[gmx-users] Performance of NVIDIA GTX980 in PCI-e 3.0 x8 or x16 slots ?

[Mirco Wahab]

On 11.06.2015 14:07, David McGiven wrote:
> Your 1-3% claim is based on the webpage you linked ?
>
> Is it reliable to compare GPU performances for gromacs with those of 3D
> videogames ?

OK, you got me on this. As much as I'd wish I cannot
really back up my claim of comparability. I have been
out of office for one week but did some tests here
for myself today.

The system is Haswell/E (i7-5820K), GPU is single GTX-980
(the "normal" Gigabyte Model), the test run is ADH-cubic-vsites
(reaction field) from the bottom of the Gromacs acceleration page
(http://www.gromacs.org/GPU_acceleration).

I can explicitly set the PCIe-x16 slots to 1.0, 2.0, and 3.0
(which I did). Theoretically (and practically), PCIe-x16 2.0
should be relatively  close in bandwidth to PCIe-x8 3.0, so this
should give some hints as what to expect.

adh-cubic-vsites/rf - ns/day:
   PCIE-x16/1.0	54.46	
   PCIE-x16/2.0	61.81	
   PCIE-x16/3.0	64.52	

percentage related to PCIE-x16/3.0
   PCIE-x16/1.0	84.4
   PCIE-x16/2.0	95.8
   PCIE-x16/3.0	100

(each value = avg. of three runs)

Therefore one could support the hypothesis
that using one card in x16 and one in x8
would probably show a performance penalty
of around  5% on the x8 card.

Regards

M.