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    SubjectRe: Status of the IO scheduler fixes for 2.4
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    On Fri, 2003-07-04 at 20:05, Andrea Arcangeli wrote:
    > On Fri, Jul 04, 2003 at 05:37:35PM -0400, Chris Mason wrote:
    > > I've also attached a patch I've been working on to solve the latencies a
    > > different way. bdflush-progress.diff changes balance_dirty to wait on
    > > bdflush instead of trying to start io. It helps a lot here (both
    > > throughput and latency) but hasn't yet been tested on a box with tons of
    > > drives.
    >
    > that's orthogonal, it changes the write throttling, it doesn't touch the
    > blkdev layer like the other patches does. So if it helps it solves a
    > different kind of latencies.

    It's also almost useless without elevator-low-latency applied ;-) One
    major source of our latencies is a bunch of procs hammering on
    __get_request_wait, so bdflush-progess helps by reducing the number of
    procs doing io. It does push some of the latency problem up higher into
    balance_dirty, but I believe it is easier to manage there.

    bdflush-progress doesn't help at all for non-async workloads.

    > However the implementation in theory can run the box oom, since it won't
    > limit the dirty buffers anymore. To be safe you need to wait 2
    > generations. I doubt in practice it would be easily reproducible though ;).

    Hmmm, I think the critical part here is to write some buffers after
    marking a buffer dirty. We don't check the generation number until
    after marking the buffer dirty, so if the generation has incremented at
    all we've made progress. What am I missing?

    -chris


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