Out of PCIe slots? Here’s how to add another GPU anyway
Sometimes you just need an extra GPU. I’m not going to ask any questions. Even though SLI and CrossFire are dead and buried, there are still plenty of legitimate uses for a second GPU, and in fact, these use cases are increasing. Unfortunately, looking out the window of your computer case, you notice that your motherboard only has one PCIe slot long enough for a GPU.
The easiest solution is to buy a new motherboard, but not only is that expensive, it means effectively rebuilding your PC and that’s no fun. So before ordering a motherboard, there are a few alternatives to consider.
Use the M.2 slot with a PCIe riser adapter
Yes, really!
If your motherboard has open M.2 slots that work with NVMe drives, you can access the (usually) four PCIe lanes for that slot using a PCIe riser adapter.
Yes, this only gives your second GPU access to four PCIe lanes, but that’s not necessarily a problem. If we’re talking about PCIe 5.0 lanes, that’s the same bandwidth as eight PCIe 4.0 lanes or 16 PCIe 3.0 lanes. In other words, don’t worry too much about the number of lanes, but whether the total bandwidth is enough for this GPU to do the job you have in mind.
The only issue here is the number of PCIe lanes your CPU and motherboard offer. Filling up a spare M.2 slot can cause your PCIe 16x slot where your primary GPU resides to drop by half the speed. This is also only a problem if your GPU needs more bandwidth than that. So consult your motherboard and CPU documentation to check how many channels you have and how the motherboard distributes them depending on what is plugged in.
Thunderbolt/USB4
Vanilla options
If your motherboard (or an expansion card) has Thunderbolt over USB-C or USB4, you may have the option to purchase an external GPU enclosure that can accommodate any card you already have on hand. Assuming it matches the physical case and the power supply is up to par. Alternatively, you can purchase a dedicated eGPU which is an all-in-one solution but cannot be upgraded later.
Since this method gives you about four PCIe 3.0 lanes of bandwidth at best, it’s not worth connecting a particularly powerful card this way. Aim too high and you’ll simply be hampered by limited bandwidth. However, if you choose your card wisely based on the available bandwidth, you can still get balanced performance. If your GPU’s workload is not bandwidth dependent (e.g. AI models loaded entirely into VRAM), then this is not a problem and you can simply choose the best card you can afford.
Use OCuLink (if your card supports it)
Well, if you have it
OCuLink is pretty much the same as using Thunderbolt or USB4, but it provides more bandwidth for the eGPU’s needs and is generally the better choice. The problem is that your motherboard almost certainly doesn’t have OCuLink built-in. This is still relatively rare, especially on desktop systems.
However, that doesn’t make it irrelevant by a long shot. If you have an open PCIe 1x, 4x, or 8x slot on your motherboard, you can purchase an OCuLink PCIe card that will give you a proportional amount of bandwidth. For example, an OCuLink 8x card could provide eight lanes of PCIe 4.0 bandwidth. A lot for a powerful GPU.
USB graphics adapters (with major limitations)
This can work in a pinch
If the reason you’re trying to add a second GPU has nothing to do with GPU power and you’re just trying to connect more displays, you can use USB adapters to get the job done. There are a few options here. If you have USB-C ports that support DisplayPort Alt mode, that’s ideal. Simply get the adapter of your choice and get a direct PCIe tunnel to your GPU.
If you don’t have DP Alt Mode ports, you can use DisplayLink USB adapters, but these are software emulated, and while they are fine for web browsing and other basic tasks, don’t expect to play video games or anything that requires acceleration.
Turn another PC into a GPU node
The advanced solution
Another option exists, although it is somewhat unorthodox. You can use a second computer as a GPU node. Some software supports network rendering, so if you have a dedicated 2.5Gbps or 10Gbps connection to a second computer, you can offload some GPU tasks like video processing or anything that doesn’t necessarily have to happen in real time.
The setup can be complex and different for each task, but it is possible with some effort.
Before I forget, if you have an iGPU, you may have the option to enable it in tandem with your graphics card. Some modern iGPUs are quite capable and you can use them to do light tasks while using your main card for heavy tasks, if that configuration meets your needs.
