Disable the default Armadillo in C++ when compiled with -fopenmp

Question

In Armadillo C++, is there any way to disable the default parallelization when compiled with -fopenmp. I would like the parallelization to be on other parts of the code.

The function I'm particularly interested in is eig_sym().

Thanks very much, Yantao

Answer 1

Armadillo isn't parallelized with OpenMP, with slight caveats:

• The underlying LAPACK or BLAS implementation may be paralellized. If you are using OpenBLAS, it is.
• The Armadillo gmm_diag implementation uses OpenMP.

So the simplest way to go is "don't use OpenBLAS, instead use a singlethreaded BLAS". But that's not the only way to go.

It sounds to me like you want to disable nested parallelism, so that the only parts of the code that are parallelized are at the higher levels of your code and not in eig_sym(). Here's some documentation on OMP_NESTED:

https://docs.oracle.com/cd/E19205-01/819-5270/aewbc/index.html

So you could either set the environment OMP_NESTED to false at runtime, or with omp_set_nested() in your code.