Question

I have the following matlab code:

randarray = gpuArray(rand(N,1));
N = 1000;

tic
g=0;
for i=1:N

    if randarray(i)>10
        g=g+1;
    end

end
toc

secondrandarray = rand(N,1);
 g=0;

 tic 
for i=1:N

    if secondrandarray(i)>10
        g=g+1;
    end

end
toc



Elapsed time is 0.221710 seconds.
Elapsed time is 0.000012 seconds.

1) Why is the if clause so slow on the GPU? It is slowing down all my attempts at optimisation

2) What can I do to get around this limitation?

Thanks

Answer 1

This is typically a bad thing to do no matter if you are doing it on the cpu or the gpu.

The following would be a good way to do the operation you are looking at.

N = 1000;
randarray = gpuArray(100 * rand(N,1));
tic
g = nnz(randarray > 10);
toc

I do not have PCT and I can not verify if this actually works (number of functions supported on GPU are fairly limited).

However if you had Jacket, you would definitely be able to do the following.

N = 1000;
randarray = gdouble(100 * rand(N, 1));
tic
g = nnz(randarray > 10);
toc

Full disclosure: I am one of the engineers developing Jacket.