On Thu, Jan 30, 2020 at 10:52:35AM +0000, Stefan Hajnoczi wrote:
> On Thu, Jan 30, 2020 at 01:29:16AM +0100, Paolo Bonzini wrote:
> > On 29/01/20 16:44, Stefan Hajnoczi wrote:
> > > On Mon, Jan 27, 2020 at 02:10:31PM +0100, Cornelia Huck wrote:
> > >> On Fri, 24 Jan 2020 10:01:57 +0000
> > >> Stefan Hajnoczi <stefa...@redhat.com> wrote:
> > >>> @@ -47,10 +48,15 @@ static void vhost_scsi_pci_realize(VirtIOPCIProxy
> > >>> *vpci_dev, Error **errp)
> > >>> {
> > >>> VHostSCSIPCI *dev = VHOST_SCSI_PCI(vpci_dev);
> > >>> DeviceState *vdev = DEVICE(&dev->vdev);
> > >>> - VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(vdev);
> > >>> + VirtIOSCSIConf *conf = &dev->vdev.parent_obj.parent_obj.conf;
> > >>> +
> > >>> + /* 1:1 vq to vcpu mapping is ideal because it avoids IPIs */
> > >>> + if (conf->num_queues == VIRTIO_SCSI_AUTO_NUM_QUEUES) {
> > >>> + conf->num_queues = current_machine->smp.cpus;
> > >> This now maps the request vqs 1:1 to the vcpus. What about the fixed
> > >> vqs? If they don't really matter, amend the comment to explain that?
> > > The fixed vqs don't matter. They are typically not involved in the data
> > > path, only the control path where performance doesn't matter.
> >
> > Should we put a limit on the number of vCPUs? For anything above ~128
> > the guest is probably not going to be disk or network bound.
>
> Michael Tsirkin pointed out there's a hard limit of VIRTIO_QUEUE_MAX
> (1024). We need to at least stay under that limit.
>
> Should the guest have >128 virtqueues? Each virtqueue requires guest
> RAM and 2 host eventfds. Eventually these resource requirements will
> become a scalability problem, but how do we choose a hard limit and what
> happens to guest performance above that limit?>From the UX perspective, I think it's safer to use a rather low upper limit for the automatic configuration. Users of large VMs (>=32 vCPUs) aiming for the optimal performance are already facing the need of manually tuning (or relying on a software to do that for them) other aspects of it, like vNUMA, IOThreads and CPU pinning, so I don't think we should focus on this group. On the other hand, the increase in host resource requirements may have unforeseen in some environments, specially to virtio-blk users with multiple disks. All in all, I don't have data that would justify setting the limit to one value or the other. The only argument I can put on the table is that, so far, we only had one VQ per device, so perhaps a conservative value (4? 8?) would make sense from a safety and compatibility point of view. Thanks, Sergio.
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