Re: [Qemu-devel] IO performance test on the tcm-vhost scsi

[Asias He <asias@redhat.com>]

On 06/14/2012 08:07 PM, Stefan Hajnoczi wrote:
> On Thu, Jun 14, 2012 at 05:45:22PM +0800, Cong Meng wrote:
>> On Thu, 2012-06-14 at 09:30 +0100, Stefan Hajnoczi wrote:
>>> On Wed, Jun 13, 2012 at 11:13 AM, mengcong <mc@linux.vnet.ibm.com> wrote:
>>>>                     seq-read        seq-write       rand-read     rand-write
>>>>                     8k     256k     8k     256k     8k   256k     8k   256k
>>>> ----------------------------------------------------------------------------
>>>> bare-metal          67951  69802    67064  67075    1758 29284    1969 26360
>>>> tcm-vhost-iblock    61501  66575    51775  67872    1011 22533    1851 28216
>>>> tcm-vhost-pscsi     66479  68191    50873  67547    1008 22523    1818 28304
>>>> virtio-blk          26284  66737    23373  65735    1724 28962    1805 27774
>>>> scsi-disk           36013  60289    46222  62527    1663 12992    1804 27670
>>>
>>>>
>>>> unit: KB/s
>>>> seq-read/write = sequential read/write
>>>> rand-read/write = random read/write
>>>> 8k,256k are blocksize of the IO
>>>
>>> What strikes me is how virtio-blk performs significantly worse than
>>> bare metal and tcm_vhost for seq-read/seq-write 8k.  The good
>>> tcm_vhost results suggest that the overhead is not the virtio
>>> interface itself, since tcm_vhost implements virtio-scsi.
>>>
>>> To drill down on the tcm_vhost vs userspace performance gap we need
>>> virtio-scsi userspace results.  QEMU needs to use the same block
>>> device as the tcm-vhost-iblock benchmark.
>>>
>>> Cong: Is it possible to collect the virtio-scsi userspace results
>>> using the same block device as tcm-vhost-iblock and -drive
>>> format=raw,aio=native,cache=none?
>>>
>>
>> virtio-scsi-raw     43065  69729    52052  67378    1757 29419    2024 28135
>>
>> qemu ....\
>> -drive file=/dev/sdb,format=raw,if=none,id=sdb,cache=none,aio=native \
>> -device virtio-scsi-pci,id=mcbus \
>> -device scsi-disk,drive=sdb
>>
>> there is only one scsi HBA.
>> /dev/sdb is the disk on which all tests have been done.
>>
>> Is this what you want?
>
> Perfect, thanks.  virtio-scsi userspace is much better than virtio-blk
> here.  That's unexpected since they both use the QEMU block layer.  If
> anything, I would have expected virtio-blk to be faster!
>
> I wonder if the request patterns being sent through virtio-blk and
> virtio-scsi are different.  Asias discovered that the guest I/O
> scheduler and request merging makes a big difference between QEMU and
> native KVM tool performance.  It could be the same thing here which
> causes virtio-blk and virtio-scsi userspace to produce quite different
> results.

Yes. Cong, can you try this:

echo noop > /sys/block/$disk/queue/scheduler
echo 2 > /sys/block/$disk/queue/nomerges

This will disable the merge in guest kernel. The host side IO processing 
speed has a very large impact on the guest request pattern, especially 
for sequential read and write.

> The second question is why is tcm_vhost faster than virtio-scsi
> userspace.
>
> Stefan
>


-- 
Asias