MPI 发送接收

Question

#include "mpi.h"
#include <stdio.h>

int main(argc,argv)
int argc;
char *argv[];  {
int numtasks, rank, dest, source, rc, count, tag=1;
MPI_Status Stat;

MPI_Init(&argc,&argv);
MPI_Comm_size(MPI_COMM_WORLD, &numtasks);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);

if (rank%2 == 1) {
dest = (rank+1)%numtasks;
  source = (rank-1+numtasks)%numtasks;
  rc = MPI_Send(&outmsg, 1, MPI_CHAR, dest, tag, MPI_COMM_WORLD);
  rc = MPI_Recv(&inmsg, 1, MPI_CHAR, source, tag,MPICOMM_WORLD, &Stat);
}

else  {
  dest   = (rank-1+numtasks)%numtasks;
  source = (rank+1)%numtasks;
  rc = MPI_Recv(  &inmsg, 1, MPI_CHAR, source, tag, MPI_COMM_WORLD, &Stat );
  rc = MPI_Send( &outmsg, 1, MPI_CHAR, dest,   tag, MPI_COMM_WORLD );
}
rc = MPI_Get_count(&Stat, MPI_CHAR, &count);
printf("Task %d: Received %d char(s) from task %d with tag %d\n",
   rank, count, Stat.MPI_SOURCE, Stat.MPI_TAG);

MPI_Finalize();
}

结果：

Task 2: Received 1 char(s) from task 3 with a tag 1
Task 0: Received 1 char(s) from task 1 with a tag 1

为什么任务不能识别超过 2 个处理器？
我想在超过 2 个处理器上运行它。
我已经用环形模式更新了 mpi 程序，我认为 if(rank%2==1)

Answer 1

    #include "mpi.h"
#include <stdio.h>

int main (argc,argv)
int argc;
char *argv[]; {
int numtasks, rank, dest, source, rc, count, tag=1;
char inmsg, outmsg='x';
MPI_Status Stat;

MPI_Init (&argc, &argv);
MPI_Comm_size (MPI_COMM_WORLD, &numtasks);
MPI_Comm_rank (MPI_COMM_WORLD, &rank);

if (rank%2 == 0)
{
dest=(rank+1)%numtasks;
source=(rank+1)%numtasks;

rc = MPI_Send(&outmsg, 1, MPI_CHAR, source, tag, MPI_COMM_WORLD);
rc = MPI_Recv(&inmsg, 1, MPI_CHAR, source, tag, MPI_COMM_WORLD, &Stat);
}

else {
dest = (rank-1+numtasks)%numtasks;
source = (rank-1+numtasks)%numtasks;
rc = MPI_Recv (&inmsg, 1, MPI_CHAR, source, tag, MPI_COMM_WORLD, &Stat);
rc = MPI_Send (&outmsg, 1, MPI_CHAR, dest,  tag, MPI_COMM_WORLD);
}

{
rc = MPI_Get_count (&Stat, MPI_CHAR, &count);
printf ("Task %d: Received %d char(s) from task %d with a tag %d \n", rank, count, Stat.MPI_SOURCE, Stat.MPI_TAG);
}
MPI_Finalize();

Answer 2

count 和 Stat 仅在等级 0 和 1 上初始化，因此您基本上是在等级 2 和 3 上打印垃圾。

您的程序被硬编码为在 2 个任务上运行（它会在 1 个任务上崩溃，并在 3 个或更多任务上打印垃圾）。

我建议您使用MPI_Sendrecv() 进行调查，以便在一次 MPI 调用中发送和接收无死锁。

不确定您要达到的目标，请详细说明并更新您的问题。话虽如此，您可能会考虑使用环形模式，以便您的程序可以在任意数量的 MPI 任务上运行（例如，发送到 (rank+1)%numtasks 并从 (rank+numtasks-1)%numtasks 接收）

根据您编辑的问题，所有 MPI 等级的通信模式可以相同，如下所示

dest = (rank+1)%numtasks;
source = (rank-1+numtasks)%numtasks;
MPI_Sendrecv(&outmsg, 1, MPI_CHAR, dest, tag,
             &inmsg, 1, MPI_CHAR, source, tag,
             MPI_COMM_WORLD, &Stat);

Answer 3

可能会尝试更好地研究 MPI 机制：

不要犹豫，尝试代码，因为它将帮助您掌握 numtasks、rank-ID 的概念和相关的运行时操作.编译错误将在上面发布的代码中报告快捷方式。对于更多与 MPI 相关的实践和见解，还应该从各自的 MPI_<fun>()-calls 分配和评估 rc 返回码

问：为什么任务不能识别超过 2 个处理器？
答：它可以，但是@987654325 @code-blocks 不允许其他人产生任何可见的输出。

问：我想将 4197005 结果更改为 2、3 - 如何？
答：无法更改 MPI 报告的数字，但您可以更改代码的行为（见下文）并在可行的情况下制作更多关于谁在做什么做什么、何时、的输出实际发生的方式和在哪里。通过这种方式，我们学习理解 MPI 概念并验证实际的 MPI 代码执行。

欢迎询问更多。

#include "mpi.h"
#include <stdio.h>

int main( int   argc,
          char *argv[]
          ) {
   int        numtasks, rank, dest, source, rc, count, tag = 1;
   MPI_Status Stat;

   printf( "(INF) will MPI_Init()...\n" );

   MPI_Init( &argc, &argv );
   printf( "(INF)      MPI_Init() done.\n" );

   MPI_Comm_size( MPI_COMM_WORLD, &numtasks );
   MPI_Comm_rank( MPI_COMM_WORLD, &rank );
   printf( "(INF) I am MPI[%d of %d]\n",
            rank,
            numtasks
            );

   if ( rank  == 0 ) {
        dest   = 1;
        source = 1;
        rc     = MPI_Send( &outmsg, 1, MPI_CHAR, dest,   tag, MPI_COMM_WORLD );
        rc     = MPI_Recv(  &inmsg, 1, MPI_CHAR, source, tag, MPI_COMM_WORLD, &Stat );
        printf( "(INF) I am MPI[%d of %d] having just completed an MPI_Send( ->MPI[%d] ) + MPI_Recv( <-MPI[%d] ). Last RetCODE == %d.\n",
                 rank,
                 numtasks,
                 dest,
                 source,
                 rc
                 );
   }
   else
   if ( rank  == 1 ) {
        dest   = 0;
        source = 0;
        rc     = MPI_Recv(  &inmsg, 1, MPI_CHAR, source, tag, MPI_COMM_WORLD, &Stat );
        rc     = MPI_Send( &outmsg, 1, MPI_CHAR, dest,   tag, MPI_COMM_WORLD );
        printf( "(INF) I am MPI[%d of %d] having just completed an MPI_Recv( <-MPI[%d] ) + MPI_Send( ->MPI[%d] ). Last RetCODE == %d\n",
                 rank,
                 numtasks,
                 source,
                 dest,
                 rc
                 );
   }
   else
   if ( rank  == 2 ) {
        dest   = 3;
        source = 3;
        rc     = MPI_Recv(  &inmsg, 1, MPI_CHAR, source, tag, MPI_COMM_WORLD, &Stat );
        rc     = MPI_Send( &outmsg, 1, MPI_CHAR, dest,   tag, MPI_COMM_WORLD );
        printf( "(INF) I am MPI[%d of %d] having just completed an MPI_Recv( <-MPI[%d] ) + MPI_Send( ->MPI[%d] ). Last RetCODE == %d\n",
                 rank,
                 numtasks,
                 source,
                 dest,
                 rc
                 );
   }
   else
   if ( rank  == 3 ) {
        dest   = 2;
        source = 2;
        rc     = MPI_Send( &outmsg, 1, MPI_CHAR, dest,   tag, MPI_COMM_WORLD );
        rc     = MPI_Recv(  &inmsg, 1, MPI_CHAR, source, tag, MPI_COMM_WORLD, &Stat );
        printf( "(INF) I am MPI[%d of %d] having just completed an MPI_Send( ->MPI[%d] ) + MPI_Recv( <-MPI[%d] ). Last RetCODE == %d\n",
                 rank,
                 numtasks,
                 dest,
                 source,
                 rc
                 );
   }
   else {
        printf( "(INF) I am MPI[%d of %d] will NOP...\n",
                 rank,
                 numtasks
                 );

   }
   rc = MPI_Get_count( &Stat, MPI_CHAR, &count );
   /*
            Task  3: Received  1 char(s) from task 4197005 with a tag 0
            Task  2: Received  1 char(s) from task 4197005 with a tag 0
            Task  1: Received  1 char(s) from task 0       with a tag 1
            Task  0: Received  1 char(s) from task 1       with a tag 1                        */
   printf( "Task %d: Received %d char(s) from task %d with tag %d\n",
            rank,
            count,
            Stat.MPI_SOURCE,
            Stat.MPI_TAG
            );

   printf( "(INF) I am MPI[%d of %d]: will MPI_Finalize()...\n",
            rank,
            numtasks
            );
   MPI_Finalize();

   printf( "(INF) I am MPI[%d of %d]:      MPI_Finalize() done, will exit()...\n",
            rank,
            numtasks
            );
}