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Repeat Random Numbers in parfor-Loops

As described in Control Random Number Streams, each worker in a cluster has an independent random number generator stream. By default, therefore, each worker in a pool, and each iteration in a parfor-loop has a unique, independent set of random numbers. Subsequent runs of the parfor-loop generate different numbers.

In a parfor-loop, you cannot control what sequence the iterations execute in, nor can you control which worker runs which iterations. So even if you reset the random number generators, the parfor-loop can generate the same values in a different sequence.

To reproduce the same set of random numbers in a parfor-loop each time the loop runs, you must control random generation by assigning a particular substream for each iteration.

First, give each worker the same stream, one that supports substreams.

parpool('local',4);
spmd
    rng(0,'combRecursive');
end

Inside the parfor-loop, set the substream index by the loop index. This ensures that each iteration uses its particular set of random numbers, regardless of which worker runs that iteration or what sequence iterations run in.

r = zeros(1,16);
parfor i = 1:16
    stream = RandStream.getGlobalStream();
    stream.Substream = i;
    r(i) = rand;
end
r

r =

  Columns 1 through 8

    0.7270    0.5582    0.1666    0.3477    0.5671    0.5502    0.0130    0.9179

  Columns 9 through 16

    0.1567    0.7076    0.8577    0.8923    0.0891    0.2413    0.6074    0.2369