On 25/05/21 09:38, Ming Lei wrote:

On Tue, May 25, 2021 at 09:22:48AM +0200, Paolo Bonzini wrote:

quoted

On 24/05/21 16:59, Christoph Hellwig wrote:

quoted

On Thu, May 20, 2021 at 03:13:05PM +0100, Stefan Hajnoczi wrote:

quoted

Possible drawbacks of this approach:

- Hardware virtio_blk implementations may find virtqueue_disable_cb()
    expensive since it requires DMA. If such devices become popular then
    the virtio_blk driver could use a similar approach to NVMe when
    VIRTIO_F_ACCESS_PLATFORM is detected in the future.

- If a blk_poll() thread is descheduled it not only hurts polling
    performance but also delays completion of non-REQ_HIPRI requests on
    that virtqueue since vq notifications are disabled.

Yes, I think this is a dangerous configuration.  What argument exists
again just using dedicated poll queues?

There isn't an equivalent of the admin queue in virtio-blk, which would
allow the guest to configure the desired number of poll queues.  The number
of queues is fixed.

Dedicated vqs can be used for poll only, and I understand VM needn't to know
if the vq is polled or driven by IRQ in VM.

I tried that in v5.4, but not see obvious IOPS boost, so give up.

https://github.com/ming1/linux/commits/my_v5.4-virtio-irq-poll

Sure, but polling can be beneficial even for a single queue.  Queues 
have a cost on the host side as well, so a 1 vCPU - 1 queue model may 
not be always the best.

Paolo

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