Taken from guru3d...
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New Multi GPU Modes
Compared to prior DirectX APIs, Microsoft has made a number of changes that impact multi-GPU functionality in DirectX 12. At the highest level, there are two basic options for developers to use multi-GPU on NVIDIA hardware in DX12: Multi Display Adapter (MDA) Mode, and Linked Display Adapter (LDA) mode. LDA Mode has two forms: Implicit LDA Mode which NVIDIA uses for SLI, and Explicit LDA Mode where game developers handle much of the responsibility needed for multi-GPU operation to work successfully. MDA and LDA Explicit Mode were developed to give game developers more control. The following table summarizes the three modes supported on NVIDIA GPUs:
In LDA Mode, each GPU’s memory can be linked together to appear as one large pool of memory to the developer (although there are certain corner case exceptions regarding peer-to-peer memory); however, there is a performance penalty if the data needed resides in the other GPU’s memory, since the memory is accessed through inter-GPU peer-to-peer communication (like PCIe). In MDA Mode, each GPU’s memory is allocated independently of the other GPU: each GPU cannot directly access the other’s memory. LDA is intended for multi-GPU systems that have GPUs that are similar to each other, while MDA Mode has fewer restrictions—discrete GPUs can be paired with integrated GPUs, or with discrete GPUs from another manufacturer—but MDA Mode requires the developer to more carefully manage all of the operations that are needed for the GPUs to communicate with each other. By default, GeForce GTX 1080 SLI supports up to two GPUs. 3-Way and 4-Way SLI modes are no longer recommended. As games have evolved, it is becoming increasingly difficult for these SLI modes to provide beneficial performance scaling for end users. For instance, many games become bottlenecked by the CPU when running 3-Way and 4-Way SLI, and games are increasingly using techniques that make it very difficult to extract frame-to-frame parallelism. Of course, systems will still be built targeting other Multi-GPU software models including:
- MDA or LDA Explicit targeted
- 2 Way SLI + dedicated PhysX GPU
Wait Just A Minute... So No More 3 & 4-Way SLI?
Correct, NVIDIA no longer recommends 3 or 4 way systems for SLI and places its focus on 2-way SLI only. However, for those of you that do want more than two GPUs, there is a way (albeit complex). However... meet the enthusiast key. While NVIDIA no longer recommends 3 or 4 way systems for SLI, they know that true enthusiasts will not be swayed and in fact some games will continue to deliver great scaling beyond two GPUs. For this class of user they have developed an Enthusiast Key that can be downloaded off of NVIDIA’s website and loaded into an individual’s GPU. This process involves:
- Run an app locally to generate a signature for your GPU
- Request an Enthusiast Key from an upcoming NVIDIA Enthusiast Key website
- Download your key
- Install your key to unlock the 3 and 4-way function
Full details on the process are available on the NVIDIA Enthusiast Key website, which will be available at the time GeForce GTX 1080 GPUs are available in users’ hands."