Hi all,
I'd like to attend LSF/MM and would like to discuss polling for block drivers.
Currently there is blk-iopoll but it is neither as widely used as NAPI in the
networking field and accoring to Sagi's findings in [1] performance with
polling is not on par with IRQ usage.
On LSF/MM I'd like to whether it is desirable to have NAPI like polling in
more block drivers and how to overcome the currently seen performance issues.
It would be an interesting topic to discuss, as it is a shame that blk-iopoll
isn't used more widely.
Forgot to mention - it should only be a topic, if experimentation has
been done and results gathered to pin point what the issues are, so we
have something concrete to discus. I'm not at all interested in a hand
wavy discussion on the topic.
Hey all,
Indeed I attempted to convert nvme to use irq-poll (let's use its
new name) but experienced some unexplained performance degradations.
Keith reported a 700ns degradation for QD=1 with his Xpoint devices,
this sort of degradation are acceptable I guess because we do schedule
a soft-irq before consuming the completion, but I noticed ~10% IOPs
degradation fr QD=32 which is not acceptable.
I agree with Jens that we'll need some analysis if we want the
discussion to be affective, and I can spend some time this if I
can find volunteers with high-end nvme devices (I only have access
to client nvme devices.
I can add debugfs statistics on average the number of completions I
consume per intererupt, I can also trace the interrupt and the soft-irq
start,end. Any other interesting stats I can add?
I also tried a hybrid mode where the first 4 completions were handled
in the interrupt and the rest in soft-irq but that didn't make much
of a difference.
Any other thoughts?
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