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

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Message-ID: <x493ahzsn8p.fsf@segfault.boston.devel.redhat.com>
Date:	Mon, 10 Nov 2008 08:51:18 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <jens.axboe@...cle.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

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.

Cheers,

Jeff
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