[openib-general] Re: ANNOUNCE: First usable version of userspace verbs

[Michael S. Tsirkin]

Quoting r. Roland Dreier <roland at topspin.com>:
> Subject: Re: ANNOUNCE: First usable version of userspace verbs
> 
>     mst> but I also changed the clock sampling to use the realtime
>     mst> clock from -lrt, since it gives a more consistent timing
>     mst> results on my system.
> 
> By the way, what exactly are you using?  clock_gettime() with
> CLOCK_REALTIME?  Do you know what the difference from gettimeofday is?
> 
> I haven't followed Linux timekeeping development too closely but there
> should be some portable libc way to get high-resolution time without a
> system call (ie rdtsc on x86, mftb on ppc, etc).  
> 
>  - R.
> 

Looking at libc sources (glibc-2.3.2-200304020432) there appears
an internal macro for it, but I dont see it exported, it seems to be
used for ./malloc/memusage.c implementation. I'll look for a library
outside of libc that we can use.

clock_gettime is a syscall, so has overhead course. Still, since we call it once
per 1000 iterations, the overhead isnt big.

-- 
MST - Michael S. Tsirkin