Re: regression in page writeback

[Richard Kennedy <richard@rsk.demon.co.uk>]

On Tue, 2009-09-22 at 10:32 +0200, Peter Zijlstra wrote:
> On Tue, 2009-09-22 at 16:24 +0800, Wu Fengguang wrote:
> > On Tue, Sep 22, 2009 at 04:09:25PM +0800, Peter Zijlstra wrote:
> > > On Tue, 2009-09-22 at 16:05 +0800, Wu Fengguang wrote:
> > > > 
> > > > I'm not sure how this patch stopped the "overshooting" behavior.
> > > > Maybe it managed to not start the background pdflush, or the started
> > > > pdflush thread exited because it found writeback is in progress by
> > > > someone else?
> > > > 
> > > > -               if (bdi_nr_reclaimable) {
> > > > +               if (bdi_nr_reclaimable > bdi_thresh) {
> > > 
> > > The idea is that we shouldn't move more pages from dirty -> writeback
> > > when there's not actually that much dirty left.
> > 
> > IMHO this makes little sense given that pdflush will move all dirty
> > pages anyway. pdflush should already be started to do background
> > writeback before the process is throttled, and it is designed to sync
> > all current dirty pages as quick as possible and as much as possible.
> 
> Not so, pdflush (or now the bdi writer thread thingies) should not
> deplete all dirty pages but should stop writing once they are below the
> background limit.
> 
> > > Now, I'm not sure about the > bdi_thresh part, I've suggested to maybe
> > > use bdi_thresh/2 a few times, but it generally didn't seem to make much
> > > of a difference.
> > 
> > One possible difference is, the process may end up waiting longer time
> > in order to sync write_chunk pages and quit the throttle. This could
> > hurt the responsiveness of the throttled process.
> 
> Well, that's all because this congestion_wait stuff is borken..
> 

The problem occurred as pdflush stopped when the number of dirty pages
reached the background threshold but balance_dirty_pages kept moving
pages to writeback because the total of dirty + writeback was over the
limit. 

I tried Peter's suggestion of using bdi_thresh/2 but I didn't see any
difference on my desktop hardware, but it may help RAID setups. I don't
think anyone tried it though.

Since Jens Axboe's per-bdi code got merged in the latest kernel tree,
there's a lot of change in these code paths so I'm not sure how that
reacts and if this change is still needed or relevant. 

regards
Richard