code example
shmem SGI shmem
one-sided communication
OpenCL Open Computing Language
GPU computing

#include <mpp/shmem.h>
#include <cl.h>
#include <malloc.h>


// 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];\
}";

int main(int argc, char *argv[]) {
  int n = ...;
  start_pes(0);
  int nn = (n-1) / _num_pes();
  int n_local0 = 1 + _my_pe() * nn;
  int n_local1 = 1 + (_my_pe()+1) * nn;
  // allocate only local part + ghost zone of the arrays x,y
  float *x, *y;
  x = (float*)shmalloc((n_local1 - n_local0 + 2)*sizeof(float));
  y = (float*)shmalloc((n_local1 - n_local0 + 2)*sizeof(float));
  x -= (n_local0 - 1);
  y -= (n_local0 - 1);
  shmem_barrier_all();

  ... // fill x, y

  // fill ghost zone
  if (_my_pe() > 0)
   shmem_float_put(&y[n_local1], &y[n_local0], 1, _my_pe()-1);
  if (_my_pe() < _num_pes()-1)
   shmem_float_put(&y[n_local0-1], &y[n_local1-1], 1, _my_pe()+1);
  shmem_barrier_all();

  // 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;
  cl_command_queue queue = clCreateCommandQueue(ct, aDevices[0], 0, 0);
  // allocate GPU memory
  cl_mem fx = clCreateBuffer(ct, CL_MEM_READ_WRITE | CL_MEM_COPY_HOST_PTR,
  (n_local1-n_local0)*sizeof(cl_float), &x[n_local0], 0);
  cl_mem fy = clCreateBuffer(ct, CL_MEM_READ_ONLY | CL_MEM_COPY_HOST_PTR,
  (n_local1-n_local0+2)*sizeof(cl_float), &y[n_local0-1], 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,
  (n_local1-n_local0)/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[(n_local1-n_local0)/BLOCK];
  // call GPU
  const unsigned int size = BLOCK;
  const unsigned int dim = n_local1-n_local0+2;
  clEnqueueNDRangeKernel(queue, kern, 1, 0, &dim, &size, 0, 0, 0);
  // copy to host memory
  clEnqueueReadBuffer(queue, fx, CL_TRUE, 0,
  (n_local1-n_local0) * sizeof(cl_float), &x[n_local0], 0, 0, 0);
  clEnqueueReadBuffer(queue, fe, CL_TRUE, 0,
  (n_local1-n_local0) * sizeof(cl_float), d, 0, 0, 0);
  float ee = 0;
  for (int i=0; i<(n_local1-n_local0)/BLOCK; ++i)
   ee += d[i];
  e += ee;
  delete[] d;
  // release GPU memory
  clReleaseMemObject(fx);
  clReleaseMemObject(fy);
  clReleaseMemObject(se);
  clReleaseMemObject(fe);

  static float work[_SHMEM_REDUCE_MIN_WRKDATA_SIZE];
  static long sync[_SHMEM_REDUCE_MIN_WRKDATA_SIZE];
  static float el, es;
  el = e;
  shmem_float_sum_to_all(&es, &el, 1,
   0, 0, _num_pes(), work, sync);
  e = es;

  ... // output x, e

  x += (n_local0 - 1);
  y += (n_local0 - 1);
  shfree(x);
  shfree(y);
  return 0;
}

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