Re: [PATCH 1/1] virtio-blk-ccw: tweak the default for num_queues

[Michael Mueller <mimu@linux.ibm.com>]

On 09.11.20 19:53, Halil Pasic wrote:
> On Mon, 9 Nov 2020 17:06:16 +0100
> Cornelia Huck <cohuck@redhat.com> wrote:
> 
>>> @@ -20,6 +21,11 @@ static void virtio_ccw_blk_realize(VirtioCcwDevice *ccw_dev, Error **errp)
>>>   {
>>>       VirtIOBlkCcw *dev = VIRTIO_BLK_CCW(ccw_dev);
>>>       DeviceState *vdev = DEVICE(&dev->vdev);
>>> +    VirtIOBlkConf *conf = &dev->vdev.conf;
>>> +
>>> +    if (conf->num_queues == VIRTIO_BLK_AUTO_NUM_QUEUES) {
>>> +        conf->num_queues = MIN(4, current_machine->smp.cpus);
>>> +    }
>>
>> I would like to have a comment explaining the numbers here, however.
>>
>> virtio-pci has a pretty good explanation (use 1:1 for vqs:vcpus if
>> possible, apply some other capping). 4 seems to be a bit arbitrary
>> without explanation, although I'm sure you did some measurements :)
> 
> Frankly, I don't have any measurements yet. For the secure case,
> I think Mimu has assessed the impact of multiqueue, hence adding Mimu to
> the cc list. @Mimu can you help us out.
> 
> Regarding the normal non-protected VMs I'm in a middle of producing some
> measurement data. This was admittedly a bit rushed because of where we
> are in the cycle. Sorry to disappoint you.

I'm talking with the perf team tomorrow. They have done some 
measurements with multiqueue for PV guests and I asked for a comparison 
to non PV guests as well.

Michael

> 
> The number 4 was suggested by Christian, maybe Christian does have some
> readily available measurement data for the normal VM case. @Christian:
> can you help me out?
> 
> Regards,
> Halil
> 

-- 
Mit freundlichen Grüßen / Kind regards
Michael Müller

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