Does it matter which PCIe x16 slot I use

Does It Matter Which PCI-e x16 Slot I Use?

The quick answer is yes, it matters. It matters because not all PCI-e x16 slots are the same. They have the same physical size, but the number of lanes they provide and the generation of the slot can be different.

Most of the time, only the primary PCIe x16 slot is able to provide 16 lanes, while other PCI-e x16 slots are limited to 8 or even 4 lanes.

Understanding how the PCIe slots work and how the lanes are split between the slots is a really difficult task, and this is what we are going to do in this article. First, we will talk about what PCIe lanes are and how they work, and then we will go through a simplified explanation of what happens when you plug a card into a PCIe slot.

I am going to try my best to explain everything as simply as possible, even though this is a pretty complex topic. So let’s start.

What are PCIe lanes?

A PCIe lane is a serial communication lane that carries a bitstream of 1 or more parallel lines. This lane is physical wiring and nothing else; it has no memory or processing capabilities. In simpler terms, a PCIe lane is a piece of wire that transfers data between the CPU and the PCIe device (video card, sound card, network adapter, etc.). A single lane is composed of 2 differential signaling pairs (in each direction), so as you can already understand, the more lanes the system has, the faster data can be transferred.

PCIe lanes

A typical motherboard will have 20 to 24 PCIe lanes built into it. Most of the time, the 16 lanes will be allocated to the main PCIe slot (the one you plug in your graphics card), and all of the other PCIe slots will get a lower number of lanes. It might sound like a small number, but the truth is that except for your graphics card, all the other PCIe cards will work fine with fewer lanes (more on this in a moment).

Related Article: What Is PCIe x16 And What Does It Mean

Is The PCI-e x16 slot I use That Important?

As you can already understand, the higher number of PCIe lanes means that more data can be transferred between your CPU and your graphics card. So yes, it does matter which PCIe slot you use especially when it comes to your graphics card.

Most likely, there is one “true” PCIe x16 slot on the motherboard, and it is the one closest to the CPU. This one should be your main slot, and this is where you should plug in your graphics card.

What if I have 2 GPUs?

This is a question that gets asked a lot, especially when it comes to an NVIDIA SLI configuration. Since SLI requires 2 GPUs to work, where should you plug them?

gigabyte-pciex16-slots

Most motherboards have only 2 PCIe x16 slots, so you should plug your GPUs into each of them. But keep in mind that only one is a “true” PCIe x16 slot that will give you the maximum performance of 16 lanes. So if you plug only one graphics card into the slot closest to the CPU and leave the other one empty, then the graphics card will work at 16x speed. But if you do the opposite and plug the graphics card into the other slot and leave the first one empty, it will work fine, but it will operate with only 8 lanes as the second slot is physically limited to 8 lanes.

Finally, when you use both PCIe x16 slots, each of your graphics cards will get only 8 lanes.

Related Article: Can You Sli Two Different GPUs?

PCIe x16 with 8 and 4 lanes

As you can understand, in most motherboards, there will be only one PCIe x16 slot with 16 lanes, and the rest of the slots will be limited to 8 or even 4 lanes.

We have covered what to do if your motherboard has 2 PCIe x16 slots, with one of them having 8 lanes in the previous paragraph, but what should you do if your motherboard has a 4-lane PCIe x16 slot?

This is the case when you buy a budget motherboard that comes with 2 PCIe x16 slots. One of them will be the “true” PCIe x16 slot (with 16 lanes), and the other will be limited to 4 lanes.

All you can do if you have this type of configuration is to plug in only one GPU into the “true” PCIe x16 slot and use the other one for your sound card, network adapter, etc.

It makes no sense to use 2 graphics cards in a system with a motherboard where one of the two PCIe x16 slots is limited to 4 lanes.

Related Article: What Is A PCIe Cable And What It Is Used For

Different Versions of PCIe

To make things even more complex, PCIe slots not only have a different number of lanes but also have different versions.

When I say a different version, I mean that the PCIe Protocol has different generations just like the CPUs. The main difference between each of these generations is the speed at which data can be transferred.

You can check the table below, which shows the different speeds for each version of PCIe:

Generationx16 GB/sx8 GB/sx4 GB/sx2 GB/sx1 GB/s
5.063.01531.50815.7547.8773.938
4.031.50815.7547.8773.9381.969
3.015.7547.8773.9381.9690.985
2.08.0004.0002.0001.0000.500
1.04.0002.0001.0000.5000.250

So what does it all mean?

It means that you should check not only the number of lanes but also check the type and generation of these slots.

As an example, the motherboard below has 1 “true” PCIe x16 slot.

aorus PCIex16 slots
Gigabyte X570S AORUS MASTER PCIe slots.

The first is a PCIe 4.0 x16 with 16 lanes slot, the second is a PCIe 4.0 x16 slot with 8 lanes, and the 3rd one is a PCIe 3.0 x16 with 4 lanes.

As you can understand, the first slot with the most PCIe lanes is the fastest one, and this is the one you should use for your GPU. The second PCIe slot is a bit slower, but it will still give you enough performance.

And if you thought that things were complex, they are going to get a bit more complicated now.

I didn’t mention that even though your motherboard might support the PCIe 4.0 standard, your CPU might not support it!

I won’t get into much detail about the how and why because it will get really technical, but your CPU is not only responsible for how many PCIe lanes there are and at which speed; it also has to do with supporting different versions.

All you have to know is that older 3rd generation Ryzen processors don’t support PCIe 4.0, but newer 5th generation processors do.

Related Article: PCIe 2.0 VS 3.0 – What Is The Difference?

Are there any motherboards with 2 “true” PCIe x16 lanes?

Yes, there are some motherboards with 2 PCIe x16 slots, but they are not very common, and they come with a hefty price tag.

ROG MAXIMUS Z690 EXTREME
ROG Maximus Z690 Extreme comes with 2 real PCIe5.0 x16 slots

Before investing in a really expensive motherboard, you should think if you really need that second slot and if you don’t then it is better to buy a motherboard with just one 16-lane PCIe x16 slot. It doesn’t make sense to have so many lanes if only one slot is going to give a full performance, right?

Related Article: How Many PCIe Lanes Does Ryzen Have? [Solved]

Performance Difference Between Different Number of Lanes and Different PCIe Generations.

By this point, you know what is the difference between the number of lanes and the generation of PCIe, but how much performance difference do these actually have in real life?

To answer this question, you have to test the same GPU card on different slots and make sure that all other variables are constant.

I didn’t make the test myself, but thanks to GameNexus and PudgetSystems, there are some data from tests that they have done on similar configurations.

Related Article: What Are PCIe x1 Slots And What They Are Used For

PCIe Number of Lanes Performance Test Result

One of the two tests I mentioned focuses on the number of lanes, not the generation.

So the first test made by GamersNexus happened (Source) back in 2016 and tested a GTX 1080 Gaming X GPU on an EVGA X99 Classified motherboard.

The GPU has been placed on 2 different PCIe x16 slots (one with 8 lanes and one with 16 lanes), and in both tests, all other variables were constant.

To test the difference, the guys at GamersNexus played several games like the Witcher 3 and GTAIV and compared the FPS on the different PCIe slots.

As you can see in the video below, there was a very small performance difference between the x16 and the x8 slot

Related Article: Which PCIe Slot For GPU Is The Best?

PCIe Version Performance Test Result

The second test performed by PudgetSystems (Source) in November of 2020 is focused on the different PCIe generations.

The guys at PudgetSystems tested 2 different graphics cards, one compatible with PCIe 3.0 x16 (NVIDIA Titan RTX)and one with PCIe 4.0 x16 (GeForce RTX 3090), on a computer they could control the PCIe version from the BIOS. The motherboard of this test was a Gigabyte X570 AORUS ULTRA, and the CPU a Ryzen 5950X.

They used the Octanebench, Redshift, and V ray benchmarks to test the GPU rendering performance.

pudgetsystems_benchmark-results

As you can see, there was a huge performance difference between PCIe 4.0 and PCIe 3.0.

In the second part of this test, they used DaVinci Resolve Studio, a video editing application, and tested the performance of a GPU accelerated effect on each PCIe version.

pudgetsystems_benchmark-results-davinci

Here you can see the performance difference was not as big as it was for applications like Octanebench and V ray.

Finally, at the 3rd and last test, they used Unigine Superposition to test the game performance.

pudgetsystems_benchmark-results-unigine

As you can see, PCIe 4.0 is faster than PCIe 3.0, but the difference is even smaller.

Related Article: Can I Use PCIe 3.0 In 2.0 Slot?

Conclusion

Now you know why it matter which peripheral component interconnect express slot you use and all the different variables that can affect the performance like the number of lanes, the generation, the processor, and motherboard chipset lanes, based on testing.

So you should consider all of this if you want to buy a motherboard with more than one PCIe x16 slot.

Think about your needs, what GPU you plan to use, what kind of performance you need, and, of course, the price tag.

By the way, I hope you found this article interesting, and if so, please share it with your friends. Also, let me know your opinion on the different variables that can affect the performance between different PCIe versions and the number of lanes.

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