MPI 每个rank依次往下一个rank发送消息的循环
最后更新于：2022-04-01 09:51:24
https://www.sharcnet.ca/help/index.php/Getting_Started_with_MPI
~~~
#include <stdio.h>
#include <mpi.h>
#define BUFMAX 81
int main(int argc, char *argv[])
{
char outbuf[BUFMAX], inbuf[BUFMAX];
int rank, size;
int sendto, recvfrom;
MPI_Status status;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &size);
sprintf(outbuf, "Hello, world! from process %d of %d", rank, size);
sendto = (rank + 1) % size;
recvfrom = ((rank + size) - 1) % size;
MPI_Send(outbuf, BUFMAX, MPI_CHAR, sendto, 0, MPI_COMM_WORLD);
MPI_Recv(inbuf, BUFMAX, MPI_CHAR, recvfrom, 0, MPI_COMM_WORLD, &status);
printf("[P_%d] process %d said: \"%s\"]\n", rank, recvfrom, inbuf);
MPI_Finalize();
return(0);
}
~~~
上面的实现之所有没有死锁是依赖于数据传输的中间buffer，也就是写的时候直接写到buffer，不需要等到接收端的回应，代码是可以正确运行的，但是他的实现是并不是完全依赖MPI的标准，下面给出一个MPI 安全的例子
~~~
#include <stdio.h>
#include <mpi.h>
#define BUFMAX 81
int main(int argc, char *argv[])
{
char outbuf[BUFMAX], inbuf[BUFMAX];
int rank, size;
int sendto, recvfrom;
MPI_Status status;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &size);
sprintf(outbuf, "Hello, world! from process %d of %d", rank, size);
sendto = (rank + 1) % size;
recvfrom = ((rank + size) - 1) % size;
if (!(rank % 2))
{
MPI_Send(outbuf, BUFMAX, MPI_CHAR, sendto, 0, MPI_COMM_WORLD);
MPI_Recv(inbuf, BUFMAX, MPI_CHAR, recvfrom, 0, MPI_COMM_WORLD, &status);
}
else
{
MPI_Recv(inbuf, BUFMAX, MPI_CHAR, recvfrom, 0, MPI_COMM_WORLD, &status);
MPI_Send(outbuf, BUFMAX, MPI_CHAR, sendto, 0, MPI_COMM_WORLD);
}
printf("[P_%d] process %d said: \"%s\"]\n", rank, recvfrom, inbuf);
MPI_Finalize();
return(0);
}
~~~
这个例子和上面的最大的不同就是根据rank分成两组，这两组的接收和发送的顺序是刚好相反的，这样如果没有buffer，一样可以正确的得到结果，因此是安全的。