How to avoid variable broadcasting in a parfor loop? - matlab

Question

I'm using parfor, unfortunately two of my variables, x and y, which are both matrices, get broadcasted and don't know how to avoid it. I've read about it in the MATLAB help, but couldn't figure out a solution. How can I prevent broadcasting of x and y?
Here is my code :
parfor k=1:length(Lambda)
lambda=Lambda(k);
for p=1:length(Gamma)
gamma=Gamma(p);
for Fold=1:size(Fold_indices,2)
x_Train=x(logical(Fold_indices(:,Fold)),1:end);
Y_Train=y(logical(Fold_indices(:,Fold)),1:Num_Tasks);
% Do sth with x_Train and Y_train
end
end
end
I've tried to slice broadcasted data(x) into a cell array and it didn't solve the problem a well.
B=cell(1,J);
% Fill each entry of B with a matrix
% ...do it here
....
parfor k=1:length(Lambda)
lambda=Lambda(k);
for p=1:length(Gamma)
gamma=Gamma(p);
for Fold=1:J)
x_Train=B{1,J};
% Do sth with x_Train and Y_train
end
end
end
Interestingly, when I assign the broadacaste variable(B) to other varible(D), then it stops getting brodcated.
B=cell(1,J);
% Fill each entry of B with a matrix
% ...do it here
....
parfor k=1:length(Lambda)
D=B;
lambda=Lambda(k);
for p=1:length(Gamma)
gamma=Gamma(p);
for Fold=1:J)
x_Train=B{1,J};
% Do sth with x_Train and Y_train
end
end
end

Answer 1

First off: you need to broadcast. Each worker is a separate MATLAB instance, and it needs the data. Sending data to a worker's MATLAB instance is called broadcasting. So there's no preventing it when you use parallel computing, it's the core of it even.
Second: you can't avoid broadcasting x and y in its entirety here, since you use all of it in each separate parfor iteration. Avoiding broadcasting as a whole requires you to not need all of the matrix in each loop iteration, in which case you can slice your variables, as is presented in this answer; i.e. you'll have to rewrite your code in such a manner that you do not require all of x and y to be on each separate worker.