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?

> 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 :)

Thanks,
Fengguang