On Mon, Sep 24, 2007 at 09:35:23AM +0200, Peter Zijlstra wrote:
quoted text
> On Mon, 24 Sep 2007 11:01:10 +0800 Fengguang Wu <wfg@mail.ustc.edu.cn>
> wrote:
> 
> > > That is an interesting idea how about this:
> > 
> > It looks like a workaround, but it does solve the most important problem.
> > And it is a good logic by itself.  So I'd vote for it.
> > 
> > The fundamental problem is that the per-bdi-writeback-completion based
> > estimation is not accurate under light loads. The problem remains for
> > a light-load sda when there is a heavy-load sdb. 
> 
> Well, sure, in that case sda would get to write out a lot of small
> things. But in that case it would be fair wrt the other writers.

Hmm, I cannot agree it to be fair - but pretty acceptable ;-)
Your patch already brings great improvements in the multi-bdi case.

quoted text
> > One more workaround
> > could be to grant bdi(s) a minimal bdi_thresh. 
> 
> Ah, no, that is no good. For if there were a lot of BDIs this might
> happen:
>   nr_bdis * min_thresh > dirty_limit.

Sure it is in the extreme case. However the limit could be ensured
if we really want(which I'm really not sure;-) it:

        if (nr_reclaimable + nr_writeback < dirty_thresh &&
                bdi_nr_reclaimable + bdi_nr_writeback <= bdi_min_thresh)
                break;

quoted text
> > Or better to adjust the estimation logic?
> 
> Not sure what we can do here. The current thing is simple, fast and fair.

Agreed.

quoted text
> > > +		/*
> > > +		 * break out early when:
> > > +		 *  - we're below the bdi limit
> > > +		 *  - we're below half the total limit
> > > +		 *
> > > +		 * we let the numbers exceed the strict bdi limit if the total
> > > +		 * numbers are too low, this avoids (excessive) small writeouts.
> > > +		 */
> > > +		if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh ||
> > > +		    nr_reclaimable + nr_writeback < dirty_thresh / 2)
> > >  			break;
> > 
> > This may be slightly better:
> > 
> > 		if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
> >                         break;
> >                 /*
> >                  * Throttle it only when the background writeback cannot catchup.
> >                  */
> >                 if (nr_reclaimable + nr_writeback <
> >                                 (background_thresh + dirty_thresh) / 2)
> > 			break;
> 
> Ah, indeed. Good idea.

Thank you :-)

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