Re: regression in page writeback

[Shaohua Li <shaohua.li@intel.com>]

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.