D.10. SAMPLE MPC SESSIONS

D.9. HOW TO DEBUG MPC APPLICATIONS

Debugging mpC applications is not yet an easy task, but it is much simpler than debugging MPI applications due to the absence of indeterminism. There are at least three levels of debugging.

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At the middle level, we suggest to include printfs and barriers in the C code generated by mpcc. This is a more complex procedure, as the user needs to understand the logics of the generated C code and RTS kernel calls.

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To debug an application, which hangs without error messages, first use MPI utilities to find all the processes that do not respond (for example, LAM MPI’s mpitask). They often “die” due to a simple “C error”, such as an un-initialized variable, dividing by zero and so on. The trace is useful for finding the point in the code where the error occurs. When all processes are alive, the MPI utility will show operations where all the processes are blocked.