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Subject: Re: [OMPI users] [Open MPI] #3351: JAVA scatter error
From: Siegmar Gross (Siegmar.Gross_at_[hidden])
Date: 2012-12-15 10:46:06


> #3351: JAVA scatter error
> ---------------------+-----------------------------
> Reporter: rhc | Owner: jsquyres
> Type: defect | Status: closed
> Priority: critical | Milestone: Open MPI 1.7.1
> Version: trunk | Resolution: invalid
> Keywords: |
> ---------------------+-----------------------------
> Changes (by jsquyres):
> * status: new => closed
> * resolution: => invalid
> Comment:
> I do not believe that the sample code provided is a valid MPI program, for
> two reasons (disclaimer: I do ''not'' know Java -- I'm just reading the
> code and making some assumptions about Java):
> 1. The datatypes passed to Scatter are not valid MPI datatypes
> (MPI.OBJECT). You need to construct a datatype that is specific to the
> !MyData class, just like you would in C/C++. I think that this is the
> first error that you are seeing (i.e., that OMPI is trying to treat
> MPI.OBJECT as an MPI Datatype object, and failing (and therefore throwing
> an !ClassCastException exception).

Perhaps you are right and my small example program ist not a valid MPI
program. The problem is that I couldn't find any good documentation or
example programs how to write a program which uses a structured data
type. Therefore I sticked to the mpiJava specification which states
for derived datatypes in chapter 3.12 that the effect for MPI_Type_struct
can be achieved by using MPI.OBJECT as the buffer type and relying on
Java object serialization. "dataItem" is a serializable Java object and
I used MPI.OBJECT as buffer type. How can I create a valid MPI datatype
MPI.OBJECT so that I get a working example program?

    MPI.COMM_WORLD.Scatter (dataItem, 0, 1, MPI.OBJECT,
                            objBuffer, 0, 1, MPI.OBJECT, 0);

> 1. It looks like you're trying to Scatter a single object to N peers.
> That's invalid MPI -- you need to scatter (N*M) objects to N peers, where
> M is a positive integer value (e.g., 1 or 2). Are you trying to
> broadcast?

It is the very first version of the program where I scatter one object
to the process itself (at this point it is not the normal application
area for scatter, but should nevertheless work). I didn't continue due
to the error. I get the same error when I broadcast my data item.

tyr java 116 mpiexec -np 1 java -cp $DIRPREFIX_LOCAL/mpi_classfiles \
Exception in thread "main" java.lang.ClassCastException: MyData cannot
  be cast to [Ljava.lang.Object;
        at mpi.Intracomm.copyBuffer(
        at mpi.Intracomm.Scatter(
        at ObjectScatterMain.main(

"Broadcast" works if I have only a root process and it fails when I have
one more process.

tyr java 117 mpiexec -np 1 java -cp $DIRPREFIX_LOCAL/mpi_classfiles \

Process 0 running on
  Age: 35
  Name: Smith
  Salary: 2545.75

tyr java 118 mpiexec -np 2 java -cp $DIRPREFIX_LOCAL/mpi_classfiles \
Exception in thread "main" java.lang.ClassCastException: MyData cannot
  be cast to [Ljava.lang.Object;
        at mpi.Comm.Object_Serialize(
        at mpi.Comm.Send(
        at mpi.Intracomm.Bcast(
        at ObjectBroadcastMain.main(

> Short version -- I don't think this bug is valid. I'm closing the ticket.

If I misunderstood the mpiJava specification and I must create a special
MPI object from my Java object: How do I create it? Thank you very much
for any help in advance.

Kind regards