# Why is it not possible to pass matrix parameters as argument to GPU arrayfun and what to do?

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Anders on 27 Feb 2024
Commented: Edric Ellis on 4 Mar 2024
Background
1. The problem can be separated into a large number of independent sub-problems.
2. All sub-problems share the same matrix parameters of significant sizes.
3. Each sub-problem performs an operation involving the matrix parameters that is hard to express with linear algebra.
4. The goal is to process the sub-problems in paralell on the GPU.

Edric Ellis on 27 Feb 2024
It would be great to see a simplified example of what you're trying to do. What you can do with gpuArray/arrayfun is write a nested function to access constant data from a parent workspace, a bit like this:
function [r,c] = example
% up-level variables that we will access inside the nested
% function "iFindLocation"
mat = gpuArray(magic(4));
[m,n] = size(mat);
[r,c] = arrayfun(@iFindLocation, gpuArray(1:100));
function [r,c] = iFindLocation(in)
r = NaN; c = NaN;
for i = 1:m
for j = 1:n
% Access the matrix "mat" from the parent
% workspace
if mat(i,j) == in
r = i;
c = j;
end
end
end
end
end
Anders on 27 Feb 2024
Edited: Anders on 27 Feb 2024
99% of the code used for this project can be regarded as a black box function analogous to the function iFindLocation.
Your example code represents this problem nicely.
Follow-up question
For arrayfun to work in paralell, does it need to store copies of the matrix mat in GPU memory or is it enougth with just the one?
Edric Ellis on 4 Mar 2024
I believe the constant data used inside the nested function here does not need to be duplicated.

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