linux-kernel - Re: [PATCH 16/35] writeback: increase min pause time on concurrent dirtiers

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Date:	Mon, 13 Dec 2010 13:23:31 -0500
From:	Valdis.Kletnieks@...edu
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@....de>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	"Theodore Ts'o" <tytso@....edu>,
	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mel Gorman <mel@....ul.ie>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	linux-mm <linux-mm@...ck.org>, linux-fsdevel@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 16/35] writeback: increase min pause time on concurrent dirtiers

On Mon, 13 Dec 2010 22:47:02 +0800, Wu Fengguang said:
> Target for >60ms pause time when there are 100+ heavy dirtiers per bdi.
> (will average around 100ms given 200ms max pause time)

> --- linux-next.orig/mm/page-writeback.c	2010-12-13 21:46:16.000000000 +0800
> +++ linux-next/mm/page-writeback.c	2010-12-13 21:46:16.000000000 +0800
> @@ -659,6 +659,27 @@ static unsigned long max_pause(unsigned 
>  }
>  
>  /*
> + * Scale up pause time for concurrent dirtiers in order to reduce CPU overheads.
> + * But ensure reasonably large [min_pause, max_pause] range size, so that
> + * nr_dirtied_pause (and hence future pause time) can stay reasonably stable.
> + */
> +static unsigned long min_pause(struct backing_dev_info *bdi,
> +			       unsigned long max)
> +{
> +	unsigned long hi = ilog2(bdi->write_bandwidth);
> +	unsigned long lo = ilog2(bdi->throttle_bandwidth);
> +	unsigned long t;
> +
> +	if (lo >= hi)
> +		return 1;
> +
> +	/* (N * 10ms) on 2^N concurrent tasks */
> +	t = (hi - lo) * (10 * HZ) / 1024;

Either I need more caffeine, or the comment doesn't match the code
if HZ != 1000?

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