From: Dave Chinner <david@fromorbit.com>
Subject: Re: Excessive stall times on ext4 in 3.9-rc2
Date: Fri, 12 Apr 2013 14:50:42 +1000
Message-ID: <20130412045042.GA30622@dastard>
References: <20130402142717.GH32241@suse.de>
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 <20130411170402.GB11656@suse.de>
 <20130411183512.GA12298@thunk.org>
 <20130411213335.GE9379@quack.suse.cz>
 <20130412025708.GB7445@thunk.org>
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To: Theodore Ts'o <tytso@mit.edu>, 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>
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On Thu, Apr 11, 2013 at 10:57:08PM -0400, Theodore Ts'o wrote:
> On Thu, Apr 11, 2013 at 11:33:35PM +0200, Jan Kara wrote:
> >   I think it might be more enlightening if Mel traced which process in
> > which funclion is holding the buffer lock. I suspect we'll find out that
> > the flusher thread has submitted the buffer for IO as an async write and
> > thus it takes a long time to complete in presence of reads which have
> > higher priority.
> 
> That's an interesting theory.  If the workload is one which is very
> heavy on reads and writes, that could explain the high latency.  That
> would explain why those of us who are using primarily SSD's are seeing
> the problems, because would be reads are nice and fast.
> 
> 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.

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

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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