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SubjectRe: Slow file transfer speeds with CFQ IO scheduler in some cases
Date
Jens Axboe <jens.axboe@oracle.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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