MATLAB: How to pass structure data into parfor loop

broadcast variableparallelparforpassstructure

Hi all,
I have a simple parfor test code like this:
inpt.a1 = rand(100);
inpt.a2 = rand(100);
parfor i = 1:10000
      inpt_pass = inpt;
      otpt = sin(inpt_pass.a1);
end
I wrote inpt_pass in order to pass the structure inpt into the parfor loop. However, if inpt is a very large structure, passing it into every parfor loop would be cumbersome, and cost loads of storage.
If I do:
inpt.a1 = rand(100);
inpt.a2 = rand(100);
parfor i = 1:10000
      inpt_pass = inpt.a1;
      otpt = sin(inpt_pass.a1);
end
which only pass part of the structure into parfor loop (ideal), I got the broadcast variable warning.
So how can I do it? Many thanks!

Best Answer

You can simply extract the data that you want to pass to the workers:
inpt.a1 = rand(100);
inpt.a2 = rand(100);
A1 = inpt.a1; % Only inpt.a1 is sent to the workers
parfor i = 1:10000
      otpt = sin(A1);
end
Now there is no extra overhead of transferring data. This of course assumes a 1-by-1 vector. If you have more elements in your struct you could just make a copy of only the data that needs to be sent:
inpt.a1 = rand(100);
inpt.a2 = rand(100);
tempVar.a1 = inpt.a1;
parfor i = 1:10000
     % You will still have a warning, but you can ignore it as you only pass the data you need.
      otpt = sin(tempVar.a1); %#ok<PFBNS>     <-- Ignore the warning
end
If you get an error about how MATLAB cannot classify the variable (common with structs), place the body of the parfor loop into a local function and this can overcome this issue easily.
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