PCIe lanes: hidden sources of nasty bottlenecks
When building a new gaming PC, PCIe lanes are definitely not something people tend to stress about. Unfortunately, in some cases they do lead to some surprises. I personally know three situations where bottlenecks can happen, which I will explain so you can avoid your own share of surprises.
GPU bottlenecked by motherboard slot or CPU
PCIe bottlenecks mainly concern the GPU, as it requires the most PCIe bandwidth of any component. The motherboard's PCIe slot must match the bandwidth requirement of the GPU to avoid a bottleneck. I have provided a table below for reference. You should notice some patterns in the table: every new PCIe version has twice the bandwidth capability. PCIe 5 x8 = PCIe 4 x16, and so on.
4060 and 5060 owners beware: this unfortunately doesn't mean that plugging a PCIe 5 x8 card into a PCIe 4 x16 slot will be fully compatible. The card will run at PCIe 4 x8 speeds, as it's only physically touching 8 of the connectors.
Now let's look at the CPU. The GPU - CPU communication happens through PCIe, and the CPU has dedicated lanes for this communication. More specifically, an exact number of lanes dedicated for communication to the GPU. This is why issues can happen, as in some rare cases a CPU does not have 16 lanes dedicated to the external GPU. The most notable example of this is the Ryzen 8000G series, of which the 8500G and 8300G only has 4!! PCIe lanes dedicated to the GPU. The 8000G series was not designed to be used with an external GPU, and it will cause a serious bottleneck.
| PCIe Version | x1 Lanes | x4 Lanes | x8 Lanes | x16 Lanes |
|---|---|---|---|---|
| PCIe 3.0 | ~1 GB/s | ~4 GB/s | ~8 GB/s | ~16 GB/s |
| PCIe 4.0 | ~2 GB/s | ~8 GB/s | ~16 GB/s | ~32 GB/s |
| PCIe 5.0 | ~4 GB/s | ~16 GB/s | ~32 GB/s | ~64 GB/s |
NVMe SSD leeching lanes from the GPU
Some motherboards share PCIe lanes between NVMe SSD slots and the primary GPU slot. If the GPU and SSD are installed in shared slots, the SSD can take lanes away from the GPU. This will force the GPU to run at a lower speed. This can also happen with other PCIe devices like a PCIe soundcard.
When are these bottlenecks relevant?
A PCIe connection gets bottlenecked, if the data load is bigger than the the available bandwidth. In most games, the PCIe bandwidth will be plenty. The bottleneck usually happens, when the GPU's VRAM is nearing it's maximum capacity (newer titles, creative work, machine learning). In this case, the GPU will use the system's ram, transmitting data to the RAM and back to the GPU. It's not just games with a high VRAM usage that can create a bottleneck though. Some games just stream a lot of new data to the GPU, without using the whole VRAM.
What is the performance impact of PCIe bottlenecks?
From benchmarks it can be seen, that the in case of the bandwidth being fully saturated, the performance will decrease in a roughly linear fasion with the available bandwidth. Let's say you plug a PCIe 4 x16 GPU into a PCIe 3 x16 slot. In a scenario that requires the entire PCIe 4 bandwidth, expect to lose about 50% of performance due to the bottleneck.
Conclusion
- Check the PCIe spec of your motherboard
- Check the PCIe spec of your CPU
- Buy enough VRAM for your use case