Question

I’m sorry that currently I’m not able to boil my code down to a minimal example.
It’s a huge bunch of image processing code.

I have a loop that iterates over images (descriptors in variable stphogs) and for each image runs a detection.

function hogpatches = extractDetectionsFromImages(stphogs, poselet)
    hogpatches = cell(1,length(stphogs));

    parfor i = 1:length(stphogs)
        tmp = extractDetectionsFromImage(stphogs(i), poselet); %e.g. 1x6 struct

        if ~isempty(tmp)
            hogpatches{i} = tmp;
        end
    end

    hogpatches = cell2mat(hogpatches);
end

So this is the main loop. But the function calls in extractDetectionsFromImage go very deep.

My problem: Running this with a normal for-loop gives the correct result. When using PARFOR as above, hogpatches only contains 5 instead of 18 structs.

Where can I start to look for the error? I had a global variable the program did change. I removed that already. There is still a global variable ‘config’ which is however only read.. Any other hints? What could be the problem?

EDIT:
Even if I just run one iteration (size of stphogs is 1), the parfor fails. It doesn’t have anything to do with the isempty part. Problem persists if I remove that.

EDIT2:
Ok here I boiled it to a minimal working example.
It is indeed caused by a global variable:

function parGlobalTest()
    global testVar;

    testVar = 123;

    parfor i = 1:1
        fprintf('A Value: %d\n', testVar);
        testFunction();
    end
end

function testFunction()
    global testVar;
    fprintf('B Value: %d\n', testVar);
end

In this example. The output for A will be 123, for B it will be nothing (undefined).
Why is that?

Answer 1

Ok, here is my solution:

function syncTestVar()
    global testVar;
    save('syncvar.mat', 'testVar');
    pctRunOnAll global testVar;
    pctRunOnAll load('syncvar.mat');
end

If someone has a better approach please tell me… This one works though

Btw: The save/load is needed because in my real program, testVar is a complex struct