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

[Jeff Moyer]

Jens Axboe <jens.axboe@xxxxxxxxxx> writes:

> On Mon, Nov 10 2008, Jeff Moyer wrote:
>> Jens Axboe <jens.axboe@xxxxxxxxxx> 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.

OK, I'm not actually able to reproduce the horrible 9MB/s reported by
Vitaly.  Here are the numbers I see.

Single dd performing a cold cache read of a 1GB file from an
nfs server.  read_ahead_kb is 128 (the default) for all tests.
cfq-cc denotes that the cfq scheduler was patched with the close
cooperator patch.  All numbers are in MB/s.

nfsd threads|   1  |  2   |  4   |  8  
----------------------------------------
deadline    | 65.3 | 52.2 | 46.7 | 46.1
cfq         | 64.1 | 57.8 | 53.3 | 46.9
cfq-cc      | 65.7 | 55.8 | 52.1 | 40.3

So, in my configuration, cfq and deadline both degrade in performance as
the number of nfsd threads is increased.  The close cooperator patch
seems to hurt a bit more at 8 threads, instead of helping;  I'm not sure
why that is.

Now, the workload that showed most slowdown for cfq with respect to
other I/O schedulers was using dump(8) to backup a file system.  Here
are the numbers for that:

deadline  82241 kB/s
cfq	  34143 kB/s
cfq-cc    82241 kB/s

And a customer actually went to the trouble to write a test to approximate
dump(8)'s I/O patterns.  For that test, we also see a big speedup (as
expected):

deadline  87157 kB/s
cfq       20218 kB/s
cfq-cc    87056 kB/s

Jens, if you have any canned fio workloads that you use for regression
testing, please pass them my way and I'll give them a go on some SAN
storage.

Cheers,

Jeff
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