[gmx-developers] OpenACC Development

Millad Ghane mghane at cs.uh.edu
Fri Jul 1 20:21:31 CEST 2016

> Hi,
>> On 01 Jul 2016, at 19:42, Millad Ghane <mghane at cs.uh.edu> wrote:
>> BTW, PGI is saying
>> that they are actually transforming the OpenACC API/Pragmas into CUDA
>> code. So, practically, there shouldn't be much more performance loss on
>> GPU part.
> Based on what?

I am not saying there is no performance loss. There is, but the
performance loss shouldn't be more than like 30% "for the GPU codes".

> That’s like saying all C++ implementations of molecular dynamics should
> have the same performance because it’s the same language.
> If that was true, you should not see any performance difference when you
> disable SIMD. After all, all floating-point math on x86 is implemented
> with SSE/AVX instructions today.

It's different. By enabling and using SIMD commands you are actually
accessing and exploiting some hardware features of CPU. So, since you are
accessing high performance features, the code executes faster. But
changing languages with the same SIMD configuration, the output should be
roughly the same. However, I argue that in this case, C is a little bit
more faster than C++.

> My prediction is that OpenACC won’t even reach half our native CUDA
> performance, and then you’ll lose another 30% because PGI completely
> sucks on the CPU side.
> However, feel free to go ahead and prove us wrong ;-)


> Cheers,
> Erik
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