As games continue to become more and more graphically demanding, and other intensely taxing software applications are developed, users require a top-of-the-line graphics card to keep up.
Running sophisticated AI simulations, using advanced 3D modeling software, or running modern games at extremely high resolutions for example often requires more performance than what a single GPU can offer.
For these special performance requirements, both NVIDIA and AMD offer solutions where users can integrate and utilize multiple GPUs at once in their systems, in order to match the high computational power requirements of demanding games or software apps.
How Many GPUs Can Your PC Run at Once?
The answer is that the majority of PC Motherboards can support up to six GPUs at once, depending on your exact motherboard and GPU models. This will combine the performance of multiple GPUs into a single unit.
Note that having a bridge between the GPUs is not required to run them at once, as long as they are properly connected to the Motherboard and PSU power cables. When it comes to specialized purpose-built systems like for example crypto mining, a single rig can host up to 256 GPUs at once.
When Should You Use Multiple GPUs?
The only scenario where you would use multiple GPUs is when you connect two of the highest performing GPUs in order to solve a very specific performance requirement – such as complex 3D rendering, running of engineering physics simulations engines, or gaming at extreme resolutions and or refresh rates.
Unless your specific scenario calls for having 2, 3, or even 4 GPUs, it’s best to just stick to a single high-end GPU.
How to Use Multiple GPUs?
There are several ways to connect multiple GPUs. Some of the common ones are listed below.
Scalable Link Interface (SLI)
It is NVIDIAs first released technology that allowed users to combine multiple GPUs in a single system. Running a multi-GPU SLI setup can distribute the workload to multiple GPUs – in many scenarios boosting performance. It’s crucial to note that SLI only works when all of your GPUs have the exact amount of VRAM. Please note that SLI is not the latest technology available from NVIDIA as of 2022. You can read more about this in the section below.
SLI in particular works by scaling specialized computational elements such as geometry and texture fill rates by increasing the bandwidth of information flow between your hardware components. SLI links between GPUs offer significantly faster bandwidth than regular PCI express interfaces, in some cases by up to sixty times.
Crossfire is AMD’s proprietary multi-GPU technology. Much like the SLI technology, both AMD and NVIDIA are pushing away currently as support for the newest and future GPUs is speculated to be dropped. Crossfire will let you use either 2, 3, or four GPUs at once.
The biggest advantage of Crossfire over NVIDIAs offering is that with Crossfire you can pair GPUs with different clock speeds, VRAM, and even models, as long as they are built from the same architecture.
NVLink is the newest and most advanced technology that succeeds SLI from NVIDIA. It allows for multiple GPUs to essentially work as just a single unit, and is far superior in performance to SLI.
NVLink is tremendously faster than any past iterations of SLI, and it completely changes the ways that multiple GPUs interact with one another. This translates into up to 150 times better data transfer compared to the older SLI technology.
Most notably with NVLink there is little to no performance or latency penalty when multiple GPUs try to coordinate which GPU should process information. With SLI, the GPU in the first slot becomes the primary card, while the rest are just supporting.
Independent GPU Cards
The first and most common way for PCs to be set up is to have just a single GPU. When a single GPU is installed, it will run independently on its own and will compute all of the data itself. Whilst being the most predictable and power-friendly option for PC builds, it will never reach the full power of a multi-GPU setup.
Many GPUs can run independently in tandem, without the need for a bridge. Performance and stability might not be up to par, however. This setup depends exclusively on your exact hardware – motherboard and GPU compatibility and features. So in theory you don’t need SLI, NVlink, or Crossfire, but you will incur many downsides.
Why Single GPUs Are the Trend Right Now?
When it comes to building a brand-new PC, you should always look at the most powerful single GPU you can afford instead of looking at combining two cheaper ones with matching high-end GPU performance. This is due to the increased power consumption, lack of compatibility for some games and applications, and increased heat production.
Modern GPUs have gotten so advanced and efficient that the high-end models are designed to run games smoothly at incredibly high resolutions with ultra-high settings. SLI used to be a thing in the past. Since both NVIDIA and AMD have started to push towards single GPU setups and shy away from promoting multi-GPU rigs, the future driver and compatibility support for multi-GPUs aren’t as promising.
Additionally, as newer games and applications are developed, the support for SLI and Crossfire will dwindle down significantly.
What Are Pros and Cons of Having Multiple GPUs?
Here is a better breakdown of the pros and cons so you can get a gauge on whether it’s worth it for your specific use case:
- Near double the performance of just a single GPU in many cases
- Incredibly efficient for rendering and heavy 3D application use, great for AI simulations
- Offer better multi-monitor gaming capabilities
- Two mid-range GPUs together can almost match a high-end GPU
- Improved PC visuals
- Increased Electricity cost due to more power draw
- Heat increases within the case
- Lack of proper compatibility for some applications
- In rare cases can worsen stability and performance
- Requires a higher wattage PSU
Do Multiple GPUs Require an SLI/Crossfire Bridge?
Most of the newer GPUs can run in SLI or Crossfire without a bridge, but they would take a significant performance hit. These bridges refer to high bandwidth connector cables on each side of both GPUs to ensure smooth synchronization. We recommend that you always use an SLI/Crossfire bridge since the downsides of not having one don’t justify the small price savings.
How Does SLI VRAM Work?
You would think that if you use two NVIDIA 1650 GPUs in SLI, both having 4GB of GDDR6 memory, you would have a total of 8GB of effective VRAM, right? Wrong. VRAM does not work additively. The amount of VRAM is shared between the two GPUs, so if you have 4GB on one card, only 4GB of VRAM is available from both cards combined.
This is a very common and overlooked misconception and it has to do with how GPU architecture is built and made to run. Hopefully, in the future, one day VRAM will be additive, but until then, pay attention to the VRAM limit. This is one of the big reasons why buying two entry-level GPUs is not as good as getting a single mid-high range GPU. The single, more powerful GPU will often feature significantly more VRAM.
Don’t be discouraged though, if your games and applications only use a certain amount of VRAM, you will still get a significant FPS and performance increase in your game.
Can You Mix Different GPUs Together?
The only things that you can mix when it comes to SLI and Crossfire GPU configurations are using the same base GPU but from different manufacturers. For example, a GTX 1080 can only be run in tandem with another GTX 1080.
The only variable could be if suppose one of these cards is from EVGA while the other is from MSI. But be careful, as it’s always best to use the exact same model and type of GPU when running multiple ones.