programming issue with openmp - fortran

Question

I am having issues with openmp, described as follows:
I have the serial code like this
subroutine ...
...
do i=1,N
....
end do
end subroutine ...
and the openmp code is
subroutine ...
use omp_lib
...
call omp_set_num_threads(omp_get_num_procs())
!$omp parallel do
do i=1,N
....
end do
!$omp end parallel do
end subroutine ...
No issues with compiling, however when I run the program, there are two major issues compared to the result of serial code:
The program is running even slower than the serial code (which supposedly do matrix multiplications (matmul) in the do-loop
The numerical accuracy seems to have dropped compared to the serial code (I have a check for it)
Any ideas what might be going on?
Thanks,
Xiaoyu

Answer 1

In case of an parallelization using OpenMP, you will need to specify the number of threads your program is to use. You can do so by using the environment variable OMP_NUM_THREADS, e.g. calling your program by means of
OMP_NUM_THREADS=5 ./myprogram
to execute it using 5 threads.
Alternatively, you may set the number of threads at runtime omp_set_num_threads (documentation).
Side Notes
Don't forget to set private variables, if there are any within the loop!
Example:
!$omp parallel do private(prelimRes)
do i = 1, N
prelimRes = myFunction(i)
res(i) = prelimRes + someValue
end do
!$omp end parallel do
Note how the variable prelimRes is declared private so that every thread has its own workspace.
Depending on what you actually do within the loop (i.e. use OpenBLAS), your results may indeed vary (variations should be smaller than 1e-8 with regard to double precision variables) due to the differing, parellel processing.
If you are unsure about what is happening, you should check the CPU load using htop or a similar program while your program is running.
Addendum: Setting the number of threads to automatically match the number of CPUs
If you would like to use the maximum number of useful threads, e.g. use as many threads as there are CPUs, you can do so by using (just like you stated in your question):
subroutine ...
use omp_lib
...
call omp_set_num_threads(omp_get_num_procs())
!$omp parallel do
do i=1,N
....
end do
!$omp end do
!$omp end parallel
end subroutine ...