Send objects with MPJ express

问题:

I'm new to parallel programming and I want to do it in java. I am wondering if it is possible to send and receive more complex ojects via MPI. I'm using MPJ express. However whenever I want to send a object I get a ClassCastException.

MPI.Init(args);
myrank = MPI.COMM_WORLD.Rank();
numprocs = MPI.COMM_WORLD.Size();
Vector<CustomClass> chr = new Vector<CustomClass>();
if (myrank == 0 ) { //am I the master?
    for (int i = 1; i < numprocs; i++) {
      MPI.COMM_WORLD.Send(chr, 0, chr.size(), MPI.OBJECT, i, 99); //Here's where the 
                                                                  exception occurs
    }
}
else {             
    Vector<BasicRegion> chr_received = new Vector<BasicRegion>();
    MPI.COMM_WORLD.Recv(chr_received, 0, 1, MPI.OBJECT, 0, 99 );
}

Exception:

mpi.MPIException: mpi.MPIException: java.lang.ClassCastException: java.util.Vector cannot be cast to [Ljava.lang.Object;

So my questions are: - is it possible to send/receive more complex objects with MPJ Express? - if so: what am I doing wrong?

回答1:

I am new to MPJ express as well, but seems the enclosing object needs to be primitive type - array of something. (like as you do with the C/C++ implementation in OpenMPI).

This kind of code worked for me well:

    Node t[] = new Node[4];
        ...
        count[0] = t.length;
        MPI.COMM_WORLD.Send(count, 0, 1, MPI.INT, 1, 98);
        MPI.COMM_WORLD.Send(t, 0, t.length, MPI.OBJECT, 1, 99);
    } else if( myRank == 1 ) {
        int count[] = new int[1];
        MPI.COMM_WORLD.Recv( count, 0, 1, MPI.INT, 0, 98);
        Status mps = MPI.COMM_WORLD.Recv( t, 0, count[0], MPI.OBJECT, 0, 99 );
        ...

And of course, you have to have that custom class implementing Serializable interface.

回答2:

You want to serialize it prior to sending.

import mpi.*;

import java.io.*;
import java.nio.ByteBuffer;

public class MPITest
{
    public static void main(String[] args)
    {
        MPI.Init(args);
        int me = MPI.COMM_WORLD.Rank();
        int tasks = MPI.COMM_WORLD.Size();

        MPI.COMM_WORLD.Barrier();

        if(me == 0)
        {
            Cat cat = new Cat("Tom", 15);
            cat.Speak();

            ByteBuffer byteBuff = ByteBuffer.allocateDirect(2000 + MPI.SEND_OVERHEAD);
            MPI.Buffer_attach(byteBuff);

            try
            {
                ByteArrayOutputStream bos = new ByteArrayOutputStream();
                ObjectOutput out = null;
                out = new ObjectOutputStream(bos);
                out.writeObject(cat);
                byte[] bytes = bos.toByteArray();

                System.out.println("Serialized to " + bytes.length);

                MPI.COMM_WORLD.Isend(bytes, 0, bytes.length, MPI.BYTE, 1, 0);
            }
            catch(IOException ex)
            {

            }
        }
        else
        {
            byte[] bytes = new byte[2000];
            Cat recv = null;
            MPI.COMM_WORLD.Recv(bytes, 0, 2000, MPI.BYTE, MPI.ANY_SOURCE, 0);

            ByteArrayInputStream bis = new ByteArrayInputStream(bytes);
            ObjectInput in = null;
            try
            {
                in = new ObjectInputStream(bis);
                Object obj = in.readObject();
                recv = (Cat)obj;

                recv.Speak();
            }
            catch(IOException ex)
            {

            }
            catch(ClassNotFoundException cnf)
            {

            }
        }

        MPI.COMM_WORLD.Barrier();
        MPI.Finalize();
    }
} 

This works however you may want to use Externalization and do it manually to avoid some of the extra garbage that the serialize routine will send.

HTH Brian

回答3:

import mpi.*;

/**
 * Compile: javac -cp $MPJ_HOME/lib/mpj.jar:. ObjSend.java 
 * Execute: mpjrun.sh -np 2 -dport 11000 ObjSend
 */

public class ObjSend {
    public static void main(String[] args) throws Exception {

        int peer ; 

        MPI.Init(args);

        int rank = MPI.COMM_WORLD.Rank() ;
        int size = MPI.COMM_WORLD.Size() ; 

        int tag = 100 ; 

        if(rank == 0) {

            String [] smsg = new String[1] ; 
            smsg[0] = "Hi from proc 0" ; 
            peer = 1 ; 
            MPI.COMM_WORLD.Send(smsg, 0, smsg.length, MPI.OBJECT, 
                                                        peer, tag);
            System.out.println("proc <"+rank+"> sent a msg to "+
                               "proc <"+peer+">") ; 

        } else if(rank == 1) {

            String[] rmsg = new String[1] ; 
            peer = 0 ; 
            MPI.COMM_WORLD.Recv(rmsg, 0, rmsg.length , MPI.OBJECT, 
                                                         peer, tag);
            System.out.println("proc <"+rank+"> received a msg from "+
                               "proc <"+peer+">") ; 
            System.out.println("proc <"+rank+"> received the following "+
                               "message: \""+rmsg[0]+"\"") ; 

        } 

        MPI.Finalize();

    }   
}