Problem with parfor loop due to some variable that cannot be classified.

I am unable to specify a parfor loop that works due to some variable that matlab cannot classify

I tried to implement the following function

function somefunction(N,NWorkers)
counts=zeros(1,actualNworkers);
P=myprogress(N,actualNworkers);
parfor (isim=1:N,NWorkers)                     
    local_labindex=1;
    if NWorkers    
        local_labindex=labindex;
    end
    counts(local_labindex)=counts(local_labindex)+1;
    send(P,[local_labindex,counts(local_labindex)])
end

end

But Matlab complains that "The PARFOR loop cannot run due to the way counts is used"

When this failed, I tried to use a cell array instead of a vector. But that didn't work either. Finally I tried to implement a nested function as shown below but it still does not work

function somefunction(N,NWorkers)
counts=zeros(1,actualNworkers);
P=myprogress(N,actualNworkers);
parfor (isim=1:N,NWorkers)                     
    local_labindex=1;
    if NWorkers    
        local_labindex=labindex;
    end
    o=increment(local_labindex);
    send(P,[local_labindex,o])
end
function o=increment(index)          
counts(index)=counts(index)+1;
o=counts(index);
end
end

But this time the error message is : "The nested INCREMENT cannot be called from within a PARFOR loop"

Is there any workaround this problem?

[Matt J]

What is the loop trying to do? In what way is it supposed to be parallel? The loop variable isim is never used at all.

[Ive J]

Are you aware that labindex is always 1 in a parfoor loop? Maybe you can explain what you're trying to do...

[Patrick Mboma]

I just simplified the code so that I can post it. The isim variable is irrelevant for the point here. It could be anything. The problem is with the "counts" variable

[Matt J]

You need to explain what you are trying to do with the counts variable. As shown, it doesn't make sense.

[Matt J]

Here are the different types of variable classifications that a parfor variable can have. Which type is count supposed to be? It cannot be a broadcast variable if you are going to assign to it.

[Patrick Mboma]

I have looked at that list but I could not find a solution to my problem that's why I posted it. I need to keep track of the number of iterations performed by each worker

[Patrick Mboma]

As to what I am trying to do with the counts variable, I would like to take the sum to evaluate how far along the process has come.

Answer 1

[Matt J]

It might be easier just to use for-drange loop.

spmd

    counts=0;
    for i=drange(1:N)

        counts=counts+1;

    end

end
counts{:}

[Patrick Mboma]

This is interesting. Could you, please, tell me how to integrate that with the rest of the code? Note I also need to get the correct labindex, which unfortunately is always 1, as someone pointed out, inside parfor loops.

[Matt J]

Within spmd, labindex should work as expected.

N=20;
spmd

    counts=0;
    workerID=[];
    
    for i=drange(1:N)

        counts=counts+1;
        workerID=labindex;
        
    end

end
counts{:}

ans =

5

ans =

5

ans =

5

ans =

5

workerID{:}

ans =

1

ans =

2

ans =

3

ans =

4

[Patrick Mboma]

I guess this could work. One last question, if I may : Would this code work even if there are no workers?

[Matt J]

You could try and see. Even if it does work, it might be advisable to check first to see if a pool is open before doing anything. If no pool is open, the task becomes trivial.