Large Memory Computing
Debugging

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:

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 /

include

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 /

include

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;

Mixing MPI and CUDA

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:

$ module load mvapich2 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 */
#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 */
#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;
}