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

[Michael Mueller]

On 11.11.20 13:38, Cornelia Huck wrote:
> On Wed, 11 Nov 2020 13:26:11 +0100
> Michael Mueller <mimu@linux.ibm.com> wrote:
>
> > On 10.11.20 15:16, Michael Mueller wrote:
> > > 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.
> >
> > The perf team has performed measurements for us that show that a *PV
> > KVM guest* benefits in terms of throughput for random read, random write
> > and sequential read (no difference for sequential write) by a multi
> > queue setup. CPU cost are reduced as well due to reduced spinlock
> > contention.
>
> Just to be clear, that was with 4 queues?

Yes, we have seen it with 4 and also with 9 queues.

Halil,

still I would like to know what the exact memory consumption per queue
is that you are talking about. Have you made a calculation? Thanks.

> > For a *standard KVM guest* it currently has no throughput effect. No
> > benefit and no harm. I have asked them to finalize their measurements
> > by comparing the CPU cost as well. I will receive that information on
> > Friday.
>
> Thank you for checking!
>
> > Michael
> >
> >
> > > 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
> > > >   
> > >