linux-kernel - Re: [PATCH 5/5] writeback: IO-less balance_dirty

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Message-ID: <20110823035335.GA26739@localhost>
Date:	Tue, 23 Aug 2011 11:53:35 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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 5/5] writeback: IO-less balance_dirty_pages()

> >   Because task_ratelimit_0 is initial value to begin with and we will
> >   keep on coming with new value every 200ms, we should be able to write
> >   above as follows.
> > 
> > 						      write_bw
> >   bdi->dirty_ratelimit_n = bdi->dirty_ratelimit_n-1 * --------  (8)
> > 						      dirty_bw
> > 
> >   Effectively we start with an initial value of task_ratelimit_0 and
> >   then keep on updating it based on rate change feedback every 200ms.

Ah sorry, based on the reply to Peter, there is no inherent dependency
between balanced_rate_n and balanced_rate_(n-1). bdi->dirty_ratelimit does
track balanced_rate in small steps, and hence will have some relationship
with its previous value other than equation (8).

So, although you may conduct equation (8) for balanced_rate, we'd
better not understand things in that way. Keep this fundamental
formula in mind and don't try to complicate it:

        balanced_rate = task_ratelimit_200ms * write_bw / dirty_rate

Thanks,
Fengguang
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