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    SubjectRe: regression in page writeback
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    On Tue, 2009-09-22 at 10:32 +0200, Peter Zijlstra wrote:
    > On Tue, 2009-09-22 at 16:24 +0800, Wu Fengguang wrote:
    > > On Tue, Sep 22, 2009 at 04:09:25PM +0800, Peter Zijlstra wrote:
    > > > On Tue, 2009-09-22 at 16:05 +0800, Wu Fengguang wrote:
    > > > >
    > > > > I'm not sure how this patch stopped the "overshooting" behavior.
    > > > > Maybe it managed to not start the background pdflush, or the started
    > > > > pdflush thread exited because it found writeback is in progress by
    > > > > someone else?
    > > > >
    > > > > - if (bdi_nr_reclaimable) {
    > > > > + if (bdi_nr_reclaimable > bdi_thresh) {
    > > >
    > > > The idea is that we shouldn't move more pages from dirty -> writeback
    > > > when there's not actually that much dirty left.
    > >
    > > IMHO this makes little sense given that pdflush will move all dirty
    > > pages anyway. pdflush should already be started to do background
    > > writeback before the process is throttled, and it is designed to sync
    > > all current dirty pages as quick as possible and as much as possible.
    >
    > Not so, pdflush (or now the bdi writer thread thingies) should not
    > deplete all dirty pages but should stop writing once they are below the
    > background limit.
    >
    > > > Now, I'm not sure about the > bdi_thresh part, I've suggested to maybe
    > > > use bdi_thresh/2 a few times, but it generally didn't seem to make much
    > > > of a difference.
    > >
    > > One possible difference is, the process may end up waiting longer time
    > > in order to sync write_chunk pages and quit the throttle. This could
    > > hurt the responsiveness of the throttled process.
    >
    > Well, that's all because this congestion_wait stuff is borken..
    >

    The problem occurred as pdflush stopped when the number of dirty pages
    reached the background threshold but balance_dirty_pages kept moving
    pages to writeback because the total of dirty + writeback was over the
    limit.

    I tried Peter's suggestion of using bdi_thresh/2 but I didn't see any
    difference on my desktop hardware, but it may help RAID setups. I don't
    think anyone tried it though.

    Since Jens Axboe's per-bdi code got merged in the latest kernel tree,
    there's a lot of change in these code paths so I'm not sure how that
    reacts and if this change is still needed or relevant.

    regards
    Richard




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