composite date from parallel computing

Question

I'm using parallel computing for the first time (spmd).

After starting the pool and doing some parallel computations, I only have composite variables in my workspace and can't open them. Or when I open them by double clicking they are empty. How can I use the data?

Here is my code:

matlabpool open local 4

spmd
    if labindex==1
    a = randn(300,1);
    end
    if labindex==2
     b = randn(300,1);
    end
    if labindex==3
    c = randn(300,1);
    end
    if labindex==4
     d = randn(300,1);
    end
 end

matlabpool close

Answer 1

If you want to generate 4 arrays of size (300,1) on an individual worker each, it is better to do like the following. Note that I have 4 cores on my computer/Matlab pool.

clc
clear

spmd    
    RandomArray = rand(300,1); % Matlab automatically creates a (300,1) array in each worker.
end

FinalArray = [RandomArray{:}]; % Concatenate everything outside of the spmd block.

whos % Check the results


Name               Size            Bytes  Class        Attributes

  FinalArray       300x4              9600  double                 
  RandomArray        1x4              1145  Composite              

As you see, FinalArray has a size (300,4) as you want. With your code above, it would be quite a pain to put everything together in the 2nd spmd block, because each worker is not aware of what is in the other workers and each variable would be undefined in the workers that did not use them. I don't know the right terminology sorry, but you can read the doc to get a better explanation :)

EDIT:

To answer your comment, here is a simple example. Hopefully this is what you meant :)

clc
clear

% Define different variables.
w = ones(1,10); 
x = 1:10;
y = x/2;
z = rand(1,10);

% Use different functions in each worker. Of course you could use the same function with different inputs.
spmd
    if labindex==1        
        a = w;        
    end
    if labindex==2
        b = sin(x);
    end
    if labindex==3
        c = y.^2;
    end
    if labindex==4
        d = 4*z;
    end
end

% This is the important part
FinalArray = [a{1} ;b{2}; c{3} ;d{4}];
whos 

And the output of whos is:

Name            Size            Bytes  Class        Attributes

  FinalArray      4x10              320  double                 
  a               1x4               497  Composite              
  b               1x4               497  Composite              
  c               1x4               497  Composite              
  d               1x4               497  Composite              
  w               1x10               80  double                 
  x               1x10               80  double                 
  y               1x10               80  double                 
  z               1x10               80  double