pympi-users

[Pympi-users] Converting Communicators to and from fortran modules

[Mike S. <st...@gm...>]

Hi All,

I think this is a question that Pat will have to answer, but perhaps
some of you have seen it or have the same question:

I have a fortran module wrapped using f2py.  The module in this case
is MPH, the Multi Program Handshaking utility.  It is mostly used to
help setup the MPI environment for multiple mpi executables.  I need
to call some of these library routines, some of which are supposed to
return communicators.

However, the values that are supposed to be communicators show up as
python integers.  While these may be completely usable for passing to
extension codes they are worthless for actually doing anything in
python.

I've noticed that pyMPI is capable of handling the opposite though, if
I pass a pyMPI communicator object to wrapped code expecting a fortran
communicator the wrapped code still works.

Is there any way for me to take a communicator from fortran and
"promote" it to a python communicator so that I can get information
like the size of the communicator?

If there is no pyMPI way to do it at the moment, how else could I get
this information?
Write wrappers around a few MPI functions like MPI_Comm_rank and
MPI_Comm_size in fortran or c?  Then just call those with the fortran
communicator to get the info i need?

Thanks in advance,

~Mike

Re: [Pympi-users] Converting Communicators to and from fortran modules

[Pat M. <pat...@ll...>]

> However, the values that are supposed to be communicators show up as
> python integers.  While these may be completely usable for passing to
> extension codes they are worthless for actually doing anything in
> python.

Alas, more underdocumented features of pyMPI come to light.

That functionality exists in the mpi module as the "communicator"
type object.  This is the same object used to create and
expose the internal MPI communicators.

 >>> h = myfortran.getHandle()
 >>> print h
141259752
 >>> c = mpi.communicator(h)
<communicator object at 0xb7532500>
 >>> c.rank
14

Give it a bogus handle and you may segfault in LAM or
get an "Invalid communicator" exception in MPICH.

* * * * *

For a bit more info, see the help file [below].  The idea
of persistence of communicators is just to help Python
decide whether to delete the MPI communicator when the
Python communicator is freed.  The default is to NOT
delete the communicator (that is, do NOT call MPI_Comm_free()
on it).

 >>> import mpi
 >>> help(mpi.communicator)
Help on class communicator in module __builtin__:

class communicator(object)
  |  Create communicator object from communicator or handle
  |
  |  communicator(communicator=COMM_NULL,  # Communicator to wrap
  |               persistent=1)            # If false, release MPI comm
  |   --> <communicator instance>
  |
  |  Build instance of a communicator interface.  The persistent flag (by
  |  default on) means that Python WILL NOT release the MPI communicator on
  |  delete.
  |
  |  >>> null = communicator()      # returns a (not the) NULL communicator
  |  >>> c = communicator(WORLD)    # a new interface to WORLD communicator
  |  >>> my_world = communicator(handle,0) # Python version of handle
  |                                        # MPI_Comm_free() will be called
...



-- 
Pat Miller | (925) 423-0309 | http://www.llnl.gov/CASC/people/pmiller

What hunger is in relation to food, zest is in relation to life.  -- Bertrand
Russell, philosopher, mathematician, and author (1872-1970)