Messages in this thread | | | Subject | Re: regression in page writeback | From | Peter Zijlstra <> | Date | Tue, 22 Sep 2009 08:40:12 +0200 |
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On Tue, 2009-09-22 at 13:49 +0800, Shaohua Li 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. > > Unfortunatelly I can't figure out a fix for this issue, hopefully you have more > ideas.
This whole writeback business is very fragile, the patch does indeed cure a few cases and compounds a few other cases, typical trade off.
People are looking at it.
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