# Mixing MPI and CUDA

## Combining CUDA and MPI

Mixing MPI (C) and CUDA (C++) code requires some care during linking because of differences between the C and C++ calling conventions and runtimes. One option is to compile and link all source files with a C++ compiler, which will enforce additional restrictions on C code. Alternatively, if you wish to compile your MPI/C code with a C compiler and call CUDA kernels from within an MPI task, you can wrap the appropriate CUDA-compiled functions with the `extern` keyword, as in the following example.

These two source files can be compiled and linked with both a C and C++ compiler into a single executable on Oscar using:

```bash
module load mpi cuda
mpicc -c main.c -o main.o
nvcc -c multiply.cu -o multiply.o
mpicc main.o multiply.o -lcudart
```

The CUDA/C++ compiler `nvcc` is used only to compile the CUDA source file, and the MPI C compiler `mpicc` is used to compile the C code and to perform the linking.

### `multiply.cu`

```c
#include <cuda.h>
#include <cuda_runtime.h>

__global__ void __multiply__ (const float *a, float *b)
{
    const int i = threadIdx.x + blockIdx.x * blockDim.x;
    b[i] *= a[i];
}

extern "C" void launch_multiply(const float *a, const *b)
{
    /* ... load CPU data into GPU buffers a_gpu and b_gpu */

    __multiply__ <<< ...block configuration... >>> (a_gpu, b_gpu);

    safecall(cudaThreadSynchronize());
    safecall(cudaGetLastError());

    /* ... transfer data from GPU to CPU */
}
```

Note the use of `extern "C"` around the function `launch_multiply`, which instructs the C++ compiler (`nvcc` in this case) to make that function callable from the C runtime. The following C code shows how the function could be called from an MPI task.

### `main.c`

```c
#include <mpi.h>

void launch_multiply(const float *a, float *b);

int main (int argc, char **argv)
{
    int rank, nprocs;
    MPI_Init (&argc, &argv);
    MPI_Comm_rank (MPI_COMM_WORLD, &rank);
    MPI_Comm_size (MPI_COMM_WORLD, &nprocs);

    /* ... prepare arrays a and b */

    launch_multiply (a, b);
    MPI_Finalize();
    return 1;
}
```