Why numel() is not accepted with a gpuArray

pcGpu = gpuArray(5:100);

r = arrayfun( @numel, pcGpu );

simple line of code above, although numel is supported with gpuArray, I still get following feedback:

Function passed as first input argument contains unsupported or unknown function 'numel'.

For more information see Tips.

Am I doing sth. wrong?

Thanks, Song

[Walter Roberson]

Why would you want to apply numel to each entry in a gpu Array? It would be asking for numel() of a scalar at each step, so the effect would be the same as ones(size(pcGpu), 'gpuArray')

[Song Decn]

Thanks Walter for the quick response.

Acutally what I really want to implement is to use GPU to execute a function containing numel(), like this:

function f = testFcn(a)

f = numel(a);

end

pcGpu = gpuArray(5:100);

% r = arrayfun( @numel, pcGpu );

r = arrayfun( @testFcn, pcGpu );

But still I get the following error:

Error using Untitled2 (line 24)

Function passed as first input argument contains unsupported or unknown function 'numel'.

For more information see Tips.

Error in 'testFcn' (line: 2)

So, does it mean that I can't call numel in a function for gpuArray?

I barely find examples for gpuArray calling in callback function.

[Song Decn]

I tried another way:

pcGpu = gpuArray(5:100);
a = 1:10;
% r = arrayfun( @numel, pcGpu );
f = @(x)testFcn(a,x);
arrayfun( f, pcGpu );

function f = testFcn(a, pcGpu)
for i = 1:numel(a)
    f = pcGpu + i;
end
end

still not working:

Error using Untitled2 (line 26)

Use of functional workspace is not supported.

For more information see Tips.

[Walter Roberson]

Are you applying numel to the entire array, or are you applying numel to each element of an array? When you use arrayfun then by definition the function is passed one element of the array.

arrayfun over a gpuArray does not support anonymous functions with captured variables.

[Walter Roberson]

for i = 1:numel(a)
    f = pcGpu + i;
end

That overwrites all of f each iteration, so if you could get it to work, the result would be the same as if you had done

i = numel(a);
f = pcGpu + numel(a);

Reminder: you can pass numel(a) in as a parameter.

Answer 1

[Walter Roberson]

https://www.mathworks.com/help/parallel-computing/gpuarray.arrayfun.html

numel is not supported inside any function run on a gpuArray by using arrayfun. The above link lists all supported functions.

Remember that arrayfun on a gpuArray runs all aspects on the gpu. Variables not specifically configured as gpuArray have to be converted to gpuArray internally because the entire function runs on the gpu.

[Song Decn]

Thanks a lot Walter. I learned a lot and now have to rewrite my code now.