code example
MPI message passing interface
two-sided communication.
POSIX threads
OpenCL Open Computing Language
GPU computing

#include <mpi.h>
#include <pthread.h>
#include <cl.h>
#include <malloc.h>

float *x, *y;
#define PROC (8)
float e_vec[PROC];
int n_thread0, n_thread1;

// kernel
#define BLOCK (512)
const char *source =
"__kernel void sub1(__global float* fx,\
    __global const float* fy,\
    __local float* se, __global float* fe) {\
  const unsigned int t = get_global_id(0);\
  const unsigned int b = get_group_id(0);\
  const unsigned block = 512;\
  const unsigned int i = block*b+t;\
  float e;\
  /* do computation */\
  fx[t] += ( fy[t+2] + fy[t] )*.5;\
  e = fy[t+1] * fy[t+1];\
  /* reduction */\
  se[t] = e;\
  barrier(CLK_LOCAL_MEM_FENCE);\
  if (t<256) {\
   se[t] += se[t+256];\
   barrier(CLK_LOCAL_MEM_FENCE);\
  }\
  if (t<128) {\
   se[t] += se[t+128];\
   barrier(CLK_LOCAL_MEM_FENCE);\
  }\
  if (t<64) {\
   se[t] += se[t+64];\
   barrier(CLK_LOCAL_MEM_FENCE);\
  }\
  if (t<32) {\
   se[t] += se[t+32];\
   se[t] += se[t+16];\
   se[t] += se[t+8];\
   se[t] += se[t+4];\
   se[t] += se[t+2];\
   se[t] += se[t+1];\
  }\
  if (t==0)\
   fe[b] = se[0];\
}";

void *thread1(void *arg) {
  int p = (int)arg;
  int n0 = n_thread0 + (p * (n_thread1-n_thread0)) / PROC;
  int n1 = n_thread0 + ((p+1) * (n_thread1-n_thread0)) / PROC;
  // allocate GPU
  cl_context ct = clCreateContextFromType(0, CL_DEVICE_TYPE_GPU, 0, 0, 0);
  size_t ctsize;
  clGetContextInfo(ct, CL_CONTEXT_DEVICES, 0, 0, &ctsize);
  cl_device_id *aDevices = (cl_device_id*)malloc(ctsize);
  clGetContextInfo(ct, CL_CONTEXT_DEVICES, ctsize, aDevices, 0);
  // compile kernel
  cl_program prog = clCreateProgramWithSource(ct, 1, &source, 0, 0);
  clBuildProgram(prog, 0, 0, 0, 0, 0);
  cl_kernel kern = clCreateKernel(prog, "sub1", 0);
  float e = 0;
  // pick GPU
  cl_command_queue queue = clCreateCommandQueue(ct, aDevices[p], 0, 0);
  // allocate GPU memory
  cl_mem fx = clCreateBuffer(ct, CL_MEM_READ_WRITE | CL_MEM_COPY_HOST_PTR,
  (n1-n0)*sizeof(cl_float), &x[n0], 0);
  cl_mem fy = clCreateBuffer(ct, CL_MEM_READ_ONLY | CL_MEM_COPY_HOST_PTR,
  (n1-n0+2)*sizeof(cl_float), &y[n0-1], 0);
  cl_mem se = clCreateBuffer(ct, CL_MEM_READ_WRITE,
  BLOCK*sizeof(cl_float), 0, 0);
  cl_mem fe = clCreateBuffer(ct, CL_MEM_WRITE_ONLY,
  (n1-n0)/BLOCK*sizeof(cl_float), 0, 0);
  clSetKernelArg(kern, 0, sizeof(cl_mem), (void *)&fx);
  clSetKernelArg(kern, 1, sizeof(cl_mem), (void *)&fx);
  clSetKernelArg(kern, 2, sizeof(cl_mem), (void *)&se);
  clSetKernelArg(kern, 3, sizeof(cl_mem), (void *)&fe);
  float *d = new float[(n1-n0)/BLOCK];
  // call GPU
  const unsigned int size = BLOCK;
  const unsigned int dim = n1-n0+2;
  clEnqueueNDRangeKernel(queue, kern, 1, 0, &dim, &size, 0, 0, 0);
  // copy to host memory
  clEnqueueReadBuffer(queue, fx, CL_TRUE, 0,
  (n1-n0) * sizeof(cl_float), &x[n0], 0, 0, 0);
  clEnqueueReadBuffer(queue, fe, CL_TRUE, 0,
  (n1-n0) * sizeof(cl_float), d, 0, 0, 0);
  float ee = 0;
  for (int i=0; i<(n1-n0)/BLOCK; ++i)
   ee += d[i];
  e += ee;
  delete[] d;
  // release GPU memory
  clReleaseMemObject(fx);
  clReleaseMemObject(fy);
  clReleaseMemObject(se);
  clReleaseMemObject(fe);
  e_vec[p] = e;
  return (void*) 0;
}

int main(int argc, char *argv[]) {
  int n = ...;
  MPI_Init(&argc, &argv);
  int numproc, me;
  MPI_Comm_size(MPI_COMM_WORLD, &numproc);
  MPI_Comm_rank(MPI_COMM_WORLD, &me);
  int p_left = -1, p_right = -1;
  if (me > 0)
   p_left = me-1;
  if (me < numproc-1)
   p_right = me+1;
  int n_local0 = 1 + (me * (n-1)) / numproc;
  int n_local1 = 1 + ((me+1) * (n-1)) / numproc;
  // allocate only local part + ghost zone of the arrays x,y
  float *x, *y;
  x = new float[n_local1 - n_local0 + 2];
  y = new float[n_local1 - n_local0 + 2];
  x -= (n_local0 - 1);
  y -= (n_local0 - 1);

  ... // fill x, y

  // fill ghost zone
  MPI_Status s;
  if (p_left != -1)
    MPI_Send(&y[n_local0], 1, MPI_FLOAT, p_left,
      1, MPI_COMM_WORLD);
  if (p_right != -1) {
    MPI_Recv(&y[n_local1], 1, MPI_FLOAT, p_right,
      1, MPI_COMM_WORLD, &s);
    MPI_Send(&y[n_local1-1], 1, MPI_FLOAT, p_right,
      2, MPI_COMM_WORLD);
  }
  if (p_left != -1)
    MPI_Recv(&y[n_local0-1], 1, MPI_FLOAT, p_left,
      2, MPI_COMM_WORLD, &s);
 

  pthread_t threads[PROC];
  pthread_attr_t attr;
  pthread_attr_init(&attr);
  n_thread0 = n_local0;
  n_thread1 = n_local1;
  float e = 0;
  // start threads and wait for termination
  for (int p=0; p<PROC; ++p)
   pthread_create(&threads[p], &attr, thread1, (void *)p);
  for (int p=0; p<PROC; ++p) {
   pthread_join(threads[p], NULL);
   e += e_vec[p];
  }

  float e_local = e;
  MPI_Allreduce(&e_local, &e, 1, MPI_FLOAT, MPI_SUM, MPI_COMM_WORLD);

  ... // output x, e

  x += (n_local0 - 1);
  y += (n_local0 - 1);
  delete[] x, y;
  MPI_Finalize();
  return 0;
}

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