1
我正在參加並行編程課程。老師給了我們一個涉及模板計算域分區的作業。對於這種類型的計算（有限差分），並行化代碼的最常見方式是對域進行分區並創建一些鬼區域（光暈）。
爲了更好地理解在MPI中創建重影區域，我編程了這個簡單的例子，它初始化了一些內部值爲123和邊界值爲88的數組。在所有通信結束時，所有重影值都應該保持爲8.在一個節點我得到123個值。
串行（沒有鬼）：
123 - 123 - ... - 123 - 123
兩個分區：
123 - 123 - ... - 88 ||| 88 - ... - 123 - 123
三分區：
123 - 123 - ... - 88 ||| 88 - ... - 123 - 123 - 88 ||| 88 - ... - 123 - 123
除此之外，這裏的主要問題是如何正確方法來創建和維護更新的幽靈區域。是否有此除了我凌亂的，如果（身份識別碼== ....否則，如果（身份識別碼= ...其他類型的實現？怎麼人們通常實現這種並行的？
#include<mpi.h>
#include<stdio.h>
#include<stdlib.h>
int WhichSize(int mpiId, int numProc, int tam);
int main(int argc, char *argv[]){
int i;
int localSize;
int numProc;
int myid;
int leftProc;
int rightProc;
int * myArray;
int fullDomainSize = 16;
MPI_Request request;
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &numProc);
MPI_Comm_rank(MPI_COMM_WORLD, &myid);
// Lets get each partition size.
localSize = WhichSize(myid, numProc, fullDomainSize);
// Allocate arrays acording to proc number.
if(numProc == 1){
//printf("Allocating Array for serial usage\n");
myArray = (int*)malloc(localSize*sizeof(int));
} else if(numProc == 2) {
//printf("Allocating Array for 2 proc usage\n");
myArray = (int*)malloc((localSize+ 1)*sizeof(int));
} else if(numProc > 2) {
if (myid == 0 || myid == numProc - 1){
//printf("Allocating array for boundary nodes usage\n");
myArray = (int*)malloc((localSize+ 1)*sizeof(int));
} else {
//printf("Allocating array for inner nodes usage\n");
myArray = (int*)malloc((localSize+ 2)*sizeof(int));
}
}
// Now we will fill the arrays with a dummy value 123. For the
// boundaries (ghosts) we will fill than with 80. Just to differe
// ntiate.
if(numProc == 1){
//printf("----------------------------------------\n");
//printf("Filling the serial array with values... \n");
for (i = 0; i<localSize; i++){
myArray[i] = 123;
}
} else if(numProc == 2) {
////printf("------------------------------------------------\n");
//printf("Filling array for two proc usage with values... \n");
for (i = 0; i<localSize; i++){
myArray[i] = 123;
}
// ghost.
myArray[localSize+1] = 8;
} else if(numProc > 2) {
if (myid == 0 || myid == numProc - 1){
//printf("--------------------------------------------------\n");
//printf("Filling boundary node arrays usage with values... \n");
for (i = 0; i<localSize; i++){
myArray[i] = 123;
}
// ghosts.
myArray[localSize+1] = 8;
} else {
//printf("--------------------------------------------------\n");
//printf("Filling inner node arrays usage with values... \n");
for (i = 0; i<localSize; i++){
myArray[i] = 123;
}
// ghosts.
myArray[localSize+1] = 8;
myArray[0] = 8;
}
}
// Now lets comunicate the ghosts with MPI_Sendrecv().
if(numProc == 1){
//printf("Serial usage, no ghost to comunicate \n");
} else if(numProc == 2) {
if (myid == 0){
//printf("Sending ghost value from proc %d to %d\n", myid, myid + 1);
MPI_Isend(&myArray[localSize+1],
1,
MPI_INT,
1,
12345,
MPI_COMM_WORLD,
&request);
} else if (myid == 1) {
//printf("Receiving ghost value from proc %d to %d\n", myid-1, myid);
MPI_Irecv(&myArray[localSize+1],
1,
MPI_INT,
0,
12345,
MPI_COMM_WORLD,
&request);
}
} else if(numProc > 2) {
if (myid == 0){
rightProc = myid + 1;
if (myid == 0){
//printf("-------------------------------\n");
//printf("Communicating Boundary ghosts !\n");
//printf("-------------------------------\n");
//printf("Sending ghost value from proc %d to %d\n", myid, myid + 1);
MPI_Isend(&myArray[localSize+1],
1,
MPI_INT,
rightProc,
12345,
MPI_COMM_WORLD,
&request);
} else if (myid == rightProc) {
//printf("Receiving ghost value from proc %d to %d\n", myid-1, myid);
MPI_Irecv(&myArray[localSize+1],
1,
MPI_INT,
0,
12345,
MPI_COMM_WORLD,
&request);
}
} else if (myid == numProc - 1) {
leftProc = myid - 1;
if (myid == numProc - 1){
//printf("-------------------------------\n");
//printf("Communicating Boundary ghosts !\n");
//printf("-------------------------------\n");
////printf("Sending ghost value from proc %d to %d\n", myid, myid + 1);
MPI_Isend(&myArray[localSize+1],
1,
MPI_INT,
leftProc,
12345,
MPI_COMM_WORLD,
&request);
} else if (myid == leftProc) {
rightProc = myid + 1;
//printf("Receiving ghost value from proc %d to %d\n", myid-1, myid);
MPI_Irecv(&myArray[localSize+1],
1,
MPI_INT,
rightProc,
12345,
MPI_COMM_WORLD,
&request);
}
} else {
//printf("-------------------------------\n");
//printf("Communicating Inner ghosts baby\n");
//printf("-------------------------------\n");
leftProc = myid - 1;
rightProc = myid + 1;
// Communicate tail ghost.
if (myid == leftProc) {
MPI_Isend(&myArray[localSize+1],
1,
MPI_INT,
rightProc,
12345,
MPI_COMM_WORLD,
&request);
} else if (myid == rightProc){
MPI_Irecv(&myArray[localSize+1],
1,
MPI_INT,
leftProc,
12345,
MPI_COMM_WORLD,
&request);
}
// Communicate head ghost.
if (myid == leftProc) {
MPI_Isend(&myArray[0],
1,
MPI_INT,
rightProc,
12345,
MPI_COMM_WORLD,
&request);
} else if (myid == rightProc){
MPI_Irecv(&myArray[0],
1,
MPI_INT,
leftProc,
12345,
MPI_COMM_WORLD,
&request);
}
}
}
// Now I Want to see if the ghosts are in place !.
if (myid == 0){
printf("The ghost value is: %d\n", myArray[localSize + 1]);
} else if (myid == numProc - 1){
printf("The ghost value is: %d\n", myArray[0]);
} else {
printf("The head ghost is: %d\n", myArray[0]);
printf("The tail ghost is: %d\n", myArray[localSize + 1]);
}
MPI_Finalize();
exit(0);
}
int WhichSize(int mpiId, int numProc, int tam){
double resto;
int tamLocal;
tamLocal = tam/numProc;
resto = tam - tamLocal*numProc;
if (mpiId < resto) tamLocal = tamLocal + 1;
return tamLocal;
}
感謝一個清潔的解決方案你們！
謝謝您的回覆！ –