Broadcast / Sliced variable implementation

I have a question about the implementation of broadcast variables in a parfor loop. Specifically, for the following code:

% Make some fake data and preallocate output
R = rand(100,100);
output = NaN(100,1);

parfor t = 1:100

    % Dynamically choose indices in dimension 1.
    rows = someFunction(t);

    % Do a calculation using the relevant rows. 
    output(t) = anotherFxn( R(rows,t) );
end

I get the code Analyzer warning: "The array or structure R is a broadcast variable. This might result in unnecessary communication overhead."

I'm curious if the entire array R is being broadcast to each worker, or if one column of R is being broadcast to each worker. Since each column of R is sliced, I would have thought that only one column of R would be sent to each worker. But the code analyzer message seems to suggest differently.

Practically, is there any difference between the above code and the following:

% Make some fake data and preallocate output
R = rand(100,100);
output = NaN(100,1);

parfor t = 1:100
    
    % Slice a column
    Rslice = R(:,t);

    % Dynamically choose indices in dimension 1.
    rows = someFunction(t);

    % Do a calculation using the relevant rows. 
    output(t) = anotherFxn( Rslice(rows) );
end

with respect to how much of R is sent to each worker?

Thanks!

Answer 1

[Edric Ellis]

You can use ticBytes and tocBytes to show that in your first example, the warning is accurate and R will not be sliced. This limitation is documented as follows:

• Form of Indexing. Within the first-level of indexing for a sliced variable, exactly one indexing expression is of the form i, i+k, i-k, or k+i. The index i is the loop variable and k is a scalar integer constant or a simple (non-indexed) broadcast variable. Every other indexing expression is a positive integer constant, a simple (non-indexed) broadcast variable, a nested for-loop index variable, colon, or end.

The final sentence is the restriction you're falling foul of here - in your first case, rows is not one of those types of indexing expression.

[Jonathan King]

Got it, thank you!