Pickling of MPI Comm

[Feng Yu]

While a general MPI communicator is not serializable, the predefined constant communicators like COMM_SELF, COMM_NULL, and COMM_WORLD can probably be serialized and deserialized in a well defined manner.

We are reusing a MPI4py based python module with distributed, which sends pickled objects over the network for distributed computing. In our setup the MPI'd module is running strictly single rank with MPI.COMM_SELF to solve a small problem.

I understand this is not a correct way of using MPI, but allowing a picklable COMM_SELF seems to be the cleanest solution given these constraints.

[Lisandro Dalcin]

I'm afraid I'm not ready to implement these features yet. While I acknowledge the case of MPI.Comm is rather trivial, other MPI classes would require much more care. For example, think of MPI datatypes, we would need to handle the predefined ones, but as MPI provides datatype decode functionality, we should be able to serialize reconstruct back any MPI datatype. The case of MPI.Op is similar, we should need to support pickling for a bunch of predefined operations, but I think for user-defined operations we cannot do much (unless the routine implemented the operation is implemented in Python and that routine is pickleable). At the risk of upsetting you, I have to reject your request at this time.

However, you can workaround things easily on your side by using the copyreg module. Here you have an example that would allow you to pickle/unpickle the predefined communicators, and you can easily extend the support to other predefined handles. You just need to make sure to put this code in a module, and import it in both client and server sides before attempting to transfer communicators over the network.

try:
    import copyreg
except ImportError:  # Python 2
    import copy_reg as copyreg
from mpi4py import MPI


def unpickle(name):
    return getattr(MPI, name)


def comm_pickle(obj):
    if obj == MPI.COMM_NULL:
        return unpickle, ('COMM_NULL',)
    if obj == MPI.COMM_SELF:
        return unpickle, ('COMM_SELF',)
    if obj == MPI.COMM_WORLD:
        return unpickle, ('COMM_WORLD',)
    raise TypeError("cannot pickle object")


copyreg.pickle(MPI.Comm, comm_pickle, unpickle)
copyreg.pickle(MPI.Intracomm, comm_pickle, unpickle)


if __name__ == '__main__':
    import pickle
    for obj in (MPI.COMM_NULL,
                MPI.COMM_SELF,
                MPI.COMM_WORLD):
        assert pickle.loads(pickle.dumps(obj)) == obj