The state of Multi GPU (SLI/Crossfire) in DX12

[VincentHanna]

So we're finally starting to see a trickle of DX12 titles available to play:

Gears of War: Ultimate Edition
Rise of the Tomb Raider
HITMAN
Ashes of the Singularity

Now, I know it's very early days but I thought DX12 was supposed to make it easier for developers to integrate Multiadapter/Multi-GPU options to the point where you can even run AMD and Nvidia GPU's together.

Having tested 3 of the 4 titles listed above (I don't own Ashes of the Singularity) it seems that none of them currently support SLI using the DX12 renderer.

I understand the controversy over the Windows 10 store restrictions which could impact Gears of War, and it seems Crossfire won't work on any Windows store games until they allow the games to run Exclusive Fullscreen (correct me if I am wrong)?

But with regards to both HITMAN and Rise of the Tomb Raider, they work fine with SLI in DX11.

Tomb Raider in particular always shows 99% usage on both my Titan X GPU's in DX11 and yet MSI afterburner confirms only one GPU in use in DX12

I only ran the HITMAN benchmark quickly, and can see SLI needs work as the usage fluctuates wildly, but again it is utilizing both graphics cards in DX11 and only one in DX12

So is this currently a developer issue or do Nvidia/AMD need to release some updated DX12 specific drivers with SLI/Crossfire profiles?

 

[VincentHanna]

But if multi GPU works in your game in DX11, why doesn't it work in DX12?

 

[VincentHanna]

It can't just be drivers. They couldnt get Arkham Knight working properly with SLI.

 

[VincentHanna]

HITMAN runs better in DX12 than DX11 SLI, despite the frame rates being objectively higher overall with dual GPU's

 

[VincentHanna]

I missed this Anandtech article last week discussing the state of DX12 with Microsoft and Oxide Games (Ashes of the Singularity)

http://www.anandtech.com/show/10136/discussing-the-state-of-directx-12-with-microsoft-oxide-games

Some interesting bits from the interview:

Baker said that Oxide’s next game may go DX12-exclusive, as adoption is strong and doing so would give Oxide’s developers the freedom to implement some new rendering strategies that they can’t properly implement in a game that needs to support both DX11 and DX12.

So it sounds like the real gains will comes in a few years once we are out of the transitional period whereby games support both DX11 and DX12 due to gamers sticking with Windows 7/8.1

The low-level nature of DX12 means that more control over optimizations will be in the hands of developers – and they will need to rise up to the challenge for best results – as opposed to video card drivers.

memory management under DirectX 12 is still a challenge, albeit one that’s evolving. Under DirectX 11 memory management was typically a driver problem, and the drivers usually got it right – though as Baker noted in our conversation, even now they do sometimes fail when dealing with issues such as memory fragmentation. DX12 on the other hand gives all of this control over to developers, which brings both great power and great responsibility. PC developers need to be concerned with issues such as memory overcommitment, and how to gracefully handle it. Mantle users will be familiar with this matter: most Mantle games would slow to a crawl if memory was overcommitted, which although better than crashing, is not necessarily the most graceful way to handle the situation.

Lets hope that developers can rise to that challenge, as it sounds like the gains we used to get with new drivers may become a thing of the past.

While every game will be unique, in the case of Ashes Oxide has already run into situations where they are both CPU memory bandwidth and CPU core count limited. Much of this has to do with the game’s expensive AI and simulation code paths, but as Baker was all too proud to recount, Ashes’ QA team had to go track down a more powerful system for multi-GPU testing, as their quad core systems were still CPU limited. DX12’s low-level nature is going to reduce CPU usage in some ways, but with its multithreading capabilities it’s going to scale it back up again in other ways that may very well push the limits of conventional quad core CPUs in other games as well.

Still sounds like those 6/8 core systems might come in to their own in the future

 

[VincentHanna]

Nvidia has the biggest PC market share, but the console's both run AMD GPUs.

Really DX12 looks to favour AMD at the moment as most games are developed for the PS4 and ported over.