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

[yukuai (C)]

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