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

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Date:   Mon, 28 Dec 2020 09:56:15 +0800
From:   "yukuai (C)" <yukuai3@...wei.com>
To:     Ming Lei <ming.lei@...hat.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,

On 2020/12/27 19:58, Ming Lei wrote:
> 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?

The details of the environment that we found the problem are as follows:

  total driver tags: 128
  number of disks: 13 (network drive, and they form a dm-multipath)
  default queue_depth: 32
  disk performance: when test with 4k randread and single thread, iops is
                    300. And can up to 4000 with 32 thread.
  test cmd: fio -ioengine=psync -numjobs=32 ...

We found that mpath will issue sg_io periodically(about 15s)，which lead
to active_queues setting to 13 for about 5s in every 15s.

By the way, I'm not sure this is a common scenario, however, sq don't
have such problem,

Thanks
Yu Kuai