using a parallel.pool.Constant variable in multiple parfor loops

I have ClassA as follows

classdef ClassA < handle
    properties (Access = public)
        myParameter = 'initValue';
    end
    methods (Access = public)
        function obj = ClassA()
        end
        function [] = changeMyParameter(obj)
            obj.myParameter = 'changedValue';
        end
    end
end

and ClassB as follows

classdef ClassB < handle
    properties (Access = public)
        myClassAList;
    end
    methods (Access = public)
        function obj = ClassB()
            clc

              for i = 1 : 20
                  obj.myClassAList{i} = ClassA();
              end

              c = parallel.pool.Constant(obj.myClassAList);

              parfor i = 1 : 20
                  c.Value{i}.changeMyParameter();
                  disp(c.Value{i}.myParameter);
              end

              disp('do something in the client')

              parfor i = 1 : 20
                  disp(c.Value{i}.myParameter);
              end
          end
      end
  end

If I execute ClassB, I expect the disp command to produce the same results in both of the parfor loops, a list of 'changedValue'. This is what happens in the first loop, however, in the second loop I see a list that both have 'changedValue' and 'initValue' elements as well. Why?

Answer 1

[Edric Ellis]

In this case, the parallel.pool.Constant you're creating has a cell array of 20 elements on each worker. In your first parfor loop, each worker gets some of the 20 loop iterates, and modifies those elements of the cell array inside the Constant value. If you modify your parfor loops, hopefully this will help explain what's happening. For the first loop,

parfor i = 1 : 20
    t = getCurrentTask();
    c.Value{i}.changeMyParameter();
    fprintf('Changing element %d on worker %d\n', i, t.ID);
end

This will show you which elements of c.Value are modified on which worker. (Note that each worker has its own independent copy of c.Value).

Then, when you run the second parfor loop, the iterates are not necessarily dispatched to the same workers in the same order, so if you run:

parfor i = 1 : 20
    t = getCurrentTask();
    fprintf('Checking element %d on worker %d\n', i, t.ID);
    disp(c.Value{i}.myParameter);
end

You can see that in some cases, you're checking an element of c.Value on a worker where it wasn't modified.