Why is Second x16 Slot in SLI PCIe x8?

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The simplest reason for the second SLI PCIe slot to be x8 is because PCs have a limited amount of PCIe lanes coming from the CPU and therefore, they have to be shared among the two graphics cards.

A full x16 slot uses 16 lanes. If you have two full x16 slots that will amount to 32 lanes and no commercial computers have that many amount of PCIe lanes available!

Hence, while ideally all graphics cards are built to occupy 16 lanes, in the case of SLI, the CPU lanes are divided equally among the top and second PCIe slot i,e x8/x8.

Fortunately, most of graphics cards do not fully saturate an x16 PCIe connection. In fact, even a high end graphics card will not saturate the v3.0 x16 PCIe slot let alone the newer PCIe v4. Hence dividing the lanes count often has minimal affect on the overall performance. However, in certain situations the different can be quite noticeable.

In the following text we talk in detail about why is second x16 slot in SLI PCIe x8, how the motherboard slots are configured and whether it has any effect on the GPU performance.

So What is the x16 Slot and its Type?

does it matter which PCIe slot I use

Image: The second PCIe x16 slot is NOT ALWAYS x8. It could also be x4, as is the case with the Gigabyte GA-P67A-UD3 motherboard above. Always be sure to check the motherboard specsheet out.

x16 slot is the largest slot that is you can find on any motherboard. However, it is also the most confusing in terms of how many PCIe lane count it offers.

There are basically three types of PCIe x16 slot,

  1. Full x16 Slot – This is the primary x16 slot with 16 PCIe lanes. If you have single graphics card, this is the slot you want to use. The first PCIe x16 slot always offers full 16 lanes coming from the CPU.
  2. x16(x8) – The x16(x8) is the topic of this article today. This is often found on motherboards built of NVIDIA SLI setup. This is not the primary, but the second x16 slot on your motherboard. This slot also receives lanes from the CPU, but shares the lanes with the primary x16 slot.
  3. x16(x4) – This is a an x16 slot that merely has 4 PCIe lanes coming from the motherboard chipset, not from the CPU. This is used for medium sized cards like an SSD expansion card etc.

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How are SLI Motherboards Configured?

MSI MPG Z490 Gaming Carbon WiFi

Image: MSI MPG Z490 Gaming Carbon WiFi

The MSI MPG Z490 Gaming Carbon WiFi motherboard above is a great example of how an SLI motherboard with multiple x16 slots but with varying lane count is configured.

Here the first x16 slot has 16 physical lanes and the second has 8 physical lanes. Both the first and second x16 slots get their PCIe lanes from the CPU. The third slot at the bottom is an x4 slot and receives its lanes from the mother chipset (Z490 chipset)

Now for an NVIDIA Graphics Card to work, it requires at least an x8 slot! Hence, plugging an NVIDIA Graphics card in the last x16(x4) slot will not allow it work.

This motherboard essentially operates in the following mode as far as its x16 slots are concerned:

  1. Mode 1: x16/0/x4
  2. Mode 2: x8/x8/x4

The key takeaway here is that if you only have a SINGLE graphics card, use the top x16 slot.

Also, whether you install a graphics card in the top x16 slot or leave it empty, occupying the second x16(x8) slot will clock the first x16 slot to x8 bandwidth as well in any case.

So Why is Second x16 Slot in SLI PCIe x8?

To understand why is second SLI PCIe 8, it’s crucial to have an understanding of what PCIe lanes are and how they matter to the performance of an SLI setup.

What are PCIe Lanes?

PCIe slots transmit data through channels that are known as PCIe lanes. The lanes run from the PCIe slots and offer an unshared connection to the CPU and the motherboard chipset. Since they have high data throughput rate (speed), the are used by high speed devices like a graphics cards.

However, there are a limited amount of PCIe lanes that come from the CPU, which are the primary drivers of the graphics cards. A typical processor has 16 – 24 PCIe lanes depending on whether you are using the Intel or AMD, but not all of these lanes are user accessible.

As a rule of thumb at the moment, any given commercial processor offers 16 lanes for the graphics card.

Intel Core i7-10700K

Image: PCIe lanes on Intel Core i7-10700K

Only the workstation grade processors like the Intel X-Series processor or the AMD Threadripper processors, which can cost as much as an entire gaming desktop itself, have very high lane count.

Intel Core i9-9980XE lane count

Image: Intel Core i9-9980XE boasts 44 PCIe lanes.

So on workstation grade desktop you can have multiple x16 slots with full 16 lane bandwidth. However, workstation desktops are not for an average Joe and hence not the topic of today’s discussion.

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Motherboard Lane Limitations for SLI

Like with CPUs, with motherboards you also get a fixed number of PCIe lanes coming from the chipset. The amount of PCIe lanes that you get from the motherboard depends upon the chipset.

However, the maximum amount of lanes bifurcated by motherboard at a time for a single PCIe slot is 4. NVIDIA graphics card, as mentioned earlier, require at least a single x8 slot.

Hence, the motherboard PCIe lanes are not suitable in this situation.

Brief Note on AMD Crossfire GPU Which Can Use 4 Lanes

Just as NVIDIA has SLI, AMD offers an alternative for their multi GPUs setup known as Crossfire.

The best part about AMD crossfire is that it can be done on a mere x4 slot and thus on relatively cheap motherboard you can achieve a dual graphics card setup. Since AMD GPU can work on 4 lanes, it can use the x4 slot coming from the motherboard as well.

Albeit the performance of a graphics card is greatly reduced in an x4 slot.

Is There Any Performance Difference When Using Graphics Card on X8?

Yes, there can be a massive difference between using the cards in x16 or x8 slot. However, it all depends upon the load and the application you run.

It also depends upon whether you have a single graphics card in the x8 slot or if you have SLI configuration x8/x8.

PudgetSystems Performance Report

Pudgest Systems has a comprehensive study on this subject. They used an NVIDIA Titan X for their study.

The results they found were mixed and quite dependent on the situation. Having a single graphics card in the x16 or in the x8 slot does not make a big difference. For instance, on games and certain applications like Davinci Resolve, the performance difference between plugging in a single card in the x16 or the x8 slot was minimal.

When running DUAL graphics cards, things start to become more interesting. On DUAL graphics cards, you could see upto 30% performance drop when using the two in x8/x8 slot instead of x16/x16 slots.

Read the full study here.

GamerNexus Performance Report

Gamersnexus.net has a great guide on this as well where they explore the performance drop of a single graphics card. Here they test the MSI GTX 1080 Gaming X graphics card on various game using both the x16 and x8 PCIe slot.

why is second x16 slot in SLI PCIe x8

Image Credit: Gamernexus.net

You can see that there is a maximum of 1% performance drop when using a graphics card in an x8 slot. Quite negligible.

However, this test does not take into account the effect of TWO graphics card in SLI configuration on x8/x8 slots.

Getting Around the Bottleneck

So depending upon the situation, you can expect a bottleneck in performance.

The only way to get round this is to invest in a workstation grade desktop. As mentioned earlier, workstation CPUs can feature anywhere between 44 to 128 PCIe lanes.

These CPUs fit on specialized motherboards that offer multiple full x16 slots capable of running graphics cards at their full 16 lane throughput requirement.

gigabyte designare EX

Image: Gigabyte Designer EX is a workstation motherboard with the Intel X299 chipset built of Intel X-Series processors. IT has two full x16 slots.

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Final Words

The main reason as to why is second x16 slot in SLI PCIe x8 hinges on the PCIe lane limitations on average gaming desktop. Several factors go into this as we have seen above.

The number of lanes you can use for your SLI setup on a computer depends on the number of lanes that the processor can support.

While there is a performance difference in using the card in an x8 slot, it is only noticeable in extreme cases and on certain applications as discussed in the Pudget Systems’ study. For gamers, the difference is generally negligible.

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Atif Qazi
Atif Qazi is the founder of PCGuide101. He is a digital nomad who loves everything PC. He is a PC builder, tech enthusiast, engineer, and a lover of single player lore-rich RPG games.

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