different result between gpuArray / matrix

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emar
emar el 2 de Mayo de 2017
Comentada: Joss Knight el 13 de Mayo de 2017
Hello, I tried to compare the result between two codes two inputs with the same function:
function local_sum_A = local_sum(A,m,n)
B = padarray(A,[m n]);
s = cumsum(B,1);
c = s(1+m:end-1,:)-s(1:end-m-1,:);
s = cumsum(c,2);
local_sum_A = s(:,1+n:end-1)-s(:,1:end-n-1)
It's a function that i found on Matlab. When i compile :
A=rand(20);
s=local_sum(A,21,22);
gpuDevice;
A2=gpuArray(A);
s2=local_sum(A2,21,22);
I have two different results ( difference by a factor 1e-13), but I do a lot of calculations after, so the difference become important (1e-02).
Could you tell me why there is such difference ?
Thank you in advance
  2 comentarios
Adam
Adam el 2 de Mayo de 2017
gpu does calculations differently to the cpu so there will be different levels of accuracy. 1e-13 is insignificant, but the more calculations you do certainly the more the error will inevitably accumulate.
Joss Knight
Joss Knight el 13 de Mayo de 2017
To be clear, the error here is between the computed result and the theoretically correct result, not between the CPU result and the GPU result. The CPU result is no more correct than the GPU one. All floating point computation has error, and this error always accumulates the more operations you do.

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