HTML version of Scripted Foils prepared 11 November 1996

Foil 10 Collective Computation

From CPS615-Completion of MPI foilset and Application to Jacobi Iteration in 2D Delivered Lectures of CPS615 Basic Simulation Track for Computational Science -- 7 November 96. by Geoffrey C. Fox *
Secs 315.3
1 One can often perform computing during a collective communication
2 MPI_REDUCE performs reduction operation of type chosen from
  • maximum(value or value and location), minimum(value or value and location), sum, product, logical and/or/xor, bit-wise and/or/xor
  • e.g. operation labelled MPI_MAX stores in location result of processor rank the global maximum of original in each processor as in
  • call MPI_REDUCE(original,result,1,MPI_REAL,MPI_MAX,rank,comm,ierror)
  • One can also supply one's own reduction function
3 MPI_ALLREDUCE is as MPI_REDUCE but stores result in all -- not just one -- processors
4 MPI_SCAN performs reductions with result for processor r depending on data in processors 0 to r
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