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Date:	Fri, 20 Jun 2014 07:42:14 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Mel Gorman <mgorman@...e.de>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	Jan Kara <jack@...e.cz>, Johannes Weiner <hannes@...xchg.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH 1/4] cfq: Increase default value of target_latency

On Thu, Jun 19, 2014 at 02:38:44PM -0400, Jeff Moyer wrote:
> Mel Gorman <mgorman@...e.de> writes:
> 
> > The existing CFQ default target_latency results in very poor performance
> > for larger numbers of threads doing sequential reads.  While this can be
> > easily described as a tuning problem for users, it is one that is tricky
> > to detect. This patch the default on the assumption that people with access
> > to expensive fast storage also know how to tune their IO scheduler.
> >
> > The following is from tiobench run on a mid-range desktop with a single
> > spinning disk.
> >
> >                                       3.16.0-rc1            3.16.0-rc1                 3.0.0
> >                                          vanilla          cfq600                     vanilla
> > Mean   SeqRead-MB/sec-1         121.88 (  0.00%)      121.60 ( -0.23%)      134.59 ( 10.42%)
> > Mean   SeqRead-MB/sec-2         101.99 (  0.00%)      102.35 (  0.36%)      122.59 ( 20.20%)
> > Mean   SeqRead-MB/sec-4          97.42 (  0.00%)       99.71 (  2.35%)      114.78 ( 17.82%)
> > Mean   SeqRead-MB/sec-8          83.39 (  0.00%)       90.39 (  8.39%)      100.14 ( 20.09%)
> > Mean   SeqRead-MB/sec-16         68.90 (  0.00%)       77.29 ( 12.18%)       81.64 ( 18.50%)
> 
> Did you test any workloads other than this?  Also, what normal workload
> has 8 or more threads doing sequential reads?  (That's an honest
> question.)

I'd also suggest that making changes basd on the assumption that
people affected by the change know how to tune CFQ is a bad idea.
When CFQ misbehaves, most people just switch to deadline or no-op
because they don't understand how CFQ works, nor what what all the
nobs do or which ones to tweak to solve their problem....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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