On Thu, Jan 16, 2020 at 12:53:49PM +0100, Cornelia Huck wrote:
> On Thu, 16 Jan 2020 10:58:40 +0000
> Stefan Hajnoczi <stefa...@redhat.com> wrote:
>
> > Automatically size the number of request virtqueues to match the number
> > of vCPUs. This ensures that completion interrupts are handled on the
> > same vCPU that submitted the request. No IPI is necessary to complete
> > an I/O request and performance is improved.
> >
> > Remember that virtqueue numbering assumptions are being removed from the
> > virtio-pci proxy object, so the Control and Event virtqueues are counted
> > by ->get_num_virtqueues() and we only add 1 for the Configuration Change
> > interrupt:
> >
> > if (vpci_dev->nvectors == DEV_NVECTORS_UNSPECIFIED) {
> > - vpci_dev->nvectors = vs->conf.num_queues + 3;
> > + vpci_dev->nvectors = vdc->get_num_virtqueues(VIRTIO_DEVICE(vdev))
> > + 1;
> > }
> >
> > Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>
> > ---
> > hw/core/machine.c | 3 +++
> > hw/scsi/vhost-scsi.c | 2 ++
> > hw/scsi/vhost-user-scsi.c | 2 ++
> > hw/scsi/virtio-scsi.c | 18 ++++++++++++++++++
> > hw/virtio/vhost-scsi-pci.c | 4 ++--
> > hw/virtio/vhost-user-scsi-pci.c | 4 ++--
> > hw/virtio/virtio-scsi-pci.c | 4 ++--
> > include/hw/virtio/virtio-scsi.h | 1 +
> > 8 files changed, 32 insertions(+), 6 deletions(-)
> >
>
> > @@ -878,6 +879,18 @@ static struct SCSIBusInfo virtio_scsi_scsi_info = {
> > .load_request = virtio_scsi_load_request,
> > };
> >
> > +static uint32_t virtio_scsi_get_num_virtqueues(VirtIODevice *vdev)
> > +{
> > + VirtIOSCSICommon *s = VIRTIO_SCSI_COMMON(vdev);
> > + uint32_t request_queues = s->conf.num_queues;
> > +
> > + if (s->conf.num_queues == 1 && s->conf.auto_num_queues) {
> > + request_queues = current_machine->smp.cpus;
> > + }
> > +
> > + return VIRTIO_SCSI_VQ_NUM_FIXED + request_queues;
>
> I'm not sure doing this at the device level is the right thing to do.
> For now, only virtio-pci will call this function, and there basing the
> number of virtqueues off the number of cpus makes sense; but that's a
> property of the transport, not of the device.
>
> Consider e.g. a virtio-scsi-ccw device: If we wanted to introduce a way
> to automatically pick a good number of virtqueues there, this functions
> likely would not return a particularly useful value, as queue interrupt
> processing does not really relate to the number of cpus with adapter
> interrupts. It's not a problem right now, as only virtio-pci calls
> this, but someone looking at this callback is likely getting the
> impression that this is a generically useful function.That's a great point. Maybe the direction should be reversed so that the device asks the transport to suggest the optimal number of queues when auto-num-queues is enabled. Stefan
signature.asc
Description: PGP signature
