On Mon, 3 Feb 2020 11:25:29 +0100
Sergio Lopez <slp@redhat.com> wrote:

> 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 <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?  
> 
> 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.

Yes... what happens on systems that have both a lot of vcpus and a lot
of disks? We don't know how many other disks are there in the
configuration, and they might be hotplugged later, anyway.

> 
> 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.

The more I think about it, the more I agree. Aiming a bit lower will
hopefully give more performance with less opportunity for unforeseen
breakage due to resource exhaustion.