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

Ming Lei
2020-12-28T08:30:22Z

On Mon, Dec 28, 2020 at 09:56:15AM +0800, yukuai (C) wrote:
> 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

Looks the tagset depth is a bit low.

> number of disks: 13 (network drive, and they form a dm-multipath)
> default queue_depth: 32

Another candidate solution may be to always return true from hctx_may_queue()
for this kind of queue because queue_depth has provided fair allocation for
each LUN, and looks not necessary to do that again.

> 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.

BTW, I just observe sg_io on rhel8 & rhel7 on mpath over scsi_debug, looks not
see any such activity.

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

If it is done by mpath at default setting, I think it can be thought as
one common case.

Thanks,
Ming