From: Theodore Ts'o <tytso@mit.edu>
Subject: Re: Excessive stall times on ext4 in 3.9-rc2
Date: Fri, 12 Apr 2013 11:19:52 -0400
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Cc: Jan Kara <jack@suse.cz>, Mel Gorman <mgorman@suse.de>,
	linux-ext4@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>, Jiri Slaby <jslaby@suse.cz>
To: Dave Chinner <david@fromorbit.com>
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On Fri, Apr 12, 2013 at 02:50:42PM +1000, Dave Chinner wrote:
> > If that is the case, one possible solution that comes to mind would be
> > to mark buffer_heads that contain metadata with a flag, so that the
> > flusher thread can write them back at the same priority as reads.
> 
> Ext4 is already using REQ_META for this purpose.

We're using REQ_META | REQ_PRIO for reads, not writes.

> I'm surprised that no-one has suggested "change the IO elevator"
> yet.....

Well, testing to see if the stalls go away with the noop schedule is a
good thing to try just to validate the theory.

The thing is, we do want to make ext4 work well with cfq, and
prioritizing non-readahead read requests ahead of data writeback does
make sense.  The issue is with is that metadata writes going through
the block device could in some cases effectively cause a priority
inversion when what had previously been an asynchronous writeback
starts blocking a foreground, user-visible process.

At least, that's the theory; we should confirm that this is indeed
what is causing the data stalls which Mel is reporting on HDD's before
we start figuring out how to fix this problem.

   	 	      	     	 - Ted

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