Re: [ESPResSo-users] CUDA warning for slower atomicAdd emulation

[Vincent Ustach]

Thanks Georg, good news. I thought I would ask because atomic add is a slow operation, therefore I want to avoid making the slow step slower!

Cheers,

Vincent

On Jan 29, 2014 11:44 PM, "Georg Rempfer" <address@hidden> wrote:

Your GPU has compute_capability 2.0, as you can see here:

    http://en.wikipedia.org/wiki/Nvidia_Tesla#Specifications_and_configurations

2014-01-30 Georg Rempfer <address@hidden>

Hello Vincent,

the Espresso build system creates Cuda binaries for compute capability 1.1 as well as 2.0. Compute capability 1.1 does not allow for float atomic adds, so in this case we use a little workaround, which is slower than the native float atomic add present in later compute capabilities. You always get this warning message, unless you remove the "-gencode arch=compute_11,code=compute_11" from configure.ac". Most likely it has no impact on your performance, since Cuda determines at runtime which executable to use (depending on the compute performance of your Cuda device). So unless you have some pretty old GPU, you will be using the 2.0 binary, which in turn makes use of the native atomic add for floats.

Greetings,

Georg

2014-01-30 Vincent Ustach <address@hidden>

Hi All,

Upon running make on a new build of the developer's version of Espresso, I saw this and several similar warnings:

../../src/lbgpu_cuda.cu:123:2: warning: #warning Using slower atomicAdd emulation

Is it a major concern for losing performance, or since it is related to an emulation is it only for debugging?

See attached for the results of make. I have the configure results as well, if that will help. By the way I am using cudatoolkit-5.5 and the GPU card is a Tesla M2050 GPU

Best Regards,

--Vincent Ustach
  University of California, Davis