# Reference BLAS/LAPACK from NETLIB is twice as fast as MKL for complex numbers

I'm solving the Helmholtz equation using PETSc. I found with the PETSc configure option --download-f-blas-lapack my program runs twice as fast over running it with MKL. Is this a common trend or are there other factors at play? I'm using gcc and Open MPI with PETSc.

This is usually caused by trying to use a threaded MKL combined with MPI, resulting in over-subscription. Either explicitly configure PETSc to use non-threaded MKL or add MKL_NUM_THREADS=1 to your environment.