我正在尝试实现一个 MPI 程序，以迭代地将数组中的每个元素设置为自身及其邻居的平均值（在前一个时间步），同时保持第一个和最后一个元素为常数。对于一个过程，这很好用；但是，对于多个进程，我没有得到正确的答案，特别是第一个数组元素总是被覆盖。
我的初始化步骤似乎工作正常，至少就“计算前”输出而言，无论使用的进程数是 1 还是更多，它都会打印相同的向量。
我不完全确定的一件事是我是否正确使用了 MPI_Request 和 MPI_Status；要注意的变量是sendL、sendR和status。
我试图只包含代码的相关部分；“...” 标记缺少的地方。其中一些省略号带有注释以解释删除的内容。给出了并行和单进程实现以供比较。
...
#include "mpi.h"
... //definition of f() for initialization
int main(int argc, char **argv) {
int id, p, i, j, k, n, t, m, v, vp,
lbound, ubound, block_size, offset;
double startwtime, endwtime;
float time;
MPI_Request *sendL, *sendR;
MPI_Status *status; /* return status for receive */
double *prev, *cur, *temp;
... // initialize MPI; get PE rank and size
.... // set the following:
// n = vector length, m = num iterations, k = buffer size
// v = verbose (true/false)
// Memory allocation for output from MPI functions
// Note that I never actually initialized these. Is this a problem?
sendL = (MPI_Request *) malloc(sizeof(MPI_Request));
sendR = (MPI_Request *) malloc(sizeof(MPI_Request));
status = (MPI_Status *) malloc(sizeof(MPI_Status));
// Memory allocation for data array.
block_size = (n/p+2*k);
prev = (double *) malloc( sizeof(double) * block_size);
cur = (double *) malloc( sizeof(double) * block_size);
... //malloc error handling
t = 0;
/* The following block is for a single process. It works correctly. */
if(p==1){
// Initialization
startwtime = MPI_Wtime();
for(i=0;i<n;i++) prev[i] = f(i,n);
cur[0] = f(0,n); cur[n-1] = f(n-1,n);
if(v){
printf("Before calculation\n");
for(i=0;i<n;i++) printf("%f ",prev[i]);
printf("\n");
}
while (t < m) {
for ( i=1 ; i < n-1 ; i++ ) {
cur[i] = (prev[i-1]+prev[i]+prev[i+1])/3;
}
temp = prev; prev = cur; cur = temp; t++;
}
if(v){
printf("After calculation:\n");
for(i=0;i<n;i++) printf("%f ",prev[i]);
printf("\n");
}
endwtime = MPI_Wtime();
time = endwtime-startwtime;
printf("Sequential process complete, time: %f\n", time);
return MPI_Finalize();
}
/* Here is my parallel implementation. It has problems. */
else{
if (id == 0){
startwtime = MPI_Wtime();
}
// Initialization
offset = id*(n/p)-k;
for(i=0;i<block_size;i++) prev[i] = f(i+offset,n);
cur[0] = f(0,n); cur[block_size-1] = prev[block_size-1];
if (id == 0){
for (i=0;i<k;i++){
prev[i] = f(0,n);
cur[i] = prev[i];
}
}
if (id == p-1){
for (i=block_size-k;i<block_size;i++){
prev[i] = f(n-1,n);
cur[i] = prev[i];
}
}
if(v && id == 0){
printf("Before calculation:\n");
for(j=k;j<(n/p)+k;j++) printf("%f ",prev[j]);
for(i=1;i<p;i++){
MPI_Recv(prev+k,(n/p),MPI_DOUBLE_PRECISION,i,2,MPI_COMM_WORLD,status);
for(j=k;j<(n/p)+k;j++) printf("%f ",prev[j]);
}
printf("\n");
}
else if (v){
MPI_Isend(prev+k,(n/p),MPI_DOUBLE_PRECISION,0,2,MPI_COMM_WORLD,sendL);
}
lbound = (id == 0) ? (k+1) : (1);
ubound = (id == p-1) ? (block_size-k-2) : (block_size-2);
while (t < m) {
for ( i=lbound ; i < ubound ; i++ ) {
cur[i] = (prev[i-1]+prev[i]+prev[i+1])/3;
}
temp = prev; prev = cur; cur = temp; t++;
if (t%k == 0){
if (id > 0){
// send to left
MPI_Isend(prev+k,k,MPI_DOUBLE_PRECISION,id-1,0,MPI_COMM_WORLD,sendL);
}
if (id < p-1) {
// send to right
MPI_Isend(prev+block_size-2*k,k,
MPI_DOUBLE_PRECISION,id+1,1,MPI_COMM_WORLD,sendR);
}
if (id < p-1){
// receive from right
MPI_Recv(prev+block_size-k,k,
MPI_DOUBLE_PRECISION,id+1,0,MPI_COMM_WORLD,status);
}
if (id > 0) {
// receive from left
MPI_Recv(prev,k,MPI_DOUBLE_PRECISION,id-1,1,MPI_COMM_WORLD,status);
}
}
}
if(v && id == 0){
printf("After calculation\n");
for(j=k;j<(n/p)+k;j++) printf("%f ",prev[j]);
for(i=1;i<p;i++){
MPI_Recv(prev+k,(n/p),MPI_DOUBLE_PRECISION,i,2,MPI_COMM_WORLD,status);
for(j=k;j<(n/p)+k;j++) printf("%f ",prev[j]);
}
printf("\n");
}
else if (v){
MPI_Isend(prev+k,(n/p),MPI_DOUBLE_PRECISION,0,2,MPI_COMM_WORLD,sendL);
}
if (id == 0){
endwtime = MPI_Wtime();
time = endwtime-startwtime;
printf("Process 0 complete, time: %f\n", time);
}
return MPI_Finalize();
}
}