What happens when we call a function containing a parfor loop within another parfor loop?

Consider

answer = zeros(1,5);

parfor x1 = 1:5
    answer(x1) = fun();
end



function output = fun()

R = zeros(1,10)
    parfor x2 = 1:10
        R(x2) = x2;
    end
output = sum(R(:));

end

Is the parallelization applied only to 'x1', or is it applied to both 'x1' and 'x2'?

Answer 1

[Torsten]

This page has the answer to your question:

https://uk.mathworks.com/help/parallel-computing/nested-parfor-loops-and-for-loops.html

From the page:

You can also use a function that uses parfor and embed it in a parfor-loop. Parallelization occurs only at the outer level. In the following example, call a function MyFun.m inside the outer parfor-loop. The inner parfor-loop embedded in MyFun.m runs sequentially, not in parallel.

parfor i = 1:10
    MyFun(i)
end

function MyFun(i)
parfor j = 1:5
    ...
end
end

Tip

Nested parfor-loops generally give you no computational benefit.