linux-kernel - Re: [PATCH 3/5] writeback: dirty rate control

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Message-ID: <20110810141535.GD29724@localhost>
Date:	Wed, 10 Aug 2011 22:15:35 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Andrea Righi <arighi@...eler.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] writeback: dirty rate control

On Wed, Aug 10, 2011 at 01:02:02AM +0800, Peter Zijlstra wrote:
> On Sat, 2011-08-06 at 16:44 +0800, Wu Fengguang wrote:
> 
> > +       pos_bw = bw * pos_ratio >> BANDWIDTH_CALC_SHIFT;
> > +       pos_bw++;  /* this avoids bdi->dirty_ratelimit get stuck in 0 */
> > +
> 
> > +       pos_ratio *= bdi->avg_write_bandwidth;
> > +       do_div(pos_ratio, dirty_bw | 1);
> > +       ref_bw = bw * pos_ratio >> BANDWIDTH_CALC_SHIFT; 
> 
> when written out that results in:
> 
>            bw * pos_ratio * bdi->avg_write_bandwidth
>   ref_bw = -----------------------------------------
>                          dirty_bw
> 
> which would suggest you write it like:
> 
>   ref_bw = div_u64((u64)pos_bw * bdi->avg_write_bandwidth, dirty_bw | 1);
> 
> since pos_bw is already bw * pos_ratio per the above.

Good point. Oopse I even wrote a comment for the over complex calculation:

         * balanced_rate = pos_rate * write_bw / dirty_rate

Thanks,
Fengguang
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