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Some comments and tweaks for SLI by LM

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A quick note on SLI:

- If you have multiple monitors and multiple views, then you'll generally be better off driving a display from each video card than using SLI. If you have 2 cards and 1 monitor than it's worth giving SLI a shot.

- We're still waiting on NVidia to create an SLI driver profile for Prepar3D v2 so the drivers may not be picking the best SLI mode for our app be default.

- We still have more work to do to improve SLI performance, but we chose to focus on autogen vegetation performance for this release as it was a big contributor to OOMs and CPU boundedness.

- If you want to check out what the best-case improvement of SLI might be in the future, you can try this:

1. Make a copy of your Prepar3D.exe and rename it AFR-FriendlyD3D.exe. Now make a new shortcut to the AFR-FriendlyD3D.exe and launch that. This is a developer trick for telling the SLI drivers not to copy any resources between GPUs each frame. (See NVidia's SLI Best Practices Doc for more info: In a perfect world, we wouldn't need any resources to be copied and this would result in a nice perf boost with no side effects. In the real world, we do have some features that rely on resources that persist frame-to-frame. HDR is one of them so you'll probably want to turn that off to avoid getting an odd pulsing effect. You may also see some garbage geometry streak across the screen here and there. (at least that's what I was seeing on a dual titan rig with the drivers I was using at the time.)


Also, any time you enable/disable SLI in the driver settings you should clear out your shader cache or you may get driver crashes. This really shouldn't happen since cached shaders are compiled in a non-device-specific way, but it does seem to happen nonetheless. This is also sometimes true of doing driver updates.


Beau Hollis

Rendering System Lead - Prepar3D® Team


As per the thread here:


So any if you with SLI and game enough to test it out, there may be some beneficial SLI tweaks for you.

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