parfor loop output and input are not matched in order
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I have a code segment here:
so I wish to use parfor to calculate a function called OFreduced. The OFreduced takes in 10 numbers each time as input.
I have stored all input variables in modified_geo as a matrix of 3*10, where each entire row is one input to the function. So in total I have 3 sets of input that are needed to be calculated in parfor.
The output from the function OFreduced is simply just a number, and I stored that in another array v, which should contain the 3 results from the input.
It should produce three different answers and stored in v, and I could perfectly run this in serial with no problem. However parfor gives me the same number for the three inputs (i.e. three numbers are indentical in v). It seems like I have not matched the input index to the output index, and the results are "overwriting".
Please advise how shall I modify this to "match" the outputs and the inputs in the correct order. Many thanks in advance!
% compute cost using simplified OF
modified_cost1 = zeros(3,1);
v = zeros(3,1);
% run OF with temporary variable
parfor j = 1:3
v(j) = OFreduced(modified_geo(j,:));
end
modified_cost1 = v(1,1:3);
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Edric Ellis
2023년 1월 23일
By reproduction steps - I mean code that we can execute and see the problem. It sounds like that's not going to be easily possible if you're calling an external executable.
When you execute "avl.exe" - how are you getting the results out? If it creates a file - then that's a potential problem right there. Your workers might well all be clashing trying to write the same file - unless you take steps to prevent that. One simple way to avoid that is to have the workers change to a unique directory prior to running the body of the loop, like this:
parfor j = 1:3
oldWd = pwd();
newDirName = tempname();
mkdir(newDirName);
cd(newDirName);
... do stuff
cd(oldWd);
end
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Rik
2023년 1월 23일
I suspect the source of the problem is the interaction with the file system.
You should consider generating a random file name (either with randi indexing from allowed characters, or with tempname). That way you ensure you don't overwrite the file that the other process is using.
I would suggest avoiding cd if at all possible. You can use tempdir if you need the temp folder location.
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