Parfor unnecessary communication overhead

Hi all,

I have the below parfor loop which is good when I am wanting to pass thru negative values to evaluate. I am getting the following error/warning from MATLAB which I wonder if there is a faster/optimal way to get to the same point improving the below structure.

Error: The Entire Array or Structure "yA_vals"/"yC_vals"/"yD_vals" is a broadcast variable. This might result in unnecessary communication overhead.

yA_vals = -5:5; n_yA = length(yA_vals);

yC_vals = 0:10; n_yC = length(yC_vals);

yD_vals = -10:0; n_yD = length(yD_vals);

parfor yAidx = 1:n_yA

yA = yA_vals(yAidx);

fprintf('>Job 1 %d of %d at %s...\n',yA,yA_vals(end),datestr(now));

for yCidx = 1:n_yC

yC = yC_vals(yCidx);

fprintf('>>Job 2 %d of %d...\n',yC,yC_vals(end));

for yDidx = 1:n_yD

yD = yD_vals(yDidx);

fprintf('>>>Job 3 %d of %d...\n',yD,yD_vals(end));

[~,~,Total(yAidx,yCidx,yDidx)] = Function(Input,yA,yC,yD);

end

end

end

Answer 1

[Raymond Norris]

This is a warning, not an error, displayed by the Code Analyzer. Not sure how MATLAB would "throw" this warning.

Depending on the size of the matrix/structure, this could be an issue. Specifically, for large size variables. Of course "large" is relative. But in your case, the vectors are very small -- this won't be an issue for you -- so you can use broadcast variables. To see how much data is being sent back and for, look at ticBytes and tocBytes.

If the variables are large, you could consider a couple of options

• Create additional variables that represent the portition of the larger variable you need. The Code Analyzer gives an example of this.
• Create temporay variables (created within the parfor). It's not how you'd write MATLAB serial code, but would avoid the transfer. For example

yA_vals = -5:5; n_yA = length(yA_vals);
parfor yAidx = 1:n_yA
    % Create the temporary variable yA_vals in the parfor to avoid the
    % transfer.
    yA_vals = -5:5;
    yA = yA_vals(yAidx);
    fprintf('>Job 1 %d of %d at %s...\n',yA,yA_vals(end),datestr(now));
end

[IDN]

Thanks, this is a great way to understand it!