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

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'?

Answer 1

[Edric Ellis]

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.

[Atanu]

Thank you!

Answer 2

[Drew]

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

[Atanu]

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?

[Walter Roberson]

@Atanu

It was due to the ending period being interpreted as part of the URL. I have corrected the link.