Re: [PATCH 2/6] writeback: reduce calls to global_page_state in balance_dirty_pages()

[Jan Kara <jack@suse.cz>]

On Tue 27-07-10 11:59:41, Wu Fengguang wrote:
> > > This patch slightly changes behavior by replacing clip_bdi_dirty_limit()
> > > with the explicit check (nr_reclaimable + nr_writeback >= dirty_thresh)
> > > to avoid exceeding the dirty limit. Since the bdi dirty limit is mostly
> > > accurate we don't need to do routinely clip. A simple dirty limit check
> > > would be enough.
> > > 
> > > The check is necessary because, in principle we should throttle
> > > everything calling balance_dirty_pages() when we're over the total
> > > limit, as said by Peter.
> > > 
> > > We now set and clear dirty_exceeded not only based on bdi dirty limits,
> > > but also on the global dirty limits. This is a bit counterintuitive, but
> > > the global limits are the ultimate goal and shall be always imposed.
> >   Thinking about this again - what you did is rather big change for systems
> > with more active BDIs. For example if I have two disks sda and sdb and
> > write for some time to sda, then dirty limit for sdb gets scaled down.
> > So when we start writing to sbd we'll heavily throttle the threads until
> > the dirty limit for sdb ramps up regardless of how far are we to reach the
> > global limit...
> 
> The global threshold check is added in place of clip_bdi_dirty_limit()
> for safety and not intended as a behavior change. If ever leading to
> big behavior change and regression, that it would be indicating some
> too permissive per-bdi threshold calculation.
> 
> Did you see the global dirty threshold get exceeded when writing to 2+
> devices? Occasional small exceeding should be OK though. I tried the
> following debug patch and see no warnings when doing two concurrent cp
> over local disk and NFS.
  Oops, sorry. I've misread the code. You're right. There shouldn't be a big
change in the behavior.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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