linux-kernel - Re: [PATCH 03/13] writeback: per-task rate limit on balance_dirty

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Date:	Wed, 24 Nov 2010 11:49:28 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Theodore Ts'o <tytso@....edu>,
	Chris Mason <chris.mason@...cle.com>,
	Mel Gorman <mel@....ul.ie>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 03/13] writeback: per-task rate limit on
 balance_dirty_pages()

On Wed, 2010-11-24 at 18:43 +0800, Wu Fengguang wrote:
> On Wed, Nov 24, 2010 at 06:23:07PM +0800, Peter Zijlstra wrote:
> > On Wed, 2010-11-17 at 12:27 +0800, Wu Fengguang wrote:
> > > +       if (unlikely(current->nr_dirtied >= current->nr_dirtied_pause ||
> > > +                    bdi->dirty_exceeded)) {
> > > +               balance_dirty_pages(mapping, current->nr_dirtied);
> > > +               current->nr_dirtied = 0;
> > >         } 
> > 
> > Was it a conscious choice to use
> >   current->nr_dirtied = 0
> > over 
> >   current->nr_dirtied -= current->nr_dirtied_pause
> > ?
> > 
> > The former will cause a drift in pause times due to truncation of the
> > excess.
> 
> It should be fine in either way, as long as the "truncated" number is
> passed to balance_dirty_pages():
> 
> +               balance_dirty_pages(mapping, current->nr_dirtied);
> +               current->nr_dirtied = 0;
> 
> or
> 
> +               balance_dirty_pages(mapping, current->nr_dirtied_pause);
> +               current->nr_dirtied -= current->nr_dirtied_pause;

ok, just wanted to make sure you'd considered it.

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