[openib-general] [PATCH/RFC 1/2] IB: Return "maybe_missed_event" hint from ib_req_notify_cq()

[Michael S. Tsirkin]

Quoting r. Roland Dreier <rdreier at cisco.com>:
>
> I would really like to understand why ehca does worse with NAPI.  In
> my tests both mthca and ipath exhibit various degrees of improvement
> depending on the test -- but I've never seen performance get worse.
> This is the main thing holding back merging NAPI.

Documentation/netowkring/NAPI_HOWTO.txt says:

APPENDIX 3: Scheduling issues
As seen NAPI moves processing to softirq level. Linux uses the ksoftirqd as the
general solution to schedule softirq's to run before next interrupt and by
putting them under scheduler control. Also this prevents consecutive softirq's
from monopolize the CPU. This also have the effect that the priority of ksoftirq
needs to be considered when running very CPU-intensive applications and
networking to get the proper balance of softirq/user balance. Increasing
ksoftirq priority to 0 (eventually more) is reported cure problems with low
network performance at high CPU load.

So I wonder
1. Was this tried? Its clear that we have high CPU load.
2. Could this be the reason that e.g. e1000 disables NAPI by default?

The issue seem sufficiently tricky that we may yet find ourselves debugging
NAPI performance problems in the field.
Maybe we still need a module option ...

-- 
MST