MATLAB: GPU time slower than CPU time, what went wrong with the GPU implementation

cuda performanceflopsgpugpu computingmatrix multiplicationParallel Computing Toolbox

Hi all, I have been testing the GPU computing feature in MATLAB. The code below is running and timing large matrix multiplications (1024×1024) using CPU and GPU computing:
    A=rand(1024);
    gA=gpuArray(A);
    %warming up
    for i=1:10
        C=A*A;
        gC=gA*gA;
    end
    tic,C=A*A;toc;
    tic,gC=gA*gA; toc;
After many trials, the results using CPU turns out to be faster than GPU time. I am surprised because this guy on stackoverflow forum did the exact testing and he proved that using GPU is faster:
    >> A = rand(1024); gA = gpuArray(A);
    % warm up by executing the operations a couple of times, and then:
    >> tic, C = A * A; toc
    Elapsed time is 0.075396 seconds.
    >> tic, gC = gA * gA; toc
    Elapsed time is 0.008621 seconds.
The only reason I can think of is that we are using different GPUs. The other guy has a Tesla C2070 while the laptop I am using is Dell Inspirion17R (NVIDIA GeForce GT 525M).
Could it be possible that by using a lesser GPU, the computation is actually slower than using CPU ?
Thank you! Ruby

Best Answer

Hi Ruby,
I've just uploaded a benchmarking tool to the File Exchange which runs a whole load of these type of timings to put your GPU in context with others in the market:
http://www.mathworks.com/matlabcentral/fileexchange/34080-gpubench
One thing to bear in mind is that virtually all GPUs that aren't explicitly designed for scientific computing are optimized for single-precision maths (as is used by OpenGL etc.). GeForce cards, mobile or otherwise, are quite good for single-precision performance but usually about 8x worse for double. MATLAB defaults to using double-precision everywhere. Of the NVIDIA cards, only the Tesla and top-end Quadro series do well at double-precision. Add to that the fact that a mobile GPU typically has far fewer cores than a desktop one, and I'd be amazed if you saw any significant speed-ups compared to a modern mobile CPU when doing double-precision maths.
Anyway, give the benchmark a try and let us all know what you find.
Cheers
Ben
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