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    SubjectRe: regression in page writeback
    On Tue, Sep 22, 2009 at 06:49:15PM +0800, Wu, Fengguang wrote:
    > Shaohua,
    >
    > On Tue, Sep 22, 2009 at 01:49:13PM +0800, Li, Shaohua wrote:
    > > Hi,
    > > Commit d7831a0bdf06b9f722b947bb0c205ff7d77cebd8 causes disk io regression
    > > in my test.
    > > My system has 12 disks, each disk has two partitions. System runs fio sequence
    > > write on all partitions, each partion has 8 jobs.
    > > 2.6.31-rc1, fio gives 460m/s disk io
    > > 2.6.31-rc2, fio gives about 400m/s disk io. Revert the patch, speed back to
    > > 460m/s
    > >
    > > Under latest git: fio gives 450m/s disk io; If reverting the patch, the speed
    > > is 484m/s.
    > >
    > > With the patch, fio reports less io merge and more interrupts. My naive
    > > analysis is the patch makes balance_dirty_pages_ratelimited_nr() limits
    > > write chunk to 8 pages and then soon go to sleep in balance_dirty_pages(),
    > > because most time the bdi_nr_reclaimable < bdi_thresh, and so when write
    > > the pages out, the chunk is 8 pages long instead of 4M long. Without the patch,
    > > thread can write 8 pages and then move some pages to writeback, and then
    > > continue doing write. The patch seems to break this.
    >
    > Do you have trace/numbers for above descriptions?
    No. Just guess, because there is less io merge. And watch each bdi's states,
    bdi_nr_reclaimable < bdi_thresh seems always true.

    > > Unfortunatelly I can't figure out a fix for this issue, hopefully
    > > you have more ideas.
    >
    > Attached is a very verbose writeback debug patch, hope it helps and
    > won't disturb the workload a lot :)
    Hmm, the log buf will get overflowed soon, there is > 400m/s io. I tried
    to produce this issue in a system with two disks, but fail. Anyway, I'll try
    it out tomorrow.


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