# How to set good environment variables for the Intel MKL library¶

The Intel MKL library can be linked in a sequential or in a multi-threaded mode (default); the latter is advantageous for multi-CPU architectures.

In order to benefit from the parallelization of MKL, the user should provide appropriate environment variables.

## Examples¶

Safe-play default for processor unit with N cores:

export MKL_NUM_THREADS="N"
export MKL_DYNAMIC="FALSE"
export OMP_NUM_THREADS=1

Allow for 4 threads per core in MKL/BLAS routines:

export MKL_NUM_THREADS=4
export MKL_DYNAMIC="FALSE"
export OMP_DYNAMIC="FALSE"

Note that is important to properly balance MPI and MKL threads. If all cores are already taken by the MPI, the the “unwanted” feature of the threaded-MKL is (sometimes) significant slow down of the code’s execution run.

As an example of proper splitting of CPUs between MPI and MKL, assume a 16 core node where 8 cores are assigned to MPI and the rest to MKL threads:

export MKL_NUM_THREADS="2"
export MKL_DYNAMIC="FALSE"
export OMP_DYNAMIC="FALSE"

If you would like to run a sequential job on the same node it would then read:

export MKL_NUM_THREADS="16"
export MKL_DYNAMIC="FALSE"
export OMP_DYNAMIC="FALSE"

You can find recommended settings for calling Intel MKL routines from multi-threaded applications on the Intel web page.

## Troubleshooting¶

Sometimes you could get this error:

OMP: Error #18: Setting environment variable "__KMP_REGISTERED_LIB_12973" failed:
OMP: Hint: Seems application required too much memory.