Re: 2.6.23-rc6-mm1 -- mkfs stuck in 'D'

Peter Zijlstra
2007-09-24T07:35:41Z

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.

> 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.

> Or better to adjust the estimation logic?

Not sure what we can do here. The current thing is simple, fast and fair.

> > + /*
> > + * 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.