[SOLVED] Why does combining GPUs cause a decrease in the FPS ?

[thecringytechguy]

So if you take any SLI GPUs (for example 2 x RTX 3090s in SLI) and connect it to 4 x 8k monitors, then it would be 16k.
But my question is not about 16k or any insane setups, but I wanted to know why combining 2 x GPUs using Nvdia Surround, the FPS decreases?
I mean if the display resolution increases then why not the FPS too?

 

[Deleted member 2731765]

If you are specifically talking about the GAMING performance in SLI mode, then that's a different story. Not many games SCALE well on any SLI/CFX setup, and some don't even benefit 1% gain in performance, despite what you do with your rig. The resolution is not always directly related to the increase in game FPS. There are other factors as well.

Most importantly, scaling depends on the Game, and the ENGINE which it is using. The developers need to code it individually. But, what all PC games have tried playing in SLI Mode ? List all the names of these titles.

I usually don't recommend using any multi-GPU setup, be it SLI or CrossFireX. Gone are the days when multi-GPU setup was a norm. DX12 multi-GPU explicit support is a different story though. Like mentioned above, not many PC games scale that well on such a setup. Depends on the Game's Engine/code though, but you would be much better off using a SINGLE powerful GPU instead. Less hassle, less power consumption, less heat, and better performance in games as well.

SLI and CrossFireX technologies are kind of dead in the water, and developers have been slowly abandoning these multi-GPU technologies, due to lack of proper scaling seen in games, and also because of some driver issues, among other factors like adoption rate. Each game developer also needs to code the game accordingly to take proper advantage of any SLI or CFX setup. However, DX-12 explicit multi-GPU support feature is not totally abandoned, but not many game developers have been using this m-GPU DX12 technology as well (might change in future).

This will only help if GAMES are also going to take advantage of any new multi-GPU feature, provided the Game developers implement this in DX12 API. Using a SINGLE powerful GPU is more likely a much better and feasible option for any high-end Gaming setup, instead of using two cards in tandem. Multi-GPU setup is usually not recommended for obvious reasons

 

[chaoyang]

So if you take any SLI GPUs (for example 2 x RTX 3090s in SLI) and connect it to 4 x 8k monitors, then it would be 16k.
But my question is not about 16k or any insane setups, but I wanted to know why combining 2 x GPUs using Nvdia Surround, the FPS decreases?
I mean if the display resolution increases then why not the FPS too?

Not all games or design software are optimised for Multi-GPU processing. A lot of additional calculations are required to sync frames together, which can decrease performance and becomes counter-productive if the games you are playing is unoptimised for it.

 

[hftvhftv]

If you are specifically talking about the GAMING performance in SLI mode, then that's a different story. Not many games SCALE well on any SLI/CFX setup, and some don't even benefit 1% gain in performance, despite what you do with your rig. The resolution is not always directly related to the increase in game FPS. There are other factors as well.

Most importantly, scaling depends on the Game, and the ENGINE which it is using. The developers need to code it individually. But, what all PC games have tried playing in SLI Mode ? List all the names of these titles.

I usually don't recommend using any multi-GPU setup, be it SLI or CrossFireX. Gone are the days when multi-GPU setup was a norm. DX12 multi-GPU explicit support is a different story though. Like mentioned above, not many PC games scale that well on such a setup. Depends on the Game's Engine/code though, but you would be much better off using a SINGLE powerful GPU instead. Less hassle, less power consumption, less heat, and better performance in games as well.

SLI and CrossFireX technologies are kind of dead in the water, and developers have been slowly abandoning these multi-GPU technologies, due to lack of proper scaling seen in games, and also because of some driver issues, among other factors like adoption rate. Each game developer also needs to code the game accordingly to take proper advantage of any SLI or CFX setup. However, DX-12 explicit multi-GPU support feature is not totally abandoned, but not many game developers have been using this m-GPU DX12 technology as well (might change in future).

This will only help if GAMES are also going to take advantage of any new multi-GPU feature, provided the Game developers implement this in DX12 API. Using a SINGLE powerful GPU is more likely a much better and feasible option for any high-end Gaming setup, instead of using two cards in tandem. Multi-GPU setup is usually not recommended for obvious reasons

Exactly, the last game I played that worked nearly perfectly was GTA V with my old SLI GTX 770 setup.

 

[thecringytechguy]

If you are specifically talking about the GAMING performance in SLI mode, then that's a different story. Not many games SCALE well on any SLI/CFX setup, and some don't even benefit 1% gain in performance, despite what you do with your rig. The resolution is not always directly related to the increase in game FPS. There are other factors as well.

Most importantly, scaling depends on the Game, and the ENGINE which it is using. The developers need to code it individually. But, what all PC games have tried playing in SLI Mode ? List all the names of these titles.

I usually don't recommend using any multi-GPU setup, be it SLI or CrossFireX. Gone are the days when multi-GPU setup was a norm. DX12 multi-GPU explicit support is a different story though. Like mentioned above, not many PC games scale that well on such a setup. Depends on the Game's Engine/code though, but you would be much better off using a SINGLE powerful GPU instead. Less hassle, less power consumption, less heat, and better performance in games as well.

SLI and CrossFireX technologies are kind of dead in the water, and developers have been slowly abandoning these multi-GPU technologies, due to lack of proper scaling seen in games, and also because of some driver issues, among other factors like adoption rate. Each game developer also needs to code the game accordingly to take proper advantage of any SLI or CFX setup. However, DX-12 explicit multi-GPU support feature is not totally abandoned, but not many game developers have been using this m-GPU DX12 technology as well (might change in future).

This will only help if GAMES are also going to take advantage of any new multi-GPU feature, provided the Game developers implement this in DX12 API. Using a SINGLE powerful GPU is more likely a much better and feasible option for any high-end Gaming setup, instead of using two cards in tandem. Multi-GPU setup is usually not recommended for obvious reasons

thanks for the answers but I was wondering if it would also combine the vram of the gpu's

 

[DSzymborski]

thanks for the answers but I was wondering if it would also combine the vram of the gpu's

No.

 

[Deleted member 2731765]

Nope. Basically in traditional SLI and CrossFireX setups, the GPU's VRAM doesn't get stacked, which means if you have 2 cards with 4GB VRAM, then the game won't see it as a single 8GB Frame Buffer. Nope. Each card will still have it's own buffer as seen by the system.

But to answer your question regarding the RTX 3090 VRAM stacking in NVLINK mode, the VRAM "stacking" is only supported in DX 12's EMA (explicit multi adapter) mode. When EMA is used, each GPU's memory is accessed individually, carrying only what it needs for its specific task. So, the RAM doesn't "stack" per se, but rather, is assigned different tasks, but that's semantics. In an EMA situation, VRAM on all utilized cards is available.

VRAM from both of your cards can be utilized independently, but only when using DX12's EMA mode. But, there are still many caveats with this EMA mode as well. I'm not 100% sure either, since I have not been following this news since long. It needs to be coded as well. Currently, few games exist that uses this feature, like for example, "Ashes of the Singularity"/AOTS. But, I haven't been following that game so I don't know for sure.

Okay, let me share this as well. We all know SLI days are over, but what about NVLink ? You must be aware that NVidia has introduced a new interface called NVLINK with the consumer Turing GPUs, instead of the old SLI. Obviously, it's the same multi-GPU bridge which can be used for gaming, but it has an interface with many times the bandwidth of an SLI connection.

Because NVLink can be used for direct memory access between cards, and not through the PCIe slots as this was creating a huge bottleneck with SLI. So I think NVlink is the future, if we go by Nvidia's theory. But I could be wrong as well, because not many Games might be able to reap the full benefits of NVlinK, because the same thing happened with SLI.

SLI bridges mostly used to have a bandwidth of 1GB/s (normal bridge), and 2GB/s (for the HB bridge), with a rough estimate. NVLink on Turing cards for example, can do 25GB/s one way, and or 50GB/s in total. But according to Nvidia, total bandwidth is 50GB/s one way, and 100GB/s total.

But all of this will only help, if GAMES are going to take advantage of this new multi-GPU feature, provided the Game developers also implement this. I think the main advantage of Nvlink is that it might help with peer-to-peer interface, VRAM stacking, because essentially the GPUs are much closer together now, also bringing the latency of a GPU-to-GPU transfer way down, IMO.

So unlike SLI, where the latency had to go through PCIe as well as memory, Nvlink behaves in a different manner. We can think of it an app that looks at one GPU, and then looks at another GPU and does something else same time. So it seems NVlink will be the future when it comes to multi-GPU setup, but sadly ONLY on the high-end GPU market segment, as other cards will lack NVLINK support. I'm still not fully sure about how this DX12 VRAM stacking will work in future.

But again, like I said before, all of this will actually depend on how well the Game's ENGINE benefits from a future multi-GPU setup. Also, assuming NVLINK will also help with VRAM stacking, the 2 GPUS should support Split Frame rendering/SFR. Unlike the previous AFR mode used mostly in SLI, Alternate frame rendering that is, in which each GPU used it's own frame buffer/VRAM, and it never got added/stacked.

According to theory,

In AFR, each GPU renders each of the other frame (either the alternate Odd or Even).
In SFR, each GPU renders half of every frame. (top/bottom, or plane division).

So I think NVLINK should also help with VRAM stacking, though we need to see how this gets implemented fully in most of the Games, either in DX12 or VULKAN API mode. Apart from this, even the price of an NVLINK bridge is kind of high, so this can be a very expensive multi-GPU setup, and not many gamers might be able to afford these. I STILL prefer having a SINGLE powerful GPU on my rig, because a lot of games don't scale well on SLI/CFX.

 

[Joseph_138]

W

So if you take any SLI GPUs (for example 2 x RTX 3090s in SLI) and connect it to 4 x 8k monitors, then it would be 16k.
But my question is not about 16k or any insane setups, but I wanted to know why combining 2 x GPUs using Nvdia Surround, the FPS decreases?
I mean if the display resolution increases then why not the FPS too?

When the resolution increases, performance goes down, not up. There are more pixels that have to be manipulated. That's why when you see video cards being tested, 4k scores are always lower than 1080p scores.

 

[Deleted member 2731765]

Exactly. The resolution is not always the deciding factor for any game's FPS. There are many more variables involved.

 

[gamerk316]

The main problem with multi-GPU is you have the fundamental problem that one GPU is always the master; all data needs to make it's way to that particular GPU to enable a frame to be output. So even if you have a second GPU doing additional work, that second GPU still needs to transmit it's data back to the first one, and this takes time, bandwidth, eats a TON of VRAM, and slows the first one down again.

Farther, DX12/Vulkan changed how multi-GPU works. Previously, you could just enable a setting in the GPU driver and call it a day, hoping one of the existing profiles would do "good enough" to get you scaling. Now, it's up to the developers to support implementing, and worse then that, they need to cover every potential use case. User wants to pair a NVIDIA 730 GT with an AMD 6800 XT? Gotta support it. For obvious reasons, devs chose not to bother.

 

[Deleted member 2731765]

Farther, DX12/Vulkan changed how multi-GPU works. Previously, you could just enable a setting in the GPU driver and call it a day, hoping one of the existing profiles would do "good enough" to get you scaling. Now, it's up to the developers to support implementing, and worse then that, they need to cover every potential use case. User wants to pair a NVIDIA 730 GT with an AMD 6800 XT? Gotta support it. For obvious reasons, devs chose not to bother.

+1. Exactly.

Btw as a side note, I was doing a bit of search, scouring the web, and it appears that currently the following games do indeed benefit from an SLI setup. Shadow of the Tomb Raider, Red Dead Redemption 2 (in the Vulkan API), Quake 2 remaster with RTX, Battlefield 1 (in DirectX 12 mode), Gears of War 4 to some extent, Strange Brigade, and Deus Ex: Mankind Divided.

It appears that Battlefield V no longer supports SLI. There are more random results posted online and the results are kind of a mixed bag. But still not many PC games take advantage of SLI these days.