FOR to PARFOR - cannot make it right

Hello everyone,

I am having troubles of adjusting FOR loop codes below to PARFOR loop. Hopefully, you can help me out. Thank you so much in advance.

    for i = 1:size(t2,1)-1
        x = find(t1.v1>t2.v1(i) & t1.v1<=t2.v1(a+1));
        t1.v2(x(t1.v2(x)~=t2.v2(a))) = 0;
    end

PS: I am not sure if I give out enough information, please let me know if you need me to clear out anything.

[Joseph Cheng]

I do not think this is a case where you can use PARFOR. in the t1.v2() line you maybe indexing the same values in x in parallel loops. what is the error it gives you when you try this?

[Hai Nguyen]

Thank you for responding. Actually I got 3 warning when I used PARFOR instead of FOR.

• The PARFOR loop cannot run due to the way variable 't1' used
• Valid indices for 't1' are restricted in PARFOR loop
• The entire array or structure 't2' is broadcast variable. This might result in unnecessary communication overhead.

Answer 1

[Edric Ellis]

There are several problems here, but I think the biggest problems you need to overcome is that the result of a PARFOR loop needs to be expressed as a sliced or a reduction variable, and that the iterations of the loop need to be provably order-independent. Simply put, a sliced output is one where MATLAB can see from the text of your program that each loop iteration assigns into a slice of an array, and the slice depends on the loop variable. So, the following is a sliced output:

x = zeros(10);
parfor idx = 1:10
  x(:, idx) = rand(10, 1);
end

In this case, each iteration of the loop assigns into a column slice of x. In your case, the indexing expression into t1.v2 is problematic firstly because it is not a simple 'sliced' form, and secondly because the indexing expression depends on previous values in t1.v2.

You might be able to run the first expression in the loop as a PARFOR, and then perform the assignment in an ordinary FOR loop later - but this will probably not get you any improvement in performance.

parfor i = 1:size(t2,1)-1
  xc{i} = find(t1.v1>t2.v1(i) & t1.v1<=t2.v1(a+1));
end
for i = 1:size(t2,1)-1
  x = xc{i};
  t1.v2(x(t1.v2(x)~=t2.v2(a))) = 0;
end