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Subject: Re: [PATCH 3/3] virtio_blk: implement blk_mq_ops->poll()
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        virtualization@lists.linux-foundation.org,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        Jason Wang <jasowang@redhat.com>, Jens Axboe <axboe@kernel.dk>,
        slp@redhat.com, sgarzare@redhat.com,
        "Michael S. Tsirkin" <mst@redhat.com>
References: <20210520141305.355961-1-stefanha@redhat.com>
 <20210520141305.355961-4-stefanha@redhat.com> <20210524145928.GA3873@lst.de>
 <7cc7f19b-34b3-1501-898d-3f41e047d766@redhat.com> <YKypgi2qcYVTgYdv@T590>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <f1997919-e059-b50b-19b3-5741e3309000@redhat.com>
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