Messages in this thread | | | Date | Tue, 22 Sep 2009 13:49:13 +0800 | From | Shaohua Li <> | Subject | regression in page writeback |
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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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