dm-multipath low performance with blk-mq

[Sagi Grimberg]

On 27/01/2016 19:48, Mike Snitzer wrote:
> On Wed, Jan 27 2016 at  6:14am -0500,
> Sagi Grimberg <sagig at dev.mellanox.co.il> wrote:
>
>>
>>>> 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...)
>
> OK thanks.
>
> BTW, I _cannot_ get null_blk to come even close to your reported 1500K+
> IOPs on 2 "fast" systems I have access to.  Which arguments are you
> loading the null_blk module with?
>
> I've been using:
> modprobe null_blk gb=4 bs=4096 nr_devices=1 queue_mode=2 submit_queues=12

$ for f in /sys/module/null_blk/parameters/*; do echo $f; cat $f; done
/sys/module/null_blk/parameters/bs
512
/sys/module/null_blk/parameters/completion_nsec
10000
/sys/module/null_blk/parameters/gb
250
/sys/module/null_blk/parameters/home_node
-1
/sys/module/null_blk/parameters/hw_queue_depth
64
/sys/module/null_blk/parameters/irqmode
1
/sys/module/null_blk/parameters/nr_devices
2
/sys/module/null_blk/parameters/queue_mode
2
/sys/module/null_blk/parameters/submit_queues
24
/sys/module/null_blk/parameters/use_lightnvm
N
/sys/module/null_blk/parameters/use_per_node_hctx
N

$ fio --group_reporting --rw=randread --bs=4k --numjobs=24 --iodepth=32 
--runtime=99999999 --time_based --loops=1 --ioengine=libaio --direct=1 
--invalidate=1 --randrepeat=1 --norandommap --exitall --name task_nullb0 
--filename=/dev/nullb0
task_nullb0: (g=0): rw=randread, bs=4K-4K/4K-4K/4K-4K, ioengine=libaio, 
iodepth=32
...
fio-2.1.10
Starting 24 processes
Jobs: 24 (f=24): [rrrrrrrrrrrrrrrrrrrrrrrr] [0.0% done] [7234MB/0KB/0KB 
/s] [1852K/0/0 iops] [eta 1157d:09h:46m:22s]