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

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Message-Id: <1177409538.26937.75.camel@twins>
Date:	Tue, 24 Apr 2007 12:12:18 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Miklos Szeredi <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, 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.

How about ensuring that vm_writeout_total increases least
2*sync_writeback_pages() during our stay in balance_dirty_pages(). That
way we have the guarantee that more pages get written out than can be
dirtied.

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