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

[Tang, Changqing]

> >
> > 	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 ? 
> ibv_poll_cq
> >   We know select() has to switch to kernel mode, does ibv_poll_cq() 
> > switch to kernel mode and may cause a context switch ?
> >   
> No - it's pure user space function

But I just check the source code, ibv_poll_cq() is actually ibv_cmd_poll_cq(),
and ibv_cmd_poll_cq() calls write() system call on the IB device.

Doesn't this write() system call switch to kernel mode and possiblely casuse
a context switch ?


--CQ



> 
> Tziporet