linux-kernel - Re: [RFC][PATCH 7/7] writeback: balance_dirty_pages() shall write more than dirtied pages

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Message-Id: <1252598069.7205.87.camel@laptop>
Date:	Thu, 10 Sep 2009 17:54:29 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Chris Mason <chris.mason@...cle.com>, Jan Kara <jack@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@...radead.org>,
	Artem Bityutskiy <dedekind1@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [RFC][PATCH 7/7] writeback: balance_dirty_pages() shall write
 more than dirtied pages

On Thu, 2009-09-10 at 23:41 +0800, Wu Fengguang wrote:
> > > So btrfs_file_write() explicitly calls
> > > balance_dirty_pages_ratelimited_nr() to get throttled.
> > 
> > Right, so what is wrong with than, and how does this patch fix that?
> > 
> > [ the only thing you have to be careful with is that you don't
> > excessively grow the error bound on the dirty limit ]
> 
> Then we could form a loop:
> 
>         btrfs_file_write():     dirty 1024 pages
>         balance_dirty_pages():  write up to 12 pages (= ratelimit_pages * 1.5)
> 
> in which the writeback rate cannot keep up with dirty rate,
> and the dirty pages go all the way beyond dirty_thresh.

Ah, ok so this is to keep the error bound on the dirty limit bounded,
because we can break out of balance_dirty_pages() early, the /* We've
done our duty */ break.

Which unbalances the duty vs the dirty ratio.

I figure that with the task dirty limit stuff we could maybe try to get
rid of this break.. worth a try.

> Sorry for writing such a vague changelog!

np, as long as we get there :-)

Change makes sense now, thanks!

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