linux-ext4 - Re: [PATCH, RFC] Don't do page stablization if !CONFIG_BLKDEV

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Date:	Thu, 8 Mar 2012 16:12:21 -0500
From:	Ted Ts'o <tytso@....edu>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Boaz Harrosh <bharrosh@...asas.com>,
	Zach Brown <zab@...bo.net>, Eric Sandeen <sandeen@...hat.com>,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH, RFC] Don't do page stablization if
 !CONFIG_BLKDEV_INTEGRITY

On Thu, Mar 08, 2012 at 03:42:52PM -0500, Jeff Moyer wrote:
> 
> So now we're back to figuring out how to tell how long I/O will take?
> If writeback is issuing random access I/Os to spinning media, you can
> bet it might be a while.  Today, you could lower nr_requests to some
> obscenely small number to improve worst-case latency.  I thought there
> was some talk about improving the intelligence of writeback in this
> regard, but it's a tough problem, especially given that writeback isn't
> the only cook in the kitchen.

... and it gets worse if there is any kind of I/O prioritization going
on via ionice(), or (as was the case in our example) I/O cgroups were
being used to provide proportional I/O rate controls.  I don't think
it's realistic to assume the writeback code can predict how long I/O
will take when it does a submission.

BTW, I'd have to check (having not looked at the application code in
depth; the bug was primarily solved by bisection and reverting the
problem commit) but I'm not entirely sure the thread doing the write
was calling fsync(); the main issue as I understand things was that
the application wasn't expecting the write(2) system call would block
unexpectedly for long periods of time while doing small buffered,
appending I/O's.  (Again, for the kind of work that distributed
systems do, 99th percentile latency is important!)

						- Ted
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