[RFC PATCH] dm: fix excessive dm-mq context switching

[Bart Van Assche]

On 02/07/16 07:41, Sagi Grimberg wrote:
>
>> FYI, here is the latest revised patch:
>> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.6&id=a5b835282422ec41991c1dbdb88daa4af7d166d2
>>
>> (revised patch header and fixed a thinko in the dm.c:rq_completed()
>> change from the RFC patch I posted earlier)
>
> Hi Mike,
>
> So I gave your patches a go (dm-4.6) but I still don't see the
> improvement you reported (while I do see a minor improvement).
>
> null_blk queue_mode=2 submit_queues=24
> dm_mod blk_mq_nr_hw_queues=24 blk_mq_queue_depth=4096 use_blk_mq=Y
>
> I see 620K IOPs on dm_mq vs. 1750K IOPs on raw nullb0.
>
> Is there something I'm missing?

Hello Sagi,

Did you run your test on a NUMA system ? If so, can you check with e.g. 
perf record -ags -e LLC-load-misses sleep 10 && perf report whether this 
workload triggers perhaps lock contention ? What you need to look for in 
the perf output is whether any functions occupy more than 10% CPU time.

Bart.