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

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Date:	Tue, 24 Apr 2007 12:19:02 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	a.p.zijlstra@...llo.nl
CC:	akpm@...ux-foundation.org, 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

> > > > 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.

No, because that's a global counter, which many writers could be
looking at.

We'd need a per-task writeout counter, but when finishing the write we
don't know anymore which task it was performed for.

Miklos
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