dm-multipath low performance with blk-mq

[Jens Axboe]

On 01/27/2016 11:16 AM, Mike Snitzer wrote:
> On Wed, Jan 27 2016 at 12:51pm -0500,
> Jens Axboe <axboe at kernel.dk> wrote:
>
>> On 01/27/2016 10:48 AM, Mike Snitzer wrote:
>>>
>>> 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
>>>
>>> On my 1 system is a 12 core single socket, single NUMA node with 12G of
>>> memory, I can only get ~500K read IOPs and ~85K write IOPs.
>>>
>>> On another much larger system with 72 cores and 4 NUMA nodes with 128G
>>> of memory, I can only get ~310K read IOPs and ~175K write IOPs.
>>
>> Look at the completion method (irqmode) and completion time
>> (completion_nsec).
>
> OK, I found that queue_mode=0 (bio-based) is _much_ faster than blk-mq
> (2, the default).  Improving to ~950K read IOPs and ~675K write IOPs (on
> the single numa node system).
>
> Default for irqmode is 1 (softirq).  2 (timer) yields poor results.  0
> (none) seems slightly slower than 1.
>
> And if I use completion_nsec=1 I can bump up to ~990K read IOPs.
>
> Seems the best, for IOPs, so far on this system is with:
> modprobe null_blk gb=4 bs=4096 nr_devices=1 queue_mode=0 irqmode=1 completion_nsec=1 submit_queues=4

That sounds a bit odd. queue_mode=0 will always be a bit faster, 
depending on how many threads, etc. But from 310K to 950K, that sounds 
very suspicious.

And 500K/85K read/write is very low. Just did a quickie on a 2 node box 
I have here, single thread performance with queue_mode=2 is around 
500K/500K read/write.

-- 
Jens Axboe