linux-kernel - Re: [PATCH 1/4] cfq: Increase default value of target

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Date:	Fri, 20 Jun 2014 12:30:25 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Dave Chinner <david@...morbit.com>
Cc:	Jeff Moyer <jmoyer@...hat.com>,
	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 Fri, Jun 20, 2014 at 07:42:14AM +1000, Dave Chinner wrote:
> 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....

Ok, that's fair enough. Tuning CFQ is tricky but as it is, the default
performance is not great in comparison to older kernels and it's something
that has varied considerably over time. I'm surprised there have not been
more complaints but maybe I just missed them on the lists.

-- 
Mel Gorman
SUSE Labs
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