Re: Excessive stall times on ext4 in 3.9-rc2

From: Dave Chinner
Date: Fri Apr 12 2013 - 00:50:55 EST

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"


Dave Chinner
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