Re: [Xen-users] Default disk I/O scheduler in linux guest

[Sadique Puthen <sputhenp@xxxxxxxxxx>]

Javier Guerra wrote:
> On Wed, Feb 4, 2009 at 5:24 PM, Jia Rao <rickenrao@xxxxxxxxx> wrote:
>   
>> Hi, all,
>>
>> I just found out that xen assigns the NOOP disk scheduler for linux guest
>> OSes. Dom0 uses cfq scheduler (it is the linux default).
>> Is there a reason for xen to turn off disk request merging in the quest OS
>> by selecting a NOOP scheduler?
>> Is it because the request optimization will be performed in dom0 or VMM ?
>>     
>
> it's an appropriate default.
>
> everything that virtualizes the IO benefits from using NOOP scheduler.
>  the point is that any (re)ordering done by the guest would be useless
> when the underlying layers (Dom0 in this case, a SAN block device in
> others) mangle the IO requests from several guests.  not only you save
> CPU cycles by not trying to be clever on the DomU, but also pushing
> the requests as early as possible to the lower layer, the best
> optimisations can be done at that layer.
>   

If the underlying hardware is RAID5 done in the hardware level, using
cfq instead of noop in the guest gives 2x performance while testing with
a simple dd command.

Noop:

# sync ; date ; time dd if=/dev/zero of=/tmp/test bs=1M count=500 ; date
; time sync ; date
Tue Apr 28 00:03:18 IST 2009
500+0 records in
500+0 records out
524288000 bytes (524 MB) copied, 10.2711 seconds, 51.0 MB/s

real 0m10.395s
user 0m0.004s
sys 0m1.288s
Tue Apr 28 00:03:29 IST 2009

real 0m43.910s
user 0m0.000s
sys 0m0.000s
Tue Apr 28 00:04:13 IST 2009

CFQ in the guest:

sync ; date ; time dd if=/dev/zero of=/tmp/test bs=1M count=500 ; date ;
time sync ; date
Tue Apr 28 00:02:09 IST 2009
500+0 records in
500+0 records out
524288000 bytes (524 MB) copied, 6.44671 seconds, 81.3 MB/s

real 0m6.451s
user 0m0.000s
sys 0m1.204s
Tue Apr 28 00:02:15 IST 2009

real 0m28.451s
user 0m0.000s
sys 0m0.000s
Tue Apr 28 00:02:44 IST 2009

Is noop still the preferred choice to be the default?

>
>   


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