On Mon, Sep 24, 2007 at 09:35:23AM +0200, Peter Zijlstra wrote:
> On Mon, 24 Sep 2007 11:01:10 +0800 Fengguang Wu <[email protected]>
> 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.
> > 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;
> > Or better to adjust the estimation logic?
>
> Not sure what we can do here. The current thing is simple, fast and fair.
Agreed.
> > > + /*
> > > + * 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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