Shadowplay using secondary GPU in SLI rig?

Shadowplay has many benefits for Nvidia users. Not the least of which is lesser impact on gaming performance than other recording software. That said there is still a noticeable impact. So an idea came mind, but I have no idea if its possible (or how to implement it if it is possible).

In a SLI rig can you disable SLI and game on the primary GPU, while recording with Shadowplay on the secondary GPU? (similar to using Physx on a second GPU with SLI disabled).

 

ShadowPlay utilizes the NVENC H.264 hardware encoder and CUDA for compression and processing. CUDA is not SLI-optimized. This is the same reason that GPU Transcode (which leverages CUDA) in id Tech 5 games such as Rage and W:TNO is incompatible with SLI unless you force CUDA to a single GPU in Nvidia Control Panel.

Therefore, in an SLI configuration, ShadowPlay only uses the primary GPU for recording. You can verify this by checking video engine load in GPU-Z or HWiNFO64. You'll notice that only the first card is loaded.

Since ShadowPlay uses CUDA on the primary GPU, the chunk of GPU performance taken will be inversely proportional to the number of CUDA cores you have on a single card. A single 650M is definitely on the low-end of the spectrum, so the performance hit will be greater than on higher-end cards.

The CUDA part of ShadowPlay directly hits the GPU core load. On my overclocked and overvolted 650M SLI setup, it's something like 20-25% of GPU usage taken up by ShadowPlay, which is not an insignificant number in demanding games.

There's also been an unresolved bug in Nvidia's drivers since last November that causes an abnormally high performance hit with ShadowPlay in SLI compared to single GPU. A lot of SLI users running a wide range of setups have reported on this.

ShadowPlay (and any kind of NVENC-based recording such as MSI Afterburner or OBS) has to be done on a GPU that's doing the graphics rendering. The dedicated PhysX card concept is different because you're offloading non-rendering physics calculations to another GPU that's not rendering.

 

Hmmm thanks for the information Octiceps. It's unfortunate that such a concept is impossible.