Passing a Global Variable to arrayfun for gpuArray

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Hi,
I would like to pass a global array to the arrayfun function. While it does work with regular CPU array, it does not work with GPU array. My original code is quite complicated, but I am pasting a simplified code to help you understand what I am trying to do.
Any help on how this can be accomplished is greatly appreciated!
M = randi(500,100,100);
global a
a = [1 2 3];
tic
C = arrayfun(@function_sum,M);
CPUTime = toc
M = gpuArray(M);
tic
C = arrayfun(@function_sum,M);
GPUTime = toc;
Boost = CPUTime/GPUTime
function y = function_sum(x)
global a
y = 0;
for i = 1 : x
y = y + 1/i + 1*a(1) + 2*a(2) + 3*a(3);
end
end

Accepted Answer

Edric Ellis
Edric Ellis on 4 Nov 2021
You can do this by using a nested function and variables in the containing parent workspace instead of global variables. Here's how you would apply this to your example. Note that I've made function_sum become a nested function, and it accesses a directly from the containing function workspace.
function repro
M = randi(500,100,100);
a = [1 2 3];
% Nested function
function y = function_sum(x)
y = 0;
for i = 1 : x
% Access 'a' directly from parent workspace
y = y + 1/i + 1*a(1) + 2*a(2) + 3*a(3);
end
end
tic
C = arrayfun(@function_sum,M);
CPUTime = toc
M = gpuArray(M);
tic
C = arrayfun(@function_sum,M);
GPUTime = toc;
Boost = CPUTime/GPUTime
end
There's a more detailed example of this approach here in the doc.
  2 Comments
Jens Risbo
Jens Risbo on 17 Feb 2022
Thanks for the example ! Just one Question.
Are these varables initialed everytime the nested function is called, or only once?
/Jens

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