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

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Date:	Mon, 24 Sep 2007 09:35:23 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Fengguang Wu <wfg@...l.ustc.edu.cn>
Cc:	Hugh Dickins <hugh@...itas.com>, Andy Whitcroft <apw@...dowen.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, spamtrap@...bisoft.de
Subject: Re: 2.6.23-rc6-mm1 -- mkfs stuck in 'D'

On Mon, 24 Sep 2007 11:01:10 +0800 Fengguang Wu <wfg@...l.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.
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