linux-kernel - Re: Slow file transfer speeds with CFQ IO scheduler in some cases

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Date:	Mon, 10 Nov 2008 14:56:18 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	"Vitaly V. Bursov" <vitalyb@...enet.dn.ua>,
	linux-kernel@...r.kernel.org
Subject: Re: Slow file transfer speeds with CFQ IO scheduler in some cases

On Mon, Nov 10 2008, Jeff Moyer wrote:
> Jens Axboe <jens.axboe@...cle.com> writes:
> 
> > On Sun, Nov 09 2008, Vitaly V. Bursov wrote:
> >> Hello,
> >> 
> >> I'm building small server system with openvz kernel and have ran into
> >> some IO performance problems. Reading a single file via NFS delivers
> >> around 9 MB/s over gigabit network, but while reading, say, 2 different
> >> or same file 2 times at the same time I get >60MB/s.
> >> 
> >> Changing IO scheduler to deadline or anticipatory fixes problem.
> >> 
> >> Tested kernels:
> >>   OpenVZ RHEL5 028stab059.3 (9 MB/s with HZ=100, 20MB/s with HZ=1000
> >>                  fast local reads)
> >>   Vanilla 2.6.27.5 (40 MB/s with HZ=100, slow local reads)
> >> 
> >> Vanilla performs better in worst case but I believe 40 is still low
> >> concerning test results below.
> >
> > Can you check with this patch applied?
> >
> > http://bugzilla.kernel.org/attachment.cgi?id=18473&action=view
> 
> Funny, I was going to ask the same question.  ;)  The reason Jens wants
> you to try this patch is that nfsd may be farming off the I/O requests
> to different threads which are then performing interleaved I/O.  The
> above patch tries to detect this and allow cooperating processes to get
> disk time instead of waiting for the idle timeout.

Precisely :-)

The only reason I haven't merged it yet is because of worry of extra
cost, but I'll throw some SSD love at it and see how it turns out.

-- 
Jens Axboe

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