Passing a Global Variable to arrayfun for gpuArray

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

Answer 1

[Edric Ellis]

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.

[FARHAN ISLAM]

Thank you so much Edric! Your method worked and it lowered my calcualtion time on the original code that I am working with by a factor of 60.

[Jens Risbo]

Thanks for the example ! Just one Question.

Are these varables initialed everytime the nested function is called, or only once?

/Jens