How to resolve warning message about broadcast variable while using 'parfor'?

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Atanu 2024-7-14，21:29

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评论： Atanu 2024-7-16，15:54

males = {'animal1', 'animal2', 'animal3'};
treatmentGrps = {'T1', 'T2', 'T3'};
nrows = size(featureLists, 1);
ncols = size(featureLists, 2);
featureLists = cell(nrows, ncols);
parfor grp = 1:nrows
    tempRow = cell(1, ncols);
   
    for animal = 1:ncols
        tempRow{animal} = individualPsychValuesPerSession('approachavoid', ...
            treatmentGrps{grp}, males{animal});
    end
    featureLists(grp, :) = tempRow;
end

When I use 'parfor' like above I get an warning message

The entire array or structure 'males' is a broadcast variable.
This might result in unnecessary communication overhead.

But, when I change 'ncols' to 'numel(males)' like following

for animal = 1:numel(males)
    tempRow{animal} = individualPsychValuesPerSession('approachavoid', ...
        treatmentGrps{grp}, males{animal});
end

for the inner loop I don't get any warning.

Can someone please explain this behavior of 'parfor'?

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Answer 1

Edric Ellis 2024-7-15，12:09

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I think you should ignore this message in this case. It looks like you really need all the values of males for each iteration of the parfor loop, and that's OK. The message is there to warn about cases where the data is being "accidentally" broadcast.

The fact is that the array males is very small (in terms of bytes), so it should not cause significant overhead. You can check using whos to see the size of the array in the workspace, and also use ticBytes and tocBytes to see how much data is being transferred to run the loop. My bet is that the overall data transfer is not being significantly changed by the size of males.

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Atanu 2024-7-16，15:54

Thank you!

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Answer 2

Drew 2024-7-14，21:54

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编辑：Walter Roberson 2024-7-14，22:44

See https://www.mathworks.com/help/parallel-computing/broadcast-variable.html It includes an example with numel().

If this answer helps you, please remember to accept the answer

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Atanu 2024-7-14，22:23

Thanks for your response. But the link is not working for me.

'The page you were looking for is not available in this version of the documentation. You can use the search box or browse the products to find related information. You can also view archived documentation for prior releases.'

Can you share the link again?