MPI Barrier C++

Question

Dear all,
I want to use MPI (MPICH2) on windows. I write this command:

MPI_Barrier(MPI_COMM_WORLD);  

And I expect it blocks all Processors until all group members have called it. But it is not happen. I add a schematic of my code:

int a;  
if(myrank == RootProc)  
   a = 4;  
MPI_Barrier(MPI_COMM_WORLD);  
cout << "My Rank = " << myrank << "\ta = " << a << endl;  

(With 2 processor:) Root processor (0) acts correctly, but processor with rank 1 doesn't know the a variable, so it display -858993460 instead of 4.

Can any one help me?
Regards

Answer 1

Variable a is not initialized - it is possible that is why it displays that number. In MPI, variable a is duplicated between the processes - so there are two values for a, one of which is uninitialized. You want to write:

int a = 4;
if (myrank == RootProc)
...

Or, alternatively, do an MPI_send in the Root (id 0), and an MPI_recv in the slave (id 1) so the value in the root is also set in the slave.

Note: that code triggers a small alarm in my head, so I need to check something and I'll edit this with more info. Until then though, the uninitialized value is most certainly a problem for you. Ok I've checked the facts - your code was not properly indented and I missed the missing {}. The barrier looks fine now, although the snippet you posted does not do too much, and is not a very good example of a barrier because the slave enters it directly, whereas the root will set the value of the variable to 4 and then enter it. To test that it actually works, you probably want some sort of a sleep mechanism in one of the processes - that will yield (hope it's the correct term) the other process as well, preventing it from printing the cout until the sleep is over.

Answer 2

You're only assigning a in process 0. MPI doesn't share memory, so if you want the a in process 1 to get the value of 4, you need to call MPI_Send from process 0 and MPI_Recv from process 1.

Answer 3

Blocking is not enough, you have to send data to other processes (memory in not shared between processes).

To share data across ALL processes use:

int MPI_Bcast(void* buffer, int count, MPI_Datatype datatype, int root, MPI_Comm comm )

so in your case:

MPI_Bcast(&a, 1, MPI_INT, 0, MPI_COMM_WORLD);

here you send one integer pointed by &a form process 0 to all other. //MPI_Bcast is sender for root process and receiver for non-root processes

You can also send some data to specyfic process by:

int MPI_Send( void *buf, int count, MPI_Datatype datatype, int dest, 
              int tag, MPI_Comm comm )

and then receive by:

int MPI_Recv(void* buf, int count, MPI_Datatype datatype, int source, int tag, MPI_Comm comm, MPI_Status *status)