Add Scatterv example based on #469 (#470)

Author: simonbyrne

docs/examples/06-scatterv.jl

@@ -0,0 +1,78 @@
+# This example shows how to use MPI.Scatterv! and MPI.Gatherv!
+# roughly based on the example from
+# https://stackoverflow.com/a/36082684/392585
+
+using MPI
+
+"""
+    split_count(N::Integer, n::Integer)
+
+Return a vector of `n` integers which are approximately equally sized and sum to `N`.
+"""
+function split_count(N::Integer, n::Integer)
+    q,r = divrem(N, n)
+    return [i <= r ? q+1 : q for i = 1:n]
+end
+
+
+MPI.Init()
+
+comm = MPI.COMM_WORLD
+rank = MPI.Comm_rank(comm)
+comm_size = MPI.Comm_size(comm)
+
+root = 0
+
+if rank == root
+    M, N = 4, 7
+
+    test = Float64[i for i = 1:M, j = 1:N]
+    output = similar(test)
+    
+    # Julia arrays are stored in column-major order, so we need to split along the last dimension
+    # dimension
+    M_counts = [M for i = 1:comm_size]
+    N_counts = split_count(N, comm_size)
+
+    # store sizes in 2 * comm_size Array
+    sizes = vcat(M_counts', N_counts')
+
+    # store number of values to send to each rank in comm_size length Vector
+    counts = vec(prod(sizes, dims=1))
+
+    test_vbuf = VBuffer(test, counts) # VBuffer for scatter
+    output_vbuf = VBuffer(output, counts) # VBuffer for gather
+else
+    # these variables can be set to `nothing` on non-root processes
+    sizes = nothing
+    output_vbuf = test_vbuf = VBuffer(nothing)
+end
+
+if rank == root
+    println("Original matrix")
+    println("================")
+    @show test sizes counts
+    println()
+    println("Each rank")
+    println("================")
+end 
+MPI.Barrier(comm)
+
+local_M, local_N = MPI.Scatter!(sizes, zeros(Int, 2), root, comm)
+local_test = MPI.Scatterv!(test_vbuf, zeros(Float64, local_M, local_N), root, comm)
+
+for i = 0:comm_size-1
+    if rank == i
+        @show rank local_test
+    end
+    MPI.Barrier(comm)
+end
+
+MPI.Gatherv!(local_test, output_vbuf, root, comm)
+
+if rank == root
+    println()
+    println("Final matrix")
+    println("================")
+    @show output
+end 

docs/make.jl

@@ -7,7 +7,8 @@ EXAMPLES = [
     "Broadcast" => "examples/02-broadcast.md",
     "Reduce" => "examples/03-reduce.md",
     "Send/receive" => "examples/04-sendrecv.md",
-    "Job Scheduling" => "examples/05-job_schedule.md"
+    "Job Scheduling" => "examples/05-job_schedule.md",
+    "Scatterv and Gatherv" => "examples/06-scatterv.md",
 ]
 
 examples_md_dir = joinpath(@__DIR__,"src/examples")