linux-kernel - Re: [PATCH v2 2/2] block/mq-deadline: Prioritize first request

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Date:   Fri, 22 Jul 2022 10:47:12 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Wang You <wangyoua@...ontech.com>, axboe@...nel.dk
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        hch@....de, jaegeuk@...nel.org, fio@...r.kernel.org,
        ming.lei@...hat.com, wangxiaohua@...ontech.com
Subject: Re: [PATCH v2 2/2] block/mq-deadline: Prioritize first request

On 7/22/22 02:51, Wang You wrote:
> The test hardware is:
> Kunpeng-920, HW-SAS3508+(MG04ACA400N * 2), RAID0.

What is MG04ACA400N? The test results suggest that it is an SSD but this 
is something that should be mentioned explicitly.

> - The test hardware is:
> Hygon C86, MG04ACA400N

What is MG04ACA400N?

> The test command is:
> fio -ioengine=psync -lockmem=1G -buffered=0 -time_based=1 -direct=1 -iodepth=1
> -thread -bs=512B -size=110g -numjobs=32 -runtime=300 -group_reporting
> -name=read -filename=/dev/sdc -ioscheduler=mq-deadline -rw=read[,write,rw]
> 
> The following is the test data:
> origin/master:
> read iops: 15463	write iops: 5949	rw iops: 574,576
> 
> nr_sched_batch = 1:
> read iops: 15082	write iops: 6283	rw iops: 783,786
> 
> nr_sched_batch = 1, use deadline_head_request:
> read iops: 15368	write iops: 6575	rw iops: 907,906

The above results are low enough such that these could come from a hard 
disk. However, the test results are hard to interpret since the I/O 
pattern is neither perfectly sequential nor perfectly random (32 
sequential jobs). Please provide separate measurements for sequential 
and random I/O.

The above results show that this patch makes reading from a hard disk 
slower. Isn't the primary use case of mq-deadline to make reading from 
hard disks faster? So why should these two patches be applied if these 
slow down reading from a hard disk?

Thanks,

Bart.