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

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Date:	Fri, 26 Aug 2011 11:04:29 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	"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>,
	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 Fri, 2011-08-26 at 08:18 +0800, Wu Fengguang wrote:
> On Thu, Aug 25, 2011 at 12:12:58AM +0800, Peter Zijlstra wrote:
> > On Wed, 2011-08-24 at 08:12 +0800, Wu Fengguang wrote:

> > > Put (6) into (4), we get
> > > 
> > >         balanced_rate_(i+1) = balanced_rate_(i) * 2
> > >                             = (write_bw / N) * 2
> > > 
> > > That means, any position imbalance will lead to balanced_rate
> > > estimation errors if we follow (4). Whereas if (1)/(5) is used, we
> > > always get the right balanced dirty ratelimit value whether or not
> > > (pos_ratio == 1.0), hence make the rate estimation independent(*) of
> > > dirty position control.
> > > 
> > > (*) independent as in real values, not the seemingly relations in equation
> > 
> > 
> > The assumption here is that N is a constant.. in the above case
> > pos_ratio would eventually end up at 1 and things would be good again. I
> > see your argument about oscillations, but I think you can introduce
> > similar effects by varying N.
> 
> Yeah, it's very possible for N to change over time, in which case
> balanced_rate will adapt to new N in similar way.

Gah.. but but but, that gives the same stuff as your (6)+(4). Why won't
you accept that for pos_ratio but you don't mind for N ?


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