Java MPI broadcast

Question

Just getting to grips with parallel programming using the Java interface for MPI. Just wondering if someone can explain very simply how the broadcast works?

I have the following:

if (me ==0) { // This is the master process
   int bvalue = 4;
   MPI.COMM_WORLD.Bcast(bvalue, 0, 4, MPI.INT, 0);
}
else { // The worker processes
   MPI.COMM_WORLD.Bcast(bvalue, 0, 4, MPI.INT, 0);
}

So I know the worker process has to call the bcast to receive the bvalue.. How would I go about using this value in the workers section?

If I do:

int workerb = MPI.COMM_WORLD.Bcast(bvalue, 0, 4, MPI.INT, 0);

I get an incompatible type error, void can't be converted into int.

Any help would be greatly appreciated. Thanks, Mike

Eric · Accepted Answer · 2015-04-27 14:33:39Z

1

I believe you might be getting the parameters wrong here. The method call to Bcast() has the following method signature (taken from here):

public void Bcast(java.lang.Object buf, int offset, int count, Datatype type, int root)

The first parameter usually describes an array of something (in this case maybe an array of integers). The second parameter describes the offset into this array from which the broadcast would start. The third parameter, count, describes how many elements from the offset to send. And the final parameter describes the rank of the sender (0 is the master node).

You are getting that error because the Bcast() method call does not return anything (returns void). Also, I believe that the call to Bcast is a blocking call, so you could essentially rewrite your code above to:

int[] bvalue = new int[1];

if (me == 0){ //this is the master process 
     bvalue[0] = 4;
}

//the master node will broadcast the value '4' and
//workers will block here waiting for the broadcast 
//to complete
MPI.COMM_WORLD.Bcast(bvalue, 0, 1, MPI.INT, 0);

//output the contents of bvalue
System.out.println("bvalue is " + bvalue[0]);

This should accomplish the behavior you are expecting I believe. Hope this helps..

edited Apr 27, 2015 at 14:33

answered Apr 27, 2015 at 12:28

Eric

614 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Wesley Bland Over a year ago

It's been a while since I've dusted off my Java, but I believe you need to allocate the array on all processes since the master node isn't the only one who will be using the array bvalue.

Eric Over a year ago

Oops! overlooked that...thanks! I updated my answer to reflect that

MikeS Over a year ago

Thank you very much for your comment! It explains alot! I'm still a little mythed about where the Bcast goes though. Within the if (me == 0) block, I have the Bcast there and in the worker block, I have the println(bvalue) however, when I run the program, it just hangs during the master block when it reaches the Bcast. Any thoughts? Thanks again.

Eric Over a year ago

You have to make sure that both master and worker processes are all calling the Bcast() method otherwise you would see the program hang. For example, you could either do something similar to the example I posted above or just have another Bcast call in the worker block like you had before. Personally, having just one MPI method call makes my debugging life easier :)

Collectives™ on Stack Overflow

Java MPI broadcast

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related