[ofa-general] performance to call ibv_poll_cq() vs. call select() on completion channel

[Tang, Changqing]

Roland or Mellanox Engineers:

	We setup completion channel for a completion queue. We want to check if there is any
event available, and suppose there is NO event on both completion channel and completion queue.

	What we can do is:

	1. call select() on completion channel with zero timeout and return 0.
	2. call ibv_poll_cq() directly and return 0.

Question:

	Which way has lower overhead ?   We know select() has to switch to kernel mode, does ibv_poll_cq()
switch to kernel mode and may cause a context switch ?


Thanks.
--CQ Tang