Question about Physx and SLI

[2is]

I'm curious is PhysX effects on an SLI setup are rendered on one or both GPUs? I currently have PhysX settings set to "auto select" in the NVidia CP, but I don't know if that means it will auto select ONE of the two GPU's or if the PhysX load is shared between both.

I'm wondering because if it's only rendered on a single card, that could potentially bottleneck my performance in more graphically intensive game since one card is having to do more work than the other. In which case, I'd consider adding a dedicated PhysX card (like a 650ti) in my 3rd PCIe slot.

Which brings me to my next question. If I were to add a 2nd card like a 650ti to be used only for PhysX, would I need an SLI bridge for it?

 

[Deders]

When I had an SLI setup I found that PhysX was only calculated on one card, I seem to remember it only being needed to be calculated the once.

I do remember most games that used a lot of PhysX (Arkham Asylum, Mafia 2) or other GPGPU calculations (Just Cause 2) worked better if I disabled SLI. I especially found that Just Cause 2 with the Nvidia only features enabled would cause microstutter due to the difference in latency between the 2 cards as one had more work to do. it was pretty much the only game I noticed it in but then I disabled SLI for any games that used PhysX or Apex or anything like that.

This was back in the g80/g92 days, I was using 9800GTX+'s and since then the latency caused from switching between graphics commands and PhysX commands have been vastly improved with Fermi and Keplar cards but even so you might still get slight latency issues with SLI enabled.

Not sure if newer drivers have enabled different clusters on each GPU to handle different calculations to even things out or not as i've not had SLI for years.

You can check for yourself just by monitoring each GPU in game with GPUz and seeing if there is much of a difference in GPU usage between the 2 cards.

You wouldn't need an SLI bridge for a seperate PhysX card, only for identical cards you wanted to run in SLI, even then you can still SLI without one as it will use the PCIe bus to communicate but obviously at the expense of bandwidth.

 

[bystander36]

Using the auto select option has no effect on games that do not use GPU PhysX. So there is no harm in most all games to leave it on auto select. In a game that does use GPU PhysX, it will run PhysX on one of them, but the frame pacing hardware/software on newer cards should keep the frames even.

I'm not sure how well performance will be between leaving it on Auto, or disabling SLI and dedicating one card for GPU PhysX. In most cases, I've found auto works best, but there may be exceptions.

 

[2is]

Thanks for the info, and yeah the part about games that don't use physx not being affected i figured I'll be sticking with just the two cards. Just checked my board and the 3rd x16 slot is sandwiched between the other two and only one slot wide.