for loops nested inside parfor loops.

Hallo everyone, I have a bit of a problem with some parfor coding. The concept is this:

parfor j = 1:1:10;
    for i = 1:1:10;
        Q(i) = i^2;
    end

      R = Q(j);
  end

Of course this code is non-sensicle, but it illustrates the problem. I cannot create an array of values inside a parfor loop that I can then use throughout the parfor loop. In a practical application the expression for Q(i) will depend on j, so taking the sub-loop outside the parfor in not an option. Therefore, for now I wish to know why such a simple example can't be parallelised. As far as the algorithm is concerned it has to individually loop through i and then compute R in parallel with no obvious interdependency between the loops. Many thanks.

Answer 1

[Titus Edelhofer]

Hi,

if you add the following line before the inner loop, it should work:

Q = zeros(10, 1);

Titus

Answer 2

[Alexantrou Serb]

It did indeed. Thanks!

Does this happen because the parfor needs to allocate space for Q for each independent thread it generates?

[Edric Ellis]

No, it's because parfor cannot immediately deduce that you're not re-using values from Q in subsequent iterations of the loop. By assigning to the whole of Q, then it can correctly deduce that Q is a temporary variable. More details about parfor variable classification here.

[BHUSHAN MUTHIYAN]

Hello Edric,

If I have to change the global variable which is declared outside the parfor loop, I am getting an error. Any reasoning for this ?

My code looks something like this :

out = zeros(dimfox, dimfoy, fis(4), ins(4), 'like', sumModel);

H = vision.Convolver('OutputSize', 'Valid', 'CustomProductDataType',numerictype([],32,frac), 'CustomOutputDataType', numerictype([],32,frac));

parfor im = 1:ins(4)

    carve = zeros(dimfox, dimfoy, 'like', sumModel);

    for f = 1:fis(4)

        for ch = 1:fis(3)
            carve(:) = carve + step(H,input(:,:,ch,im),filter(:,:,ch,f));

end

   carve(:) = carve + bias(1, f);
        out(:,:,f,im) = carve;

    end

end

I am getting error as :

The variable out in a parfor cannot be classified.