Use hardware timer to simulate interrupt moderation.
hw.nfeX.imtimer -> 0 no interrupt moderation at all.
hw.nfeX.imtimer -> -Y hardware timer simulated interrupt moderation,
the simultated interrupt moderation timer will
be set to Y.
hw.nfeX.imtimer -> Y hardware timer only interrupt moderation,
hardware timer will be set to Y.
Set hw.nfeX.imtimer to -125 (8000Hz) should work on most system.
With heavier interrupt moderation (e.g. hw.nfeX.imtimer to -250), it is
recommended to increase hw.nfe.rx_ring_count tunable to a higher value
(e.g. 512), since the default value (128) is obviously too small.