Parfor/spmd with random numbers

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JohnDapper 2016-2-2

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https://ww2.mathworks.cn/matlabcentral/answers/266320-parfor-spmd-with-random-numbers

评论： Brendan Hamm 2016-2-3

In my code I want to run a function on each worker in parallel. The function is mexified C++ and (simply put) returns an image consisting of pixels with random intensity. Something like this:

parfor i = 1 : numiters
  output(:,i) = myFn()
end

The trouble is that each output(:,i) is identical! Running the functions serially, like so:

for i = 1 : numiters
  output(:,i) = myFn()
end

there are no troubles. Each output(:,i) is different. I thought that each worker is initialized with a random seed -- but I guess not. It seems that they are all initialized to the same seed. Any tips on how to overcome this issue?

Many thanks.

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Edric Ellis 2016-2-3

Walter is exactly right that MATLAB's built-in random number generation functions like rand and randn are seeded differently on each worker, and I would recommend using these functions to generate random numbers. If you're using a C function to generate the random numbers, you need to take account of how that function is being seeded. If, for example, it is seeded using the start time of the process, then you'll get the behaviour you observe.