Matlab Parallelism toolbox : stacking loops in parfor

I'm trying to use the parfor loop in the matlab parallelism package. I'm having a similar problem to this guy : MATLAB parfor slicing issue? . The output matrix doesn't seem to be recognized as a sliced variable. In my specific case, I'm trying to stack use other for loops inside the parfor, and I have trouble applying the solution proposed in the other thread 开发者_开发问答to my problem. Here is a dummy example of what I'm trying to do :

n=175;
matlabpool;

Matred=zeros(n,n);

Matx2Cell = cell(n);

parfor i=1:n
    for j=1:n
        for k=1:n

            Matred(j,k)=exp((j+i+k)/500)
        end;
    end;
    Matx2Cell{i}=Matred;

end;
matlabpool close;

P.S. I know it would work to put the parfor on the k-loop instead of the i-loop... But I'd still like to put it on the i-loop (I believe it would be more time-efficient in my real program).

Thanks a lot Frédéric Godin

You can put Matred = zeros(n); into the parfor body, but this is very slow. Instead define a function with Matred = zeros(n); in it: effectively the same thing, but much faster:

function Matred = calcMatred(i,n)
Matred=zeros(n);
for j=1:n
    for k=1:n
        Matred(j,k)=exp((j+i+k)/500);
    end
end

Here is a time comparison:

matlabpool
n = 175;
Matx2Cell = cell(n,1);

tic
parfor i=1:n
    Matred=zeros(n);
    for j=1:n
        for k=1:n
            Matred(j,k)=exp((j+i+k)/500);
        end
    end
    Matx2Cell{i}=Matred;
end
toc

tic
parfor i=1:n
    Matx2Cell{i}=calcMatred(i,n);
end
toc

matlabpool close

On my machine, it takes 7 seconds for the first one and 0.3 seconds for the second.

Also note that I've changed the declaration of Matx2Cell to cell(n,1) since cell(n) makes an n x n cell array.

You need to move Matred into the parfor loop body. This needs to be done because each iteration of the parfor needs a new copy of Matred.

n=175;
matlabpool;

Matx2Cell = cell(n);

parfor i=1:n
    Matred=zeros(n,n);
    for j=1:n
        for k=1:n

            Matred(j,k)=exp((j+i+k)/500)
        end;
    end;
    Matx2Cell{i}=Matred;

end;
matlabpool close;