MATLAB: Are temporary variables eliminated at the end of each parfor loop on every single worker

parfor

I want to know, if the temporary variables inside the parfor loop are eliminated at the end of each loop, even on a single worker?

Best Answer

Yes, as mentioned in the documentation https://www.mathworks.com/help/parallel-computing/temporary-variable.html temporary variables are never sent back to the client. There are two ways to get results out of a parfor loop: either use sliced output variables, or reduction output variables. These variable types are explained here https://www.mathworks.com/help/parallel-computing/troubleshoot-variables-in-parfor-loops.html .

Related Solutions

MATLAB: PARFOR temporary variables warning

Uninitialized Temporaries Temporary variables in a parfor-loop are cleared at the beginning of every iteration. MATLAB can sometimes detect cases in which loop iterations use a temporary variable before it is set in that iteration.

Examples include conditionally setting a variable before any use of the variable.

function myFunction
parfor i = 1:10
    if rand < 0.5
        a = MyClass;          
    end
    disp(a.SomeValue);
end
end

Likewise, not setting a variable on all possible code paths.

function myFunction
parfor i = 1:10
    if rand < 0.5
        a = MyClass;
    else          
        % a is not set 
    end
    disp(a.SomeValue);
end
end

MATLAB: Is it possible to determine the cpu capacity of each parfor worker

parfor() breaks up the iteration range into 3 groups. In the first pass, it assigns each worker an equal share of the last 2/3 of the possible iterations (later iteration numbers are done first as an optimization on array allocation.) As each worker finishes, it is assigned a chunk which is an equal share of roughly the middle 1/4-ish of possible iterations. When those run out, each worker is assigned a chunk which is a small number of iterations. Somewhere in Answers I posted a more detailed analysis of the allocation algorithm in practice, as then implemented... ah, it was https://www.mathworks.com/matlabcentral/answers/341778-how-matlab-divide-the-number-of-iterations-of-parfor-on-the-workers-of-one-computer#comment_456761

In cases where this does not load balance well enough for your purposes, see https://www.mathworks.com/matlabcentral/answers/65350-why-does-parfor-execute-loops-in-a-random-order#comment_133329 or use parfeval()

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MATLAB: PARFOR temporary variables warning

MATLAB: Is it possible to determine the cpu capacity of each parfor worker

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