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. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
