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

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Date:	Wed, 24 Nov 2010 18:43:32 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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, 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;

Thanks,
Fengguang
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