OpenMP time and clock() give two different results
c, openmp
Solution
The `clock` function measures cpu time, the time you spend actively on the CPU, the OMP function measures the time as it has passed during execution, two completely different things.
Your process seems to be blocked in waiting somewhere.
Problem
I have sequential code to parallelize via OpenMP. I have put in the corresponding pragmas and tested it. I measure the performance gain by checking the time spent in the main function. The weird thing is the elapsed time calculated via `cpu_time()` and `omp_get_wtime()` is different. Why? The elapsed time according to `cpu_time()` is similar to the sequential time. Before computation starts: ``` ctime1_ = cpu_time(); #ifdef _OPENMP ctime1 = omp_get_wtime(); #endif ``` After computation ends: ``` ctime2_ = cpu_time(); #ifdef _OPENMP ctime2 = omp_get_wtime(); #endif ``` cpu_time() function definition: ``` double cpu_time(void) { double value; value = (double) clock () / (double) CLOCKS_PER_SEC; return value; } ``` Printing result: ``` printf("%f - %f seconds.\n", ctime2 - ctime1, ctime2_ - ctime1_); ``` Sample result: ``` 7.009537 - 11.575277 seconds. ```