[gmx-users] why two GTX780ti is slower?

[Albert]

Hello:

I've compiled Gromacs in a GPU machine with two GTX780Ti with following 
command:

env CC=icc F77=ifort CXX=icpc 
CMAKE_PREFIX_PATH=/soft/intel/mkl/include/fftw:/soft/intel/impi/4.1.3.049/intel64:/soft/intel/mkl/lib/intel64 
<http://4.1.3.049/intel64:/soft/intel/mkl/lib/intel64> cmake .. 
-DBUILD_SHARED_LIB=OFF -DBUILD_TESTING=OFF 
-DCMAKE_INSTALL_PREFIX=/soft/gromacs-5.0rc1 -DGMX_MPI=ON -DGMX_GPU=ON 
-DCUDA_TOOLKIT_ROOT_DIR=/usr/local/cuda


I've got 40ns/day for a 65,000 atoms membrane system under CHARMM36 FF 
when one GPU is used, but it is only 27ns/day if I used two GPUs. In 
another machine with two GTX690, I used ICC+openmpi instead of ICC+intel 
MPI. The same system, I obtained 40ns/day for one GPU, and 60ns/day for 
two GPU.

I am just wondering, why it is even slower with two GTX780Ti for my 
system, which doesn't happen to GTX690?

thx a lot

Albert