From: Paolo Bonzini <pbonzini@redhat.com>
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>
Subject: Re: [PATCH 3/3] virtio_blk: implement blk_mq_ops->poll()
Date: Tue, 25 May 2021 10:06:36 +0200 [thread overview]
Message-ID: <f1997919-e059-b50b-19b3-5741e3309000@redhat.com> (raw)
In-Reply-To: <YKypgi2qcYVTgYdv@T590>
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
next prev parent reply other threads:[~2021-05-25 8:08 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-20 14:13 [PATCH 0/3] virtio_blk: blk-mq io_poll support Stefan Hajnoczi
2021-05-20 14:13 ` [PATCH 1/3] virtio: add virtioqueue_more_used() Stefan Hajnoczi
2021-05-25 2:23 ` Jason Wang
2021-05-25 8:48 ` Stefan Hajnoczi
2021-05-20 14:13 ` [PATCH 2/3] virtio_blk: avoid repeating vblk->vqs[qid] Stefan Hajnoczi
2021-05-25 2:25 ` Jason Wang
2021-05-20 14:13 ` [PATCH 3/3] virtio_blk: implement blk_mq_ops->poll() Stefan Hajnoczi
2021-05-24 14:59 ` Christoph Hellwig
2021-05-25 7:22 ` Paolo Bonzini
2021-05-25 7:38 ` Ming Lei
2021-05-25 8:06 ` Paolo Bonzini [this message]
2021-05-25 13:20 ` Stefan Hajnoczi
2021-05-25 13:19 ` Stefan Hajnoczi
2021-05-25 3:21 ` Jason Wang
2021-05-25 8:59 ` Stefan Hajnoczi
2021-05-27 5:48 ` Jason Wang
2021-06-03 15:24 ` Stefan Hajnoczi
2021-05-27 2:44 ` Ming Lei
2021-06-03 15:12 ` Stefan Hajnoczi
2021-06-03 15:30 ` [PATCH 0/3] virtio_blk: blk-mq io_poll support Stefan Hajnoczi
2021-06-16 7:43 ` Christoph Hellwig
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