the priority increase.

    However, given that only CFQ understand BIO_META, I suspect that
    changing the way XFS uses BIO_SYNC to be a combination of BIO_META
    and BIO_SYNC would cause significant regressions on other


    Dave Chinner

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