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

[Christian Borntraeger]

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.
> 
> 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?
My point was to find a balance between performance gain and memory usage.
As a matter of fact, virtqueue do consume memory. So 4 looked like a
reasonable default for me for large guests as long as we do not have directed
interrupts.

Now, thinking about this again: If we want to change the default to something
else in the future (e.g. to num vcpus) then the compat handling will get
really complicated.

So we can
- go with num queues = num cpus. But this will consume memory
for guests with lots of CPUs.
- go with the proposed logic of min(4,vcpus) and accept that future compat 
handling
is harder
- defer this change