Broadcast variables in parfor loop in MATLAB

Question

The following loop results in an error in C_mat and B_mat:

%previously defined
N_RIPETIZIONI=2;
K=201;
parfor n=1:N_RIPETIZIONI*K 
    [r,k]=ind2sub([N_RIPETIZIONI,K],n);
    B=B_mat{r};
    C=C_mat{r};
end

The warning says:

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

The same for C_mat. How can I fix it so that the indices of B_mat and C_mat are no more broadcast variables?

Answer 1

The issue is that the way you index B_mat (i.e. not using n), every thread in the parfor requires the entirety of B_mat to run. The big bottleneck in parfor code is transferring copies of the data to each node.

MATLAB is basically telling you that if you were to do this, you may actually have slower code than otherwise. Its not that B_mat is some type of variable called "broadcast", its that the way you wrote the code, each n in parfor requires a copy of B_mat.

I assume this is not your real code, so we can't really help you fix it, but hopefully this explains it.