linux-kernel - Re: [PATCH 10/10] mm: per device dirty threshold

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Message-Id: <E1HgJ5u-0000aD-00@dorka.pomaz.szeredi.hu>
Date:	Tue, 24 Apr 2007 13:22:02 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	akpm@...ux-foundation.org
CC:	a.p.zijlstra@...llo.nl, miklos@...redi.hu, neilb@...e.de,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, dgc@....com,
	tomoki.sekiyama.qu@...achi.com, nikita@...sterfs.com,
	trond.myklebust@....uio.no, yingchao.zhou@...il.com
Subject: Re: [PATCH 10/10] mm: per device dirty threshold

> On Tue, 24 Apr 2007 12:12:18 +0200 Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> 
> > On Tue, 2007-04-24 at 03:00 -0700, Andrew Morton wrote:
> > > On Tue, 24 Apr 2007 11:47:20 +0200 Miklos Szeredi <miklos@...redi.hu> wrote:
> > > 
> > > > > Ahh, now I see; I had totally blocked out these few lines:
> > > > > 
> > > > > 			pages_written += write_chunk - wbc.nr_to_write;
> > > > > 			if (pages_written >= write_chunk)
> > > > > 				break;		/* We've done our duty */
> > > > > 
> > > > > yeah, those look dubious indeed... And reading back Neil's comments, I
> > > > > think he agrees.
> > > > > 
> > > > > Shall we just kill those?
> > > > 
> > > > I think we should.
> > > > 
> > > > Athough I'm a little afraid, that Akpm will tell me again, that I'm a
> > > > stupid git, and that those lines are in fact vitally important ;)
> > > > 
> > > 
> > > It depends what they're replaced with.
> > > 
> > > That code is there, iirc, to prevent a process from getting stuck in
> > > balance_dirty_pages() forever due to the dirtying activity of other
> > > processes.
> > > 
> > > hm, we ask the process to write write_chunk pages each go around the loop.
> > > So if it wrote write-chunk/2 pages on the first pass it might end up writing
> > > write_chunk*1.5 pages total.  I guess that's rare and doesn't matter much
> > > if it does happen - the upper bound is write_chunk*2-1, I think.
> > 
> > Right, but I think the problem is that its dirty -> writeback, not dirty
> > -> writeback completed.
> > 
> > Ie. they don't guarantee progress, it could be that the total
> > nr_reclaimable + nr_writeback will steadily increase due to this break.
> 
> Don't think so.  We call balance_dirty_pages() once per ratelimit_pages
> dirtyings and when we get there, we write 1.5*ratelimit_pages pages.

No, we _start_ writeback for 1.5*ratelimit_pages pages, but do not
wait for those writebacks to finish.

So for a slow device and a fast writer, dirty+writeback can indeed
increase beyond the dirty threshold.

Miklos
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