linux-kernel - Re: [PATCH] virtio_blk: set the default scheduler to none

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Date: Tue, 26 Dec 2023 17:01:40 +0800
From: Li Feng <fengli@...rtx.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Jens Axboe <axboe@...nel.dk>,
 Jason Wang <jasowang@...hat.com>,
 Paolo Bonzini <pbonzini@...hat.com>,
 Stefan Hajnoczi <stefanha@...hat.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
 "open list:BLOCK LAYER" <linux-block@...r.kernel.org>,
 linux-kernel <linux-kernel@...r.kernel.org>,
 "open list:VIRTIO BLOCK AND SCSI DRIVERS" <virtualization@...ts.linux.dev>
Subject: Re: [PATCH] virtio_blk: set the default scheduler to none

Hi MST and paolo,

mq-deadline is good for slow media, and none is good for high-speed media. 
It depends on how the community views this issue. When virtio-blk adopts
multi-queue,it automatically changes from deadline to none, which is not
uniform here.

I don't have ideas right now to answer Christoph/Paolo's question.

Thanks,
Li

> On Dec 25, 2023, at 22:20, Michael S. Tsirkin <mst@...hat.com> wrote:
> 
> On Thu, Dec 07, 2023 at 12:31:05PM +0800, Li Feng wrote:
>> virtio-blk is generally used in cloud computing scenarios, where the
>> performance of virtual disks is very important. The mq-deadline scheduler
>> has a big performance drop compared to none with single queue. In my tests,
>> mq-deadline 4k readread iops were 270k compared to 450k for none. So here
>> the default scheduler of virtio-blk is set to "none".
>> 
>> Signed-off-by: Li Feng <fengli@...rtx.com>
> 
> I dropped this for now, pls try to address comments by Christoph/Paolo
> if it's still needed
> 
>> ---
>> drivers/block/virtio_blk.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
>> index d53d6aa8ee69..5183ec8e00be 100644
>> --- a/drivers/block/virtio_blk.c
>> +++ b/drivers/block/virtio_blk.c
>> @@ -1367,7 +1367,7 @@ static int virtblk_probe(struct virtio_device *vdev)
>> 	vblk->tag_set.ops = &virtio_mq_ops;
>> 	vblk->tag_set.queue_depth = queue_depth;
>> 	vblk->tag_set.numa_node = NUMA_NO_NODE;
>> -	vblk->tag_set.flags = BLK_MQ_F_SHOULD_MERGE;
>> +	vblk->tag_set.flags = BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_NO_SCHED_BY_DEFAULT;
>> 	vblk->tag_set.cmd_size =
>> 		sizeof(struct virtblk_req) +
>> 		sizeof(struct scatterlist) * VIRTIO_BLK_INLINE_SG_CNT;
>> -- 
>> 2.42.0
>