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CUDA-x86.

syntax error ?
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alechand



Joined: 14 May 2012
Posts: 21

PostPosted: Fri May 24, 2013 4:58 pm    Post subject: Reply with quote

Thanks Mat,
i sent you an email.
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mkcolg



Joined: 30 Jun 2004
Posts: 6122
Location: The Portland Group Inc.

PostPosted: Tue May 28, 2013 8:54 am    Post subject: Reply with quote

Hi alechand,

It appears to be the difference is being caused by accumulated rounding error when using fused-multiply-add (FMA) instructions. The same issue can be seen on the CPU when using higher optimizations. Try adding the flag "-ta=nvidia,nofma" to see if this helps.

- Mat
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alechand



Joined: 14 May 2012
Posts: 21

PostPosted: Tue May 28, 2013 11:01 am    Post subject: Reply with quote

Mat,
unfortunately, this did not help.

I was thinking, if the results agree "exactly" using TIME=100000,
how can be accumulated errors ?

Do you have other idea?
I really appreciate your attention.
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mkcolg



Joined: 30 Jun 2004
Posts: 6122
Location: The Portland Group Inc.

PostPosted: Tue May 28, 2013 1:45 pm    Post subject: Reply with quote

We were able to trace this down. It looks like that at least a few x(i) and xold(j) values are within a small margin of error difference. With slight changes in precision in these cases, the dominant value may flip-flop leading to divergent values of c1 and c2. This then has a cascading effect which leads to the eventual wrong answer.

- Mat
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