http://stackoverflow.com/questions/7506952/understanding-the-different-clocks-of-clock-gettime  here

CLOCK_THREAD_CPUTIME_ID is for measuring the amount of CPU time consumed by the thread, but it's not supported by Linux. glibc emulates it badly by simply returning the amount of CPU time consumed by the process since the moment the thread was created.

I think we should probably use CLOCK_PROCESS_CPUTIME_ID instead?