[Beowulf] MPI_reduce roundoff question.

[Stern, Michael (NIH/NIA/IRP) [E]]

Hi,
 I'm a user of the NIH Biowulf cluster, and I'm looking for an answer to a question that the staff here didn't know.  I'm aware that roundoff error in MPI_reduce can vary with the number of processors, because the order of summation depends on the communication path.  My question is whether the order of summation can differ among different calls to MPI_reduce within the same program (with the same number of processors during a single run).