Re: gettimeofday is at the end of its usefulness?

On Thu, May 15, 2014 at 11:31 AM, Greg Stark <stark(at)mit(dot)edu> wrote:

> On Thu, May 15, 2014 at 8:19 AM, Benedikt Grundmann
> <bgrundmann(at)janestreet(dot)com> wrote:
> > I posted this on this mailing list before at Jane Street we have
> developed
> > very fast code to get timing information based on TSC if available. It's
> > all ocaml but well documented and mostly just calls to c functions so
> should
> > be easy to port to C and we release it under a very liberal license so it
> > should be no problem to take the ideas:
>
> What OS do you run it on though? How fast is your implementation
> compared to the kernel implementation of clock_gettime()?
>
> Are you sure your implementation is actually faster? And are you sure
> you're protected against clocks going backwards? I think you should
> put some i/o in the loop in the test and start several threads running
> it to make it more likely the thread is rescheduled to a different
> processor during the test. It suspect you'll find the rdtsc goes
> backwards sometimes or produces crazy results when switching
> processors.
>
>
There are benchmarks in the link I posted (obtained by a micro benchmarking
library we developed / use internally which takes great care to obtain
reliable numbers) . We use posix threads extensively. We internally spend
a lot of time setting up ntp and monitoring systems so that clock backwards
never happens (so with other words I wouldn't be surprised if the library
does NOT work correctly when it does -- our protection is outside). I do
not believe we have seen the tdtsc going backwards on thread context switch
you mention (and as said we use lots of threads). OS? Centos 6.5
primarily.

--
> greg
>