linux-kernel - Re: [PATCH 1/3] blk-mq: allow hardware queue to get more tag while sharing a tag set

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Message-ID: <20201227115859.GA3282759@T590>
Date:   Sun, 27 Dec 2020 19:58:59 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com,
        zhangxiaoxu5@...wei.com
Subject: Re: [PATCH 1/3] blk-mq: allow hardware queue to get more tag while
 sharing a tag set

Hi Yu Kuai,

On Sat, Dec 26, 2020 at 06:28:06PM +0800, Yu Kuai wrote:
> When sharing a tag set, if most disks are issuing small amount of IO, and
> only a few is issuing a large amount of IO. Current approach is to limit
> the max amount of tags a disk can get equally to the average of total
> tags. Thus the few heavy load disk can't get enough tags while many tags
> are still free in the tag set.

Yeah, current approach just allocates same share for each active queue
which is evaluated in each timeout period.

That said you are trying to improve the following case:
- heavy IO on one or several disks, and the average share for these
  disks become bottleneck of IO performance
- small amount IO on other disks attached to the same host, and all IOs are
submitted to disk in <30 second period.

Just wondering if you may share the workload you are trying to optimize,
or it is just one improvement in theory? And what is the disk(hdd, ssd
or nvme) and host? And how many disks in your setting? And how deep the tagset
depth is?


Thanks, 
Ming