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 # Execution in a Parallel Environment 

 Last update: 17.07.2025 

System Coupling participant library supports execution of the participant solver in a parallel Message Passing Interface (MPI) environment.

The participant library uses the Fluent's MPI wrapper library for run-time switching between the compatible MPI distributions. See Fluent documentation for a full list of available MPIs for Windows and Linux platforms. The participant library is expected to be compatible with those MPIs.

The second part of the [Heat Transfer in Square Channel Air Flow](md_17_ChannelFlowTutorial.xhtml) tutorial demonstrates the usage of the parallel APIs.

## Using a Supported MPI Distribution

### Implementing the Interfaces in Parallel

If the participant solver is using one of the supported MPIs, the relevant MPI communicator should be provided when startint the mapping workflow or when establishing the connection to System Coupling. The participant library interfaces should be implemented in each rank of that communicator. The participant library will manage all parallel processing of mesh and variable data internally. If some elements and/or nodes are duplicated in multiple partitions (e.g. due to the mesh overlap layer), the participant library can be expected to handle such scenarios robustly.

#### C++

Mapping

...

// initialize the MPI

MPI_Init(&argc, &argv);

...

// pass the MPI communicator

sysc::SystemCoupling sc(MPI_COMM_WORLD));

...



Cosimulation

...

// initialize the MPI

MPI_Init(&argc, &argv);

...

std::string scHost, scname, buildInfo;

unsigned short int scPort;

...

// pass the MPI communicator when connecting to System Coupling

sysc::SystemCoupling sc(scHost, scPort, scName, MPI_COMM_WORLD, buildInfo));

...



#### C

Mapping

...

/* initialize the MPI */

MPI_Init(&argc, &argv);

...

/* pass the MPI communicator */

syscStartStandaloneParallel(MPI_COMM_WORLD);

...



Cosimulation

...

/* initialize the MPI */

MPI_Init(&argc, &argv);

...

const char* scHost, scName, buildInfo;

unsigned short int scPort;

...

/* pass the MPI communicator when connecting to System Coupling */

syscConnectParallel(scHost, scPort, scName, MPI_COMM_WORLD, buildInfo);

...



#### Fortran

Mapping

...

integer :: err_code

...

! initialize the MPI

call MPI_INIT(err_code)

...

! pass the MPI communicator

syscStartStandaloneF(MPI_COMM_WORLD)

...



Cosimulation

...

integer :: err_code

character(len=256) :: scHost

integer :: scPort

character(len=256) :: scName

character(len=256) :: buildInfo

...

! initialize the MPI

call MPI_Init(err_code)

...

! pass the MPI communicator when connecting to System Coupling

syscConnectF(scHost, scPort, scName, MPI_COMM_WORLD, buildInfo)

...



#### Python

Parallel interfaces are not supported for Python.

### Running the Participant in Parallel

In order to link to the correct MPI wrapper at run-time, the path to the correct wrapper directory should be set in the \*\*`PATH`\*\* environment variable on Windows or \*\*`LD_LIBRARY_PATH`\*\* environment variable on Linux. For example, if the participant solver is using Intel MPI library, then those environment variables should contain the following path:

`<Ansys Installation Path>/SystemCoupling/runTime/<platform>/cnlauncher/fluent/fluent<version>/multiport/mpi_wrapper/<platform>/intel`

This will link to the MPI wrapper library in that directory (`mpi_wrapper.dll` on Windows and `libmpi_wrapper.so` on Linux).

If, for example, the Open MPI library is used, then the `openmpi` directory should be used instead of `intel` in the path above.

For examples of compiling, linking, and executing a program that uses the participant library and a supported MPI, see [Compiling, Linking, and Executing Applications That Use the Participant Library](md_16_CompilingLinkingExecuting.xhtml)

## Using an Unsupported MPI Distribution

If the participant solver is using an MPI distribution that is not supported by the Fluent's MPI wrapper library, then all participant library interfaces should be implemented/used only on one process (e.g., process with rank 0). The participant solver should manage all parallel collection of mesh and variable data for access by the participant library. In this case, the stub MPI wrapper should be used at run-time, which is located in the following directory:

`<Ansys Installation Path>/SystemCoupling/runTime/<platform>/cnlauncher/fluent/fluent<version>/multiport/mpi_wrapper/<platform>/stub`

See [Compiling, Linking, and Executing Applications That Use the Participant Library](md_16_CompilingLinkingExecuting.xhtml) for more detailed examples.