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

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Date:	Thu, 8 Mar 2012 16:20:54 -0500
From:	Chris Mason <chris.mason@...cle.com>
To:	"Ted Ts'o" <tytso@....edu>
Cc:	Jeff Moyer <jmoyer@...hat.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 04:12:21PM -0500, Ted Ts'o wrote:
> 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.

cgroups do make it much harder because it could be a simple IO priority
inversion.  The latencies are just going to be a fact of life for now
and the best choice is to skip the stable pages.

-chris
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