Question

These are some general questions I am facing while designing the error handling for an algorithm that is supposed to run in parallel using MPI (in C++):

• Do Exceptions work inside code that is executed in parallel? Is the behaviour defined?
• How do they work? Does that differ for different implementations?
• Is it good practice – or should I use return codes?

Answer 1

Exceptions work the same in an MPI code as with a serial code, but you have to be extremely careful if it is possible for the exception is not raised on all processes in a communicator or you can easily end up with deadlock.

MPI_Barrier(comm);            /* Or any synchronous call */
if (!rank) throw Exception("early exit on rank=0");
MPI_Barrier(comm);            /* rank>0 deadlocks here because rank=0 exited early */

All error handling methods have this problem, it is difficult to recover from errors that do not occur consistently across a communicator. In the case above, you could perform an MPI_Allreduce so that all ranks choose the same branch.

My preference is for calling error handlers and propagating them up the stack since this tends to give me tho most useful/verbose error message and it’s easy to catch with a breakpoint (or the error handler can attach a debugger to itself and send it to your workstation in an xterm).