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Tsirkin" , David Hildenbrand , qemu-devel@nongnu.org, Halil Pasic , qemu-s390x@nongnu.org, Michael Mueller Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Tue, 10 Nov 2020 09:47:51 +0100 Christian Borntraeger wrote: > On 09.11.20 19:53, Halil Pasic wrote: > > On Mon, 9 Nov 2020 17:06:16 +0100 > > Cornelia Huck 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. Yes, 4 does not look like a bad number, but I still don't feel really comfortable with it without at least some data. What about large guests with slow vs. fast storage? > > 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. Yes, I fear that will be messy. Just picking a value later will need compat handling, but not a really complicated one. > > So we can > - go with num queues = num cpus. But this will consume memory > for guests with lots of CPUs. I'm not sure that would be a good choice, as we don't have the benefits that pci has. > - go with the proposed logic of min(4,vcpus) and accept that future compat handling > is harder With a bit more data, I'd be way more comfortable. Might still be ok for the next rc. > - defer this change We might end up with that, given the timing :( (not blaming anyone)