Difference between debug parfor loop and run it

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Hello everyone,
I have the following question: Whats the differnce between debugging a parfor loop and run it? In the matter that i stop executing the code before the parfor loop and then execute the hole parfoor loop with pressing F9.
Because, if i debug the parfor loop it can be execute without any error but if I run it normally the following error message appears:
"for the parfor-loop that is trying to execute on the worker could not be found."
and
"Caused by:
Unrecognized function or variable 'A'.
Error using remoteParallelFunction (line 94)
Worker unable to find file.
Unrecognized function or variable 'A'."
Before the parfor-loop gets called other function gets called and the parfor-loop is also calling a other function. The called function makes some calculation and saving stuff to the file system, nothing else. So I am a bit confused, because variable 'A' is a variable that isn't changed and every iteration gets variable A.
Thanks for the help already!
  3 Comments
Nicolas Kaiser
Nicolas Kaiser on 27 May 2022
Error message: The source code (PATH TO CALLING FUNCTION) for the parfor-loop that is trying to execute on the worker could not be found.
It seems to be this problem but I can't really interprete this.

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Accepted Answer

Edric Ellis
Edric Ellis on 27 May 2022
Further to Walter's answer, parfor assumes that anything that is not statically provable to be a variable reference must be a function reference. In your case, I suspect you're using eval or load to generate the variables A etc. The parfor machinery is therefore unable to transfer A to the workers, and instead assumes A is the name of a function - which it then can't find. Here's a simple example which shows the problem:
doStuff();
Starting parallel pool (parpool) using the 'local' profile ... Connected to the parallel pool (number of workers: 2). Analyzing and transferring files to the workers ...done.
Error using solution>doStuff
The source code (solution.mlx) for the parfor-loop that is trying to execute on the worker could not be found.

Caused by:
Undefined function 'A' for input arguments of type 'double'.
Error using remoteParallelFunction
Worker unable to find file.
Undefined function 'A' for input arguments of type 'double'.
function out = doStuff()
eval('A = 1;');
parfor i = 1
out(i) = A(i);
end
end
If this is indeed the problem, then the fix is simple: don't use load or eval - ensure the variables are defined in the text of the program normally.

More Answers (2)

Walter Roberson
Walter Roberson on 27 May 2022
parfor is not always able to automatically determine which functions will be called by workers, especially if eval() or run() is used or if you have calls to an optimization or ode* function that uses a quoted string for the function name instead of using a function handle.
In order to tell parfor where to find the files you may need to attach them to the pool.
Over the longer term, if you are using quoted strings for function names you should probably rewrite to use anonymous functions.
https://www.mathworks.com/help/parallel-computing/parallel.pool.addattachedfiles.html

Nicolas Kaiser
Nicolas Kaiser on 30 May 2022
Thanks both of you! The problem was that used load to get some of the input variables for the function.

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