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

[Halil Pasic <pasic@linux.ibm.com>]

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?

Regards,
Halil