> 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.
