Negative clock cycle measurements with back-to-back rdtsc?

Question

I am writing a C code for measuring the number of clock cycles needed to acquire a semaphore. I am using rdtsc, and before doing the measurement on the semaphore, I call rdt

Answer 1

rdtsc can be used to get a reliable and very precise elapsed time. If using linux you can see if your processor supports a constant rate tsc by looking in /proc/cpuinfo to see if you have constant_tsc defined.

Make sure that you stay on the same core. Every core has its own tsc which has its own value. To use rdtsc make sure that you either taskset, or SetThreadAffinityMask (windows) or pthread_setaffinity_np to ensure that your process stays on the same core.

Then you divide this by your main clock rate which on linux can be found in /proc/cpuinfo or you can do this at runtime by

rdtsc
clock_gettime
sleep for 1 second
clock_gettime
rdtsc

then see how many ticks per second, and then you can divide any difference in ticks to find out how much time has elapsed.