From mboxrd@z Thu Jan  1 00:00:00 1970
From: Wu Fengguang <fengguang.wu@intel.com>
Subject: Re: [PATCH 3/7] writeback: introduce smoothed global dirty limit
Date: Sun, 19 Jun 2011 23:55:47 +0800
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 <20110619150510.246140117@intel.com>
 <20110619153636.GC20424@infradead.org>
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Cc: "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	Jan Kara <jack@suse.cz>, Dave Chinner <david@fromorbit.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>
To: Christoph Hellwig <hch@infradead.org>
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On Sun, Jun 19, 2011 at 11:36:37PM +0800, Christoph Hellwig wrote:
> On Sun, Jun 19, 2011 at 11:01:11PM +0800, Wu Fengguang wrote:
> > The start of a heavy weight application (ie. KVM) may instantly knock
> > down determine_dirtyable_memory() and hence the global/bdi dirty
> > thresholds.
> > 
> > So introduce global_dirty_limit for tracking the global dirty threshold
> > with policies
> > 
> > - follow downwards slowly
> > - follow up in one shot
> > 
> > global_dirty_limit can effectively mask out the impact of sudden drop of
> > dirtyable memory. It will be used in the next patch for two new type of
> > dirty limits.
> 
> This needs to be explained in more detail in comments near the actual
> code.

Good point! This is the added comment for the update_dirty_limit() function.

/*
 * The global dirtyable memory and dirty threshold could be suddenly knocked
 * down by a large amount (eg. on the startup of KVM). This may throw the
 * system into deep dirty exceeded state and throttled to "death" for a couple
 * of seconds. The solution is to maintain global_dirty_limit for tracking
 * slowly down to the knocked down dirty threshold.
 */
static void update_dirty_limit(unsigned long thresh,
                               unsigned long dirty)

Thanks,
Fengguang