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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


WARNING: multiple messages have this Message-ID (diff)
From: Paolo Bonzini <pbonzini@redhat.com>
To: Ming Lei <ming.lei@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	linux-block@vger.kernel.org,
	Stefan Hajnoczi <stefanha@redhat.com>,
	Christoph Hellwig <hch@lst.de>
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

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

  reply	other threads:[~2021-05-25  8:08 UTC|newest]

Thread overview: 42+ 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 ` Stefan Hajnoczi
2021-05-20 14:13 ` [PATCH 1/3] virtio: add virtioqueue_more_used() Stefan Hajnoczi
2021-05-20 14:13   ` Stefan Hajnoczi
2021-05-25  2:23   ` Jason Wang
2021-05-25  2:23     ` Jason Wang
2021-05-25  8:48     ` Stefan Hajnoczi
2021-05-25  8:48       ` Stefan Hajnoczi
2021-05-20 14:13 ` [PATCH 2/3] virtio_blk: avoid repeating vblk->vqs[qid] Stefan Hajnoczi
2021-05-20 14:13   ` Stefan Hajnoczi
2021-05-25  2:25   ` Jason Wang
2021-05-25  2:25     ` Jason Wang
2021-05-20 14:13 ` [PATCH 3/3] virtio_blk: implement blk_mq_ops->poll() Stefan Hajnoczi
2021-05-20 14:13   ` Stefan Hajnoczi
2021-05-24 14:59   ` Christoph Hellwig
2021-05-24 14:59     ` Christoph Hellwig
2021-05-25  7:22     ` Paolo Bonzini
2021-05-25  7:22       ` Paolo Bonzini
2021-05-25  7:38       ` Ming Lei
2021-05-25  7:38         ` Ming Lei
2021-05-25  8:06         ` Paolo Bonzini [this message]
2021-05-25  8:06           ` Paolo Bonzini
2021-05-25 13:20         ` Stefan Hajnoczi
2021-05-25 13:20           ` Stefan Hajnoczi
2021-05-25 13:19     ` Stefan Hajnoczi
2021-05-25 13:19       ` Stefan Hajnoczi
2021-05-25  3:21   ` Jason Wang
2021-05-25  3:21     ` Jason Wang
2021-05-25  8:59     ` Stefan Hajnoczi
2021-05-25  8:59       ` Stefan Hajnoczi
2021-05-27  5:48       ` Jason Wang
2021-05-27  5:48         ` Jason Wang
2021-06-03 15:24         ` Stefan Hajnoczi
2021-06-03 15:24           ` Stefan Hajnoczi
2021-05-27  2:44   ` Ming Lei
2021-05-27  2:44     ` Ming Lei
2021-06-03 15:12     ` Stefan Hajnoczi
2021-06-03 15:12       ` Stefan Hajnoczi
2021-06-03 15:30 ` [PATCH 0/3] virtio_blk: blk-mq io_poll support Stefan Hajnoczi
2021-06-03 15:30   ` Stefan Hajnoczi
2021-06-16  7:43   ` Christoph Hellwig
2021-06-16  7:43     ` Christoph Hellwig

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