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

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Date:	Wed, 24 Aug 2011 11:16:03 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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

> > > > +	x_intercept = setpoint + 2 * span;
>    ^^ BTW, why do you have 2*span here? It can result in x_intercept being
> ~3*bdi_thresh... So maybe you should use bdi_thresh/2 in the computation of
> span?

OK, I'll follow your suggestion to use

        span = 8 * write_bw, for single bdi case 
        span = bdi_thresh, for JBOD case
        x_intercept = setpoint + span;

It does make sense to squeeze the bdi_dirty fluctuation range a bit by
doubling span and making the control line more sharp.

Thanks,
Fengguang
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