regression in page writeback

[Shaohua Li]

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.

Unfortunatelly I can't figure out a fix for this issue, hopefully you have more
ideas.

Thanks,
Shaohua
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