On Tue, 2007-07-10 at 21:13 -0400, Stern, Michael (NIH/NIA/IRP) [E] wrote: > 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).
An interesting question. Technically the answer is yes it can according to the letter of the MPI spec although the spec also advises vendors that it shouldn't differ regardless of the layout of the processes. In practice I'd be very surprised if two calls to MPI_Reduce on the same communicator with the same values would produce different results although you may find that communicators with the same size but different process layout within the same job gave you slightly different answers if shared memory optimisations have been employed. I'm aware this has been a issue for people in the past and I've seen procurement contracts which state that any results obtained by the computer must be 100% repeatable which in effect means the answer to your question is no, they cannot differ. Ashley, _______________________________________________ Beowulf mailing list, Beowulf@beowulf.org To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
