Reductions

Reduction over PencilArrays using Julia functions such as minimum, maximum, sum, any or all are performed on the global data. This involves first a local reduction over each process, followed by a global reduction of a scalar quantity using MPI.Allreduce.

For example:

using MPI
using PencilArrays

MPI.Init()

comm = MPI.COMM_WORLD
nprocs = MPI.Comm_size(comm)
rank = MPI.Comm_rank(comm)
id = rank + 1

pen = Pencil((16, 32, 14), comm)
u = PencilArray{Int}(undef, pen)
fill!(u, 2 * id)

minimum(u)  # = 2
maximum(u)  # = 2 * nprocs

minimum(abs2, u)  # = 4
maximum(abs2, u)  # = (2 * nprocs)^2

all(>(0), u)   # true
all(==(2), u)  # false if nprocs > 1
any(==(2), u)  # true