It turns that C-clock() returns the >sum< of cpu usages of all
threads. Hence clock() cannot be used to measure speedups.
Here an alternative:
struct timeval tim;
*** long multi-threaded actions here ***
printf("Duration: %.3lf\\n", t1-t0);
I suspect that this works only if other processes are not interfering/ busy.
I have been doing runs three times in order to assess the quality of