source: CIVL/examples/messagePassing/diffusion1d_par.cvl@ e2f9e611

/*  Computing the solver for heat equation in multithreads, then compare the 
 *  result with the solver got in a sequencial way.
 *
 *  Command line example:
 *  civl verify -inputNPROCSB=10 -inputK=0.3 -inputNSTEPS=5 -inputNX=10 diffusion1d.cvl -showAmpleSet
 *  This diffusion1d program computing the diffusion1d equation in parallel and then compare the result with 
 *  another result of diffution1d equation computed in sequential.
 **/ 

#include<civlc.h>
#include<stdio.h>
$input int nprocs;
$input int NPROCSB;
$input double k;              /* k = alpha^2 * dt/(dx^2) */
$input int  nsteps;           /* time */
$input int  NSTEPSB;
$input int  nx;               /* the length of the array */
$input int  NXB;
$input double initialU[nx];
double seq_u[nx];
$proc __procs[nprocs];
$gcomm __MPI_COMM_WORLD;
$assume nx > nprocs;
$assume 0 < nprocs && nprocs <= NPROCSB;
$assume 0 < nsteps && nsteps <= NSTEPSB;
$assume 2 < nx && nx <= NXB;

void MPI_Process (int rank);

void send(void *buf, int count, int dest, int tag, $comm comm) {
  $message out = $message_pack(comm->place, dest, tag, buf, count*sizeof(double));
  $comm_enqueue(comm, out);
}

void recv(void *buf, int count, int source, int tag, $comm comm) {
  $message in = $comm_dequeue(comm, source, tag);
  $message_unpack(in, buf, count*sizeof(double));
}

void init() {
  __MPI_COMM_WORLD = $gcomm_create($root, nprocs);
  for (int i=0; i<nprocs; i++)
    __procs[i] = $spawn MPI_Process(i);
}

void finalize() {
  for (int i=0; i<nprocs; i++)
    $wait(__procs[i]);
}

/* computing the solver in a sequential way*/
void seqDiffusion1d(){
  double u[nx], u_new[nx];
  int i;

  //Initiate                                                              
  for(i=0; i<nx; i++){
    u[i] = initialU[i];
    u_new[i] = initialU[i];
  }
  //Jacobi Iteration             
  for(int iter=nsteps; iter>0; iter--){
    //update                                         
    for(i=1; i<nx-1; i++){
      u_new[i] = u[i] + k * (u[i-1] + u[i+1] - 2*u[i]);
    }
    for(i=1; i<nx-1; i++){
      u[i] = u_new[i];
    }
  }
  for(i=0 ;i<nx; i++){
    seq_u[i] = u[i];
  }
}

void main() {
  seqDiffusion1d();
  init();
  finalize();
}

/* update the array */
void update(double * u, double * u_new, int start, int nxl){
  int i;
  int u_length = nxl; 

  for(i=1; i<u_length+1; i++){
    u_new[i] = u[i] + k * (u[i-1] + u[i+1] - 2*u[i]);
  }
  for(i=1; i<u_length+1; i++){
    u[i] = u_new[i];
  }
}

/* Communicate with left u and right u,
   send the first interior element to left u and receive for 
   the last interior element */
void exchange_ghost_cells(double * u, int left, int right, int start, 
                          int nxl, $comm comm){
  int u_length = nxl;
  // the most left or right us just need to exchange with one side 
  if(left == -1 && right == -1)
    return;
  else if(left == -1){
    send(&u[u_length], 1, right, 0, comm); 
    recv(&u[u_length + 1], 1, right, 0, comm);
  }
  else if(right == -1){
    send(&u[1], 1, left, 0, comm); 
    recv(&u[0], 1, left, 0, comm);
  }else{
    send(&u[u_length], 1, right, 0, comm);
    send(&u[1], 1, left, 0, comm);
    recv(&u[u_length + 1], 1, right, 0, comm);
    recv(&u[0], 1, left, 0, comm);
  }
}

int firstAndCountForProc(int nx, int nprocs, int rank, int * left_u, int * right_u,
           int * first){
  int start, end, left, right;
  int nxl;
  
  nx = nx -2; // the first and last cells are not need to be dsitributed to processes
  start = (rank * nx)/nprocs + 1;
  end = ((rank+1)*nx)/nprocs + 1;
  nxl = end - start;
  left = rank - 1;
  right = rank + 1;
  if(right >= nprocs){
    right = -1;
  }
  /* return the number of interior elements */
  *first = start;
  *left_u = left;
  *right_u = right;
  return nxl;
}

/* the numbers of elements in u and u_new are 2 more than u_length because of 
   ghost elements */
void initU(double * u, double * u_new, int nxl, int first, int left, int right){
  int i, j;

  /* initiate interior array */
  j = first;
  for(i=1; i< nxl+1; i++){
    u[i] = initialU[j];
    u_new[i] = initialU[j];
    j++;
  }
  /* for the most left or right Us, initiate the first ghost element and 
     the last ghost element */
  if(left == -1){
    u[0] = initialU[0];
    u_new[0] = initialU[0];
  }
  if(right == -1){
    i = nxl + 1;
    u[i] = initialU[nx-1];
    u_new[i] = initialU[nx-1];
  }
}

void compare(int first, int nxl, double * u){
  int i;
  
  for(i=0; i<nxl; i++){
    // compare starts from u[1],u[0] is the ghost cell
    $assert ((seq_u[first + i] == u[i+1]), "i = %d\n", i);
  }
}

void MPI_Process (int rank){
  int first = -1;                            /* the index of the u*/
  int left_u = -1, right_u = -1;             /* the index of the left u 
                                                and right u*/
  int nxl = -1;                              /* number of elements in this u*/
  double * u;
  double * u_new;

  $comm MPI_COMM_WORLD = $comm_create($here, __MPI_COMM_WORLD, rank);

  nxl = firstAndCountForProc(nx, nprocs, rank, &left_u, &right_u, &first);
  u = (double*)$malloc($here, sizeof(double)*(nxl + 2));
  u_new = (double*)$malloc($here, sizeof(double)*(nxl + 2));
  initU(u, u_new, nxl, first, left_u, right_u);
  /* Jacobi Iterations*/
  for(int iter=nsteps; iter>0; iter--){
    exchange_ghost_cells(u, left_u, right_u, first, nxl, MPI_COMM_WORLD);
    update(u, u_new, first, nxl);
  }
  compare(first, nxl, u);
}