dm-multipath low performance with blk-mq

[Sagi Grimberg]

>> I don't think this is going to help __multipath_map() without some
>> configuration changes.  Now that we're running on already merged
>> requests instead of bios, the m->repeat_count is almost always set to 1,
>> so we call the path_selector every time, which means that we'll always
>> need the write lock. Bumping up the number of IOs we send before calling
>> the path selector again will give this patch a change to do some good
>> here.
>>
>> To do that you need to set:
>>
>> 	rr_min_io_rq <something_bigger_than_one>
>>
>> in the defaults section of /etc/multipath.conf and then reload the
>> multipathd service.
>>
>> The patch should hopefully help in multipath_busy() regardless of the
>> the rr_min_io_rq setting.
>
> This patch, while generic, is meant to help the blk-mq case.  A blk-mq
> request_queue doesn't have an elevator so the requests will not have
> seen merging.
>
> But yes, implied in the patch is the requirement to increase
> m->repeat_count via multipathd's rr_min_io_rq (I'll backfill a proper
> header once it is tested).

I'll test it once I get some spare time (hopefully soon...)