Re: [PATCH v2 2/4] virtio-scsi: default num_queues to -smp N

[Cornelia Huck <cohuck@redhat.com>]

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On Thu, 30 Jan 2020 10:52:35 +0000
Stefan Hajnoczi <stefanha@redhat.com> 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 <stefanha@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?

There's probably two kind of limits involved here:

- a hard limit (we cannot do more), which should be checked even for
  user-specified values, and
- a soft limit (it does not make sense to go beyond this for the
  default case), which can be overridden if explicitly specified.

VIRTIO_QUEUE_MAX (and two less for virtio-scsi) sounds like a hard
limit, maybe 128 is a reasonable candidate for a soft limit.

(I would expect systems that give 128 vcpus to the guest to also be
generously sized in other respects.)

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