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Compiling applications in Mahti

General instructions

  • Whenever possible, use the local disk on the login node for compiling software.
    • Compiling on the local disk is much faster and shifts load from the shared file system.
    • The local disk is cleaned frequently, so please move your files elsewhere after compiling.

Building MPI applications

C/C++ and Fortran applications can be built with GNU, AMD, or Intel compiler suites. The GNU compilers are loaded by default. AMD compilers can be loaded using the Modules system with the command:

module load clang
and Intel compilers with the command:
module load intel

Different applications function better with different suites, so the selection needs to be done on a case-by-case basis.

The MPI environment in Mahti is OpenMPI, and when building MPI applications all compiler suites can be used with the mpicc (C), mpicxx (C++), or mpif90 (Fortran) wrappers.

The compiler options for different suites are different. The recommended basic optimization flags are listed in the table below. It is recommended to start from the safe level and then move up to intermediate or even aggressive, while making sure the results are correct and the program's performance has improved.

Optimisation level GNU Intel AMD (clang)
Safe -O2 -march=native -O2 -fp-model precise -O2 -march=native
Intermediate -O3 -march=native -O2 -O3 -march=native
Aggressive -O3 -march=native -ffast-math -funroll-loops -O3 -fp-model fast=2 -no-prec-div -fimf-use-svml=true -O3 -march=native -ffast-math -funroll-loops

A detailed list of options for the Intel and GNU compilers can be found on the man pages (man gcc/gfortran, man icc/ifort) when the corresponding programming environment is loaded, or in the compiler manuals (see the links above).

List all available versions of the compiler suites:

module spider gcc
module spider clang
module spider intel

Building OpenMP and hybrid applications

Additional compiler and linker flags are needed when building OpenMP or MPI/OpenMP hybrid applications:

Compiler suite OpenMP flag
GNU and AMD -fopenmp
Intel -qopenmp

Building serial applications

For building serial applications, one needs to use compiler suite specific compiler command:

Compiler suite C C++ Fortran
GNU gcc g++ gfortran
AMD clang clang++ flang
Intel icc icpc ifort

Building GPU applications

The CUDA, OpenACC and OpenMP Offloading (for C++ codes) programming models are supported on Mahti. Specific modules have to be loaded in order to use them.

For example, to load the NVIDIA HPC SDK 21.2 environment:

module load nvhpc/21.2

For more detailed information about the available modules, please see module spider nvhpc.

Compilers: * The (nvc) is a C11 compiler that supports OpenACC for NVIDIA GPUs while OpenACC and OpenMP for multicore CPUs.

  • The compiler (nvc++) is a C++17 compiler which supports GPU programming with C++17 parallel algorithms, OpenACC, and OpenMP Offloading on NVIDIA GPUs. It does not support yet C++ CUDA codes.

  • The (nvcc) is the CUDA C and CUDA C++ compiler driver for NVIDIA GPUs.

  • The (nvfortran) is the CUDA Fortran compiler driver for NVIDIA GPUs, it supports OpenACC as also multicore for OpenACC and OpenMP.


To generate code for a given target device, tell the CUDA compiler what compute capability the target device supports. On Mahti, the GPUs (Ampere V100) support compute capability 8.0. Specify this using -gencode arch=compute_80,code=sm_80.

For example, compiling a CUDA kernel ( on Puhti (for C or C++ codes):

nvcc -gencode arch=compute_80,code=sm_80

Compile a CUDA Fortran code named example.cuf

nvfortran -gpu=cc80 example.cuf


To enable OpenACC support, one needs to give -acc flag to the compiler.

To generate code for a given target device, tell the compiler what compute capability the target device supports. On Puhti, the GPUs (Ampere A100) support compute capability 8.0.

For example, to compiling C code that uses OpenACC directives (example.c):

nvc -acc example.c .gpu=cc80

For information about what the compiler actually does with the OpenACC directives, use -Minfo=all.

For Fortran code:

nvfortran -acc example.F90 -gpu=cc80

For C++ code:

nvc++ -acc example.cpp -gpu=cc80

OpenMP Offloading

To enable OpenMP Offloading, the options -mp=gpu is required

For example, compile a C code with OpenMP offloading:

nvc -mp=gpu example.c -gpu=cc80

For Fortran code:

nvfortran -mp=gpu example.F90 -gpu=cc80

For C++ code:

nvc++ -mp=gpu example.cpp -gpu=cc80

The nvc++ compiler supports codes that contain OpenACC, OpenMP Offloading and C++ parallel algorithms in the same code, for such case you can compile with:

nvc++ -stdpar -acc -mp=gpu example.cpp -gpu=cc80

For MPI, load the module

module load openmpi/4.0.5

The use of the wrappers mpicc, mpic++, mpif90, executes the corresponding nvc,nvc++,nvfortran respectively.