Re: [PATCH 3/3] virtio_blk: implement blk_mq_ops->poll()

[Paolo Bonzini]

On 25/05/21 09:38, Ming Lei wrote:
> On Tue, May 25, 2021 at 09:22:48AM +0200, Paolo Bonzini wrote:
> > On 24/05/21 16:59, Christoph Hellwig wrote:
> > > On Thu, May 20, 2021 at 03:13:05PM +0100, Stefan Hajnoczi wrote:
> > > > 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