linux-kernel - Re: [PATCH] writeback: enabling-gate for light dirtied bdi

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Message-ID: <4CFB9BE1.3030902@redhat.com>
Date:	Sun, 05 Dec 2010 09:04:17 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Theodore Ts'o" <tytso@....edu>,
	Chris Mason <chris.mason@...cle.com>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
	Jens Axboe <axboe@...nel.dk>, Mel Gorman <mel@....ul.ie>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Hellwig <hch@....de>, 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] writeback: enabling-gate for light dirtied bdi

On 12/05/2010 01:44 AM, Wu Fengguang wrote:
> I noticed that my NFSROOT test system goes slow responding when there
> is heavy dd to a local disk. Traces show that the NFSROOT's bdi_limit
> is near 0 and many tasks in the system are repeatedly stuck in
> balance_dirty_pages().
>
> There are two related problems:
>
> - light dirtiers at one device (more often than not the rootfs) get
>    heavily impacted by heavy dirtiers on another independent device
>
> - the light dirtied device does heavy throttling because bdi_limit=0,
>    and the heavy throttling may in turn withhold its bdi_limit in 0 as
>    it cannot dirty fast enough to grow up the bdi's proportional weight.
>
> Fix it by introducing some "low pass" gate, which is a small (<=8MB)
> value reserved by others and can be safely "stole" from the current
> global dirty margin.  It does not need to be big to help the bdi gain
> its initial weight.

Makes a lot of sense to me.

Acked-by: Rik van Riel <riel@...hat.com>

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