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Karolina Compilation

Since Karolina's nodes are equipped with AMD Zen 2 and Zen 3 processors, we recommend to follow these instructions in order to avoid degraded performance when compiling your code:

1. Select Compilers Flags

When compiling your code, it is important to select right compiler flags; otherwise, the code will not be SIMD vectorized, resulting in severely degraded performance. Depending on the compiler, you should use these flags:

Important

-Ofast optimization may result in unpredictable behavior (e.g. a floating point overflow).

Compiler Module Command Flags
AOCC ml AOCC clang -O3 -mavx2 -march=znver2
INTEL ml intel icc -O3 -xCORE-AVX2
GCC ml GCC gcc -O3 -mavx2

The compiler flags and the resulting compiler performance may be verified with our benchmark, see Lorenz Compiler performance benchmark.

2. Use BLAS Library

It is important to use the BLAS library that performs well on AMD processors. We have measured the best performance with the MKL; however, the MKL BLAS must be ‘tricked’ to believe it is working with an Intel CPU.

ml mkl
ml KAROLINA/FAKEintel

Further, it is very important to pin the BLAS threads to the cores and also to restrict BLAS threads to run on a single socket of an AMD processor.

OMP_NUM_THREADS = 64
GOMP_CPU_AFFINITY=0:63:1

However, to get full performance, you have to execute two jobs on two Karolina sockets at the time. Other BLAS libraries may be used, however none performs as well as the MKL.

Note

Most MPI libraries do the binding automatically. The binding of MPI ranks can be inspected for any MPI by running $ mpirun -n num_of_ranks numactl --show. However, if the ranks spawn threads, binding of these threads should be done via the environment variables described above.

The choice of BLAS library and its performance may be verified with our benchmark, see Lorenz BLAS performance benchmark.

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