linux-kernel - Re: [PATCH 2/5] writeback: dirty position control

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Date:	Mon, 08 Aug 2011 16:41:41 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
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 2/5] writeback: dirty position control

On Mon, 2011-08-08 at 22:11 +0800, Wu Fengguang wrote:
> @@ -538,11 +538,6 @@ static unsigned long bdi_position_ratio(
>         goal = thresh - thresh / DIRTY_SCOPE;
>         origin = 4 * thresh;
>  
> -       if (unlikely(origin < limit && dirty > (goal + origin) / 2)) {
> -               origin = limit;                 /* auxiliary control line */
> -               goal = (goal + origin) / 2;
> -               pos_ratio >>= 1;
> -       }
>         pos_ratio = origin - dirty;
>         pos_ratio <<= BANDWIDTH_CALC_SHIFT;
>         do_div(pos_ratio, origin - goal + 1); 

So basically, pos_ratio = (4t - d) / (25/8)t, which if I'm not mistaken
comes out at 32/25 - 8d/25t. Which simply doesn't make sense at all. 


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