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

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Date:	Fri, 12 Aug 2011 15:04:19 +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 Tue, 2011-08-09 at 19:20 +0200, Peter Zijlstra wrote:
> 
> Now all of the above would seem to suggest:
> 
>   dirty_ratelimit := ref_bw
> 
> However for that you use:
> 
>   if (pos_bw < dirty_ratelimit && ref_bw < dirty_ratelimit)
>         dirty_ratelimit = max(ref_bw, pos_bw);
> 
>   if (pos_bw > dirty_ratelimit && ref_bw > dirty_ratelimit)
>         dirty_ratelimit = min(ref_bw, pos_bw);
> 
> You have:
> 
>   pos_bw = dirty_ratelimit * pos_ratio
> 
> Which is ref_bw without the write_bw/dirty_bw factor, this confuses me..
> why are you ignoring the shift in output vs input rate there? 

Could you elaborate on this primary feedback loop? Its the one part I
don't feel I actually understand well.


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