Hi I shortend this email so that you get earlier to my comments.
> > In my opinion Datatype.Vector must set the size of the > > base datatype as extent of the vector and not the true extent, because > > MPI-Java doesn't provide a function to resize a datatype. > > No, I think Datatype.Vector is doing the Right Thing in that it acts > just like MPI_Type_vector. We do want these to be *bindings*, after > all -- meaning that they should be pretty much a 1:1 mapping to the > C bindings. > > I think the real shortcoming is that there is no Datatype.Resized > function. That can be fixed. Are you sure? That would at least solve one problem. > > We should forget > > ObjectScatterMain.java for the moment and concentrate on > > ObjectBroadcastMain.java, which I have sent three days ago to the list, > > because it has the same problem. > > > > 1) ColumnSendRecvMain.java > > > > I create a 2D-matrix with (Java books would use "double[][] matrix" > > which is the same in my opinion, but I like C notation) > > > > double matrix[][] = new double[P][Q]; > > I noticed that if I used [][] in my version of the Scatter program, > I got random results. But if I used [] and did my own offset > indexing, it worked. I think if you want a 2D-matrix you should use a Java matrix and not a special one with your own offset indexing. In my opinion that is something a C programmer can/would do (I'm a C programmer myself with a little Java knowledge), but the benefit of Java is that the programmer should not know about addresses, memory layouts and similar things. Now I sound like my colleagues who always claim that my Java programs look more like C programs than Java programs :-(. I know nothing about the memory layout of a Java matrix or if the layout is stable during the lifetime of the object, but I think that the Java interface should deal with all these things if that is possible. I suppose that Open MPI will not succeed in the Java world if it requires "special" matrices and a special offset indexing. Perhaps some members of this list have very good Java knowledge or even know the exact layout of Java matrices so that Datatype.Vector can build a Java column vector from a Java matrix which even contains valid values. > If double[][] is a fundamentally different type (and storage format) > than double[], what is MPI to do? How can it tell the difference? > > > It is easy to see that process 1 doesn't get column 0. Your > > suggestion to allocate enough memory for a matrix (without defining > > a matrix) and doing all index computations yourself is in my opinion > > not applicable for a "normal" Java programmer (it's even hard for > > most C programmers :-) ). Hopefully you have an idea how to solve > > this problem so that all processes receive correct column values. > > I'm afraid I don't, other than defining your own class which > allocates memory contiguously, but overrides [] and [][] > (I'm *assuming* you can do that in Java...?). Does anybody else in this list know how it can be done? > > 2) ObjectBroadcastMain.java > > > > As I said above, it is my understanding, that I can send a Java object > > when I use MPI.OBJECT and that the MPI implementation must perform all > > necessary tasks. > > Remember: there is no standard for MPI and Java. So there is no > "must". :-) I know and I'm grateful that you try nevertheless to offer a Java interface. Hopefully you will not misunderstand my "must". It wasn't complaining, but trying to express that a "normal" Java user would expect that he can implement an MPI program without special knowledge about data layouts. > This is one research implementation that was created. We can update > it and try to make it better, but we're somewhat crafting the rules > as we go along here. > > (BTW, if we continue detailed discussions about implementation, > this conversation should probably move to the devel list...) > > > Your interface for derived datatypes provides only > > methods for discontiguous data and no method to create an MPI.OBJECT, > > so that I have no idea what I would have to do to create one. The > > object must be serializable so that you get the same values in a > > heterogeneous environment. > > > > tyr java 146 mpiexec -np 2 java ObjectBroadcastMain > > Exception in thread "main" java.lang.ClassCastException: > > MyData cannot be cast to [Ljava.lang.Object; > > at mpi.Comm.Object_Serialize(Comm.java:207) > > at mpi.Comm.Send(Comm.java:292) > > at mpi.Intracomm.Bcast(Intracomm.java:202) > > at ObjectBroadcastMain.main(ObjectBroadcastMain.java:44) > > ... > > After rooting around in the code a bit, I think I understand this > stack trace a bit better now.. > > The code line in question is in the Object_Serialize method, where > it calls: > > Object buf_els [] = (Object[])buf; > > So it's trying to cast an (Object) to an (Object[]). Apparently, > this works for intrinsic Java types (e.g., int). But it doesn't > seem to work with your own class. > > Again, here's my disclaimer that I'm not a Java guy... :-) But does > this mean you need to define an operator[] method on your class, and > that would allow this casting to work? (not that I'm sure what this > method would need to *do*, but this is a first step...) Alternatively the buffer parameter type could be changed from "Object" to "Object[]". Then everybody would know that an array is expected (even for a single value). However, I don't know if that has a consequence which breaks other things. Is a different parameter type possible or out-of any question? Kind regards Siegmar
