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

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Date:   Fri, 8 Dec 2023 13:55:30 +0800
From:   Li Feng <fengli@...rtx.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...nel.dk>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        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,

I have ran all io pattern on my another host.
Notes:
q1 means fio iodepth = 1
j1 means fio num jobs = 1

VCPU = 4, VMEM = 2GiB, fio using directio.

The results of most jobs are better than the deadline, and some are lower than the deadline。


pattern                        |  iops(mq-deadline)    | iops(none)       | diff
:-                             | -:                    | -:               | -:
4k-randread-q1-j1              | 12325             |	 13356      	|8.37%
256k-randread-q1-j1            | 1865              |	 1883       	|0.97%
4k-randread-q128-j1            | 204739            |	 319066     	|55.84%
256k-randread-q128-j1          | 24257             |	 22851      	|-5.80%
4k-randwrite-q1-j1             | 9923              |	 10163      	|2.42%
256k-randwrite-q1-j1           | 2762              |	 2833       	|2.57%
4k-randwrite-q128-j1           | 137400            |	 152081     	|10.68%
256k-randwrite-q128-j1         | 9353              |	 9233       	|-1.28%
4k-read-q1-j1                  | 21499             |	 22223      	|3.37%
256k-read-q1-j1                | 1919              |	 1951       	|1.67%
4k-read-q128-j1                | 158806            |	 345269     	|117.42%
256k-read-q128-j1              | 18918             |	 23710      	|25.33%
4k-write-q1-j1                 | 10120             |	 10262      	|1.40%
256k-write-q1-j1               | 2779              |	 2744       	|-1.26%
4k-write-q128-j1               | 47576             |	 209236     	|339.79%
256k-write-q128-j1             | 9199              |	 9337       	|1.50%
4k-randread-q1-j2              | 24238             |	 25478      	|5.12%
256k-randread-q1-j2            | 3656              |	 3649       	|-0.19%
4k-randread-q128-j2            | 390090            |	 577300     	|47.99%
256k-randread-q128-j2          | 21992             |	 23437      	|6.57%
4k-randwrite-q1-j2             | 17096             |	 18112      	|5.94%
256k-randwrite-q1-j2           | 5188              |	 4914       	|-5.28%
4k-randwrite-q128-j2           | 143373            |	 140560     	|-1.96%
256k-randwrite-q128-j2         | 9423              |	 9314       	|-1.16%
4k-read-q1-j2                  | 36890             |	 31768      	|-13.88%
256k-read-q1-j2                | 3708              |	 4028       	|8.63%
4k-read-q128-j2                | 399500            |	 409857     	|2.59%
256k-read-q128-j2              | 19360             |	 21467      	|10.88%
4k-write-q1-j2                 | 17786             |	 18519      	|4.12%
256k-write-q1-j2               | 4756              |	 5035       	|5.87%
4k-write-q128-j2               | 175756            |	 159109     	|-9.47%
256k-write-q128-j2             | 9292              |	 9293       	|0.01%

> On Dec 8, 2023, at 11:54, Ming Lei <ming.lei@...hat.com> wrote:
> 
> On Thu, Dec 07, 2023 at 07:44:37PM -0700, Keith Busch wrote:
>> On Fri, Dec 08, 2023 at 10:00:36AM +0800, Ming Lei 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".
>>> 
>>> The test result shows you may not test HDD. backing of virtio-blk.
>>> 
>>> none can lose IO merge capability more or less, so probably sequential IO perf
>>> drops in case of HDD backing.
>> 
>> More of a curiosity, as I don't immediately even have an HDD to test
>> with! Isn't it more useful for the host providing the backing HDD use an
>> appropriate IO scheduler? virtio-blk has similiarities with a stacking
>> block driver, and we usually don't need to stack IO schedulers.
> 
> dm-rq actually uses IO scheduler at high layer, and early merge has some
> benefits:
> 
> 1) virtio-blk inflight requests are reduced, so less chance to throttle
> inside VM, meantime less IOs(bigger size) are handled by QEMU, and submitted
> to host side queue.
> 
> 2) early merge in VM is cheap than host side, since there can be more block
> IOs originated from different virtio-blk/scsi devices at the same time and
> all images can be stored in single disk, then these IOs become interleaved in
> host side queue, so sequential IO may become random or hard to merge.
> 
> As Jens mentioned, it needs actual test.
> 
> 
> Thanks,
> Ming
>