[PATCH rfc] nvme: support io stats on the mpath device

[Jens Axboe]

On 9/29/22 10:25 AM, Sagi Grimberg wrote:
> 
>>>> 3. Do you have some performance numbers (we're touching the fast path here) ?
>>>
>>> This is pretty light-weight, accounting is per-cpu and only wrapped by
>>> preemption disable. This is a very small price to pay for what we gain.
>>
>> Is it? Enabling IO stats for normal devices has a very noticeable impact
>> on performance at the higher end of the scale.
> 
> Interesting, I didn't think this would be that noticeable. How much
> would you quantify the impact in terms of %?

If we take it to the extreme - my usual peak benchmark, which is drive
limited at 122M IOPS, run at 113M IOPS if I have iostats enabled. If I
lower the queue depth (128 -> 16), then peak goes from 46M to 44M. Not
as dramatic, but still quite noticeable. This is just using a single
thread on a single CPU core per drive, so not throwing tons of CPU at
it.

Now, I have no idea how well nvme multipath currently scales or works.
Would be interesting to test that separately. But if you were to double
(or more, I guess 3x if you're doing the exposed device and then adding
stats to at least two below?) the overhead, that'd certainly not be
free.

> I don't have any insight on this for blk-mq, probably because I've never
> seen any user turn IO stats off (or at least don't remember).

Most people don't care, but some certainly do. As per the above, it's
noticeable enough that it makes a difference if you're chasing latencies
or peak performance.

> My (very limited) testing did not show any noticeable differences for
> nvme-loop. All I'm saying that we need to have IO stats for the mpath
> device node. If there is a clever way to collect this from the hidden
> devices just for nvme, great, but we need to expose these stats.

 From a previous message, sounds like that's just some qemu setup? Hard
to measure anything there with precision in my experience, and it's not
really peak performance territory either.

>> So much so that I've contemplated how we can make this less expensive
>> than it currently is.
> 
> Then nvme-mpath would benefit from that as well.

Yeah, it'd be a win all around for sure...

-- 
Jens Axboe