Messages in this thread | | | Subject | Re: [PATCH 3/3] virtio_blk: implement blk_mq_ops->poll() | From | Paolo Bonzini <> | Date | Tue, 25 May 2021 10:06:36 +0200 |
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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
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