GPU wrong results

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Georgios 2011-10-27

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https://ww2.mathworks.cn/matlabcentral/answers/19609-gpu-wrong-results

Hello, I just started using GPU with matlab and I get some strange results. I run the following code

a = rand(2^10,2^10,'single');
G = gpuArray(a);
F = fft2(G);
err = isequal(gather(G), a)

Normally the value o 'err' should be 1, but in this case is not. If you call another function, e.g. instead of fft2 call atan, then the value of err is 1. Do you have any idea why this happens?

The GPU model is "GeForce GTX 580" and the matlab version is 2010b.

Thanks a lot.

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Walter Roberson 2011-10-31

It appears that the German Wischi-Waschi came down to English as "wishy-washy". I had no idea of the origin of the English term before.

Georgios 2011-10-31

Thank you all for your replies. The OS is ubuntu 10.4 and I think that we already use the latest driver (I will ask the administrator).

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回答（2 个）

Jan 2011-10-27

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https://ww2.mathworks.cn/matlabcentral/answers/19609-gpu-wrong-results#answer_25938

在 MATLAB Online 中打开

Do you expect absolutely equal results without rounding errors? This will happen as an exception only. Please check:

g = gather(G);
disp(max(abs(g(:) - a(:))));

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Jan 2011-10-31

A large FFT2 produces some heat on the GPU. Does this influence the local memory such that your data are simply corrupted?

Georgios 2011-10-31

In my case, fft2 runs for less that a sec. I don't think this produces so much heat.

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Thomas 2011-11-2

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https://ww2.mathworks.cn/matlabcentral/answers/19609-gpu-wrong-results#answer_26600

在 MATLAB Online 中打开

Georgios, I ran it on our GPU pool

    gpuDevice
tic ()
a = rand(2^10,2^10,'single');
G = gpuArray(a);
F = fft2(G);
err = isequal(gather(G), a)
toc ();

The output was ans =

parallel.gpu.CUDADevice handle
Package: parallel.gpu
Properties:
                    Name: 'Tesla M2070-Q'
                   Index: 1
       ComputeCapability: '2.0'
          SupportsDouble: 1
           DriverVersion: 4.1000
      MaxThreadsPerBlock: 1024
        MaxShmemPerBlock: 49152
      MaxThreadBlockSize: [1024 1024 64]
             MaxGridSize: [65535 65535]
               SIMDWidth: 32
             TotalMemory: 5.6366e+09
              FreeMemory: 5.5281e+09
     MultiprocessorCount: 14
            ClockRateKHz: 1147000
             ComputeMode: 'Default'
    GPUOverlapsTransfers: 1
  KernelExecutionTimeout: 0
        CanMapHostMemory: 1
         DeviceSupported: 1
          DeviceSelected: 1
err =
     1
Elapsed time is 0.054194 seconds.