PCIe "lanes" and splitting them for Graphics cards

Question

I'm a sysadmin who is lost in the world of Graphics cards and PCIe lanes. Can someone please explain how it works without assuming that I know the basics of graphics cards?

Lets take the following specifications:

1. I want to find out out how many Nvidia 2080 Ti graphics cards I can run on a certain motherboard.
2. The motherboard in question is Z370-E as specced here: https://www.asus.com/Motherboards/ROG-STRIX-Z370-E-GAMING/

The specs say that the motherboard has

2 x PCIe 3.0 x 16 SafeSlots (Support x16, x8/x8)

1 x PCIe 3.0 x 16(at x4 mode)

4 x PCIe 3.0 x 1

Questions:

1. Are # of PCIe lanes associated with the processor or the chipset in the motherboard?

2. How many PCIe lanes does a Nvidia 2080 Ti graphics card need?

3. How can these lanes be split? As in the above example board,

Am I understanding correctly that, I can can have

only 1 PCIe x16 device,

2 PCIe x8 devices on the same second slot if I were to split the slot somehow..

4 PCIe x4 devices on the third slot if I were to find a splitter dividing the 16 lanes into four x4.

and finally 4 individual PCIe x1 devices in dedicated slots

Is the above correct?

Answer 1

Yes, your understanding about the available number of PCIe devices is correct. However, it doesn't necessarily mean you can have as many NVIDIA GeForce RTX 2080 Ti GPUs.

Are # of PCIe lanes associated with the processor or the chipset in the motherboard?

Both. In modern computers, both CPU and motherboard (the South Bridge, SB) can have PCIe connectivity.

As you presented a Z370 MB, I'm taking Intel Core i7-8700K as example. According to Intel ARK, it has 16 PCIe lanes built in, which is very likely connected to the two PCIe 3.0 x 16 SafeSlots. This means you can have at most 2 PCIe devices that can communicate with CPU directly. The other 8 PCIe lanes (1x4 + 4x1) belong to the SB and are intended for peripherals like NVMe SSDs or network cards.

That doesn't mean you can't connect RTX 2080 Ti GPUs to the SB, however. Continue reading these answers to more questions.

How many PCIe lanes does a Nvidia 2080 Ti graphics card need?

This depends. If you're using those cards for machine learning or cryptocurrency mining, a single PCIe lane may suffice. But I haven't heard someone using fresh flagship card for those tasks, so I'm assuming you're using them for gaming.

For gaming, it's best for the RTX 2080 Ti card to run on full PCIe 3.0 x16 speed, as it will maximize the utilization of its outstanding performance. Running such a card with only 8 lines is likely limiting its potentials, as the data transfer rate is going to be the bottleneck when processing high-quality textures, or other I/O intensive tasks. If you lower some expectations, you may stand its performance under only PCIe 3.0 x8 speed.

How can these lanes be split? As in the above example board

If you don't get some extra pieces of hardware, you have only two options: 1x16 and 2x8 for the CPU lanes, and 1x4 for the SB lanes. This is because only the two biggest slots are connected to the CPU, so you can split the CPU lanes to at most 2 devices. The last big slot can also be used for a 3rd card if you like.

Without additional hardware, you can have at most 3 cards on that board, as there are only 3 "big slots" on the board (also take into account there's no RTX 2080 Ti GPU with single-slot thickness). But with some splitters you may be able to get more. See this thread for more information.

Answer 2

First two x16-sized slots ("SafeSlots") are driven directly by the CPU. If only the first one is populated, it works as x16. If both are in use, they are both x8.

Other slots are connected to the chipset. Their latency will be higher, because chipset acts as a middleman between CPU and devices. The third x16-sized slot provides 4 lanes (x4). Four x1 slots provide one lane each. Connecting additional PCI-E devices to M.2 slots or USB type C ports will disable some PCI-E slots. (Note that not all M.2 and USB-C devices use PCI-E.)

As far as I know, no graphics card on the market can fully saturate 8 PCI-E 3.0 lanes. So two GPUs running with 8 lanes each won't be limited by PCI-E. 4 lanes won't be sufficient for high-end cards such as 2080 Ti if you want to use it for gaming.