GPU performance with short vectors

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MatlabNinja 2016-3-30

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编辑： Joss Knight 2016-4-20

Hello - I see GPU computation underperforming when used for vector manipulation with short lengths.

>> a = rand(1000000, 100,'gpuArray'); 
>> b= gather(a);
>> tic; for i=1:100 ; eval('q = zeros(1000000,1);for i = 1:100; q = b(:,i)+q;end') ; end;doc
Elapsed time is 45.489811 seconds.
>>tic; for i=1:100 ; eval('qq = zeros(1000000,1);for i = 1:100; q = a(:,i)+q;end') ; end;toc
Elapsed time is 0.875140 seconds.

same when done for short vectors see GPU computation under performing:

>> a = rand(200, 100,'gpuArray');
>>b= gather(a);
>> tic; for i=1:100 ; eval('q = zeros(200,1);for i = 1:100; q = b(:,i)+q;end') ; end;doc
Elapsed time is 0.021727 seconds.
>>tic; for i=1:100 ; eval('qq = zeros(200,1);for i = 1:100; q = a(:,i)+q;end') ; end;toc
Elapsed time is 0.833865 seconds.

Any insight will be appreciated.

Thank you.

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Answer 1

Joss Knight 2016-4-20

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https://ww2.mathworks.cn/matlabcentral/answers/276076-gpu-performance-with-short-vectors#answer_218693

编辑：Joss Knight 2016-4-20

在 MATLAB Online 中打开

Computation in a GPU core is significantly slower than in a modern CPU core. It makes up for that by having a lot of them - thousands. If you don't give it thousands of things to do at once, you're never going to beat the CPU.

In your simple computation above you are unnecessarily using a loop. This may have been for illustrative purposes, but if it reflects your actual code, you will gain back your performance by removing the loop, i.e.

q = sum(a, [], 2);

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Answer 2

Walter Roberson 2016-3-30

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https://ww2.mathworks.cn/matlabcentral/answers/276076-gpu-performance-with-short-vectors#answer_215556

Do not use eval() for this. use timeit()

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Walter Roberson 2016-3-30

Good point.

MatlabNinja 2016-3-30

Thank you for your insight. time and gputimeit gives very similar results and shows similar trend where smaller vector(a & b above) had worse run performance when run on GPU.

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