Hi Edgar,
cid's are in fact not recycled in the block algorithm. The problem
is that comm_free is not collective, so you can not make any
assumptions whether other procs have also released that communicator.
well, that's not quite correct. The MPI standard says the following
about MPI_Comm_free (MPI 2.1, p 201, l 43): "This collective operation
marks the communication object for deallocation.". So MPI_Comm_free is
collective which makes the prescribed problem(s) easier so solve.
However, I admit that there exist an advice to implementors that
anticipates a local implementation. Personally I find this advice
rather strange and (if nobody can give a good reason for it) would
encourage its removal...
Best regards,
Christian
