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Why for loop in CPU faster than in GPU

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Khieu on 21 Jun 2017
Answered: Joss Knight on 22 Jun 2017
I am running the following command:
p = zeros(7000000,1); g_p = gpuArray(p);
1. CPU version
for i=1:size(p,1)
p(i) = 10; % just simple to see
2. GPU version
for i=1:size(g_p,1)
g_p(i) = 10; % just simple to see
It is amazing that the execution time in CPU really more faster than it in GPU. Why?


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Jan on 21 Jun 2017
@Khieu: Compare this:
tic; p(:) = 10; toc
tic; g_p(:) = 10; toc
What do you get?
Adam on 21 Jun 2017
It is very difficult often to determine where this crossover is. I have occasionally dabbled with a GPU implementation of some computation in my code, but it is always hit and miss whether it is faster or slower on a particular run since data sizes vary so I often just end up commenting it out with a vague idea I might come back to it one day and consider some fancy code to chose the CPU or GPU version of my code based on input size and any other appropriate metrics, but I never have done so far.

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Answers (1)

Joss Knight
Joss Knight on 22 Jun 2017
This isn't amazing at all. You are launching 7 million kernels, each to copy one value into GPU memory. The kernels cannot overlap because they are operating on the same data, which means each must wait for the last to finish. This is not at all an efficient way to do parallel programming!


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