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 %?
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).
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.
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.
