Optimising and why openmp is much slower than sequential way?

Question

I am a newbie in programming with OpenMp. I wrote a simple c program to multiply matrix with a vector. Unfortunately, by comparing executing time I found that the OpenMP is much

Answer 1

Because when OpenMP distributes the work among threads there is a lot of administration/synchronisation going on to ensure the values in your shared matrix and vector are not corrupted somehow. Even though they are read-only: humans see that easily, your compiler may not.

Things to try out for pedagogic reasons:

0) What happens if matrix and vector are not shared?

1) Parallelize the inner "j-loop" first, keep the outer "i-loop" serial. See what happens.

2) Do not collect the sum in result[i], but in a variable temp and assign its contents to result[i] only after the inner loop is finished to avoid repeated index lookups. Don't forget to init temp to 0 before the inner loop starts.