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

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Date:	Tue, 9 Aug 2011 06:47:42 +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 2/5] writeback: dirty position control

On Mon, Aug 08, 2011 at 10:31:49PM +0800, Peter Zijlstra wrote:
> On Mon, 2011-08-08 at 22:11 +0800, Wu Fengguang wrote:
> > It's actually dead code because (origin < limit) should never happen.
> > I feel so good being able to drop 5 more lines of code :) 
> 
> OK, but that leaves me trying to figure out what origin is, and why its
> 4 * thresh.

origin is where the control line crosses the X axis (in both the
global/bdi setpoint cases).

"4 * thresh" is merely something larger than max(dirty, thresh)
that yields reasonably gentle slope. The more slope, the larger
"gravity" to bring the dirty pages back to the setpoint.

> I'm having a horrible time understanding this stuff.

Sorry for that. Do you have more questions?

Thanks,
Fengguang
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