Re: [PATCH v2 0/4] virtio-pci: enable blk and scsi multi-queue by default

[Stefano Garzarella <sgarzare@redhat.com>]

On Fri, Jan 24, 2020 at 10:01:55AM +0000, Stefan Hajnoczi wrote:
> v2:
>  * Let the virtio-DEVICE-pci device select num-queues because the optimal
>    multi-queue configuration may differ between virtio-pci, virtio-mmio, and
>    virtio-ccw [Cornelia]
> 
> Enabling multi-queue on virtio-pci storage devices improves performance on SMP
> guests because the completion interrupt is handled on the vCPU that submitted
> the I/O request.  This avoids IPIs inside the guest.
> 
> Note that performance is unchanged in these cases:
> 1. Uniprocessor guests.  They don't have IPIs.
> 2. Application threads might be scheduled on the sole vCPU that handles
>    completion interrupts purely by chance.  (This is one reason why benchmark
>    results can vary noticably between runs.)
> 3. Users may bind the application to the vCPU that handles completion
>    interrupts.
> 
> Set the number of queues to the number of vCPUs by default.  Older machine
> types continue to default to 1 queue for live migration compatibility.
> 
> This patch improves IOPS by 1-4% on an Intel Optane SSD with 4 vCPUs, -drive
> aio=native, and fio bs=4k direct=1 rw=randread.
> 
> Stefan Hajnoczi (4):
>   virtio-scsi: introduce a constant for fixed virtqueues
>   virtio-scsi: default num_queues to -smp N
>   virtio-blk: default num_queues to -smp N
>   vhost-user-blk: default num_queues to -smp N

The series looks good to me:

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

Thanks,
Stefano