RTX 4090 into PCIe 5.0 x8 slot (Z790)

Question

I have an RTX 4090 and the ASUS Z90 Maximus Hero (Users Manual). I need to install it into the second PCIe 5.0 slot, but the manual shows that the 1st (PCIEX16(G5)_1) and 2nd (PCIEX16(G5)_2) slot are not equivalent, which I didn't know before, because ASUS stated something different on their website:

However, the manual shows:

This is indeed confusing, because they also state:

I therefore assumed that if I leave the other PCIe 5.0 slot untouched, the other will get x16. However, I can see that the 2nd slot does not have as many contacts as the 1st slot has, so I assume that the 2nd slot is never able to offer more than x8.

Will an RTX 4090 run fine on a PCIe 5.0 x8 slot?

Answer 1

PCIe generations generally double the speed of the slot over the previous generation. What that means for an end user is that a PCIe 5 x8 slot is roughly equivalent to a PCIe 4 x16 slot so you can assume that if the card worked well on a previous generation board then it should work just as well on a newer board with fewer lanes.

One of the main features of PCIe is also that the number of lanes is established and negotiated at startup so if a port only has x8 available, then both sides will go "oh, I only have 8 lanes available" and will carry on regardless, simply using less lanes that it originally expected.

At worst I might expect to see a difference in FPS measured in single digits, if you could measure any difference at all that wasn't attributable to simple measurement error.

From Wikipedia the transfer rates are as follows.

Version	Transfer rate per lane	x1	x2	x4	x8	x16
1.0	2.5 GT/s	0.250 GB/s	0.500 GB/s	1.000 GB/s	2.000 GB/s	4.000 GB/s
2.0	5.0 GT/s	0.500 GB/s	1.000 GB/s	2.000 GB/s	4.000 GB/s	8.000 GB/s
3.0	8.0 GT/s	0.985 GB/s	1.969 GB/s	3.938 GB/s	7.877 GB/s	15.754 GB/s
4.0	16.0 GT/s	1.969 GB/s	3.938 GB/s	7.877 GB/s	15.754 GB/s	31.508 GB/s
5.0	32.0 GT/s	3.938 GB/s	7.877 GB/s	15.754 GB/s	31.508 GB/s	63.015 GB/s

So instead of 63GB/s transfer rate you should get 31.5GB/s.

It absolutely should work at x8 and should work well but you will have a barely perceptible time difference in uploading texture and geometry data.

To fully fill a 24GB 4090s memory would take approximately 0.75 seconds instead of 0.425 seconds.

That a somewhat unrealistic workload for gaming and assumes that all the data is held in main RAM, in practice you will be bottlenecked by your storage device as the fastest NVMe drive is somewhere around 7.5GB/s.

For scientific workloads where you are processing data on the card and copying to RAM then you may well notice a difference, albeit very slight.

Answer 2

Two answers for the question: Will an RTX 4090 run fine on a PCIe 5.0 x8 slot?

Yes, but depending on what you will be using it for.

For most users, like gaming, it would be difficult to see a difference in performance, maybe just a single digits FPS drop. If you will be doing very demanding computations [like for science] that would require the FULL utilization of your graphics card, then your graphics card will be hitting a serious bottleneck by the x8 lanes. Let me explain.

The other two answers miss one important fundamental technicality. Since the 4090 is based on PCIe v4, it will not be able to utilize PCIe v5 speeds. So if your PCIe v5 x16 slot [max of 64GB/s] is downgraded to x8 lanes [max of 32GB/s] then the graphics card will be running at PCIe 4 x8 speeds [max of 16GB/s] (not PCIe 4 x16 speeds 32GB/s).

Here is one source that goes more in depth on the topic. https://www.youtube.com/watch?v=v2SuyiHs-O4

Answer 3

I therefore assumed that if I leave the other PCIe 5.0 slot untouched, the other will get x16. However, I can see that the 2nd slot does not have as many contacts as the 1st slot has, so I assume that the 2nd slot is never able to offer more than x8.

You are correct.

PCIEX16(G5)_2 is at most PCIe 5.0 x8.

PCIEX16(G5)_1 is at most PCIe 5.0 x16.

Both together have max x16 support. So it can be either x16/x0 or x8/x8.

Will an RTX 4090 run fine on a PCIe 5.0 x8 slot?

Yes but it will run at half the bandwidth of PCIe 5.0 x8. This is not a problem since 4090 doesn't utilize the full bandwidth anyway. In fact, 4090 doesn't work with Gen 5 directly. It will use PCIe 4.0 on PCIe 5.0 slot as PCIe are back-compatible.

Note: PCIe 5.0 is double of PCIe 4.0. So PCIe 5.0 x8 is same as PCIe 4.0 x16.

Answer 4

So basically, a RTX 4090 (PCIe 4) installed in a PCIe 5 slot will run at PCIe 4 speeds. If the slot is an x8 slot, it will run at PCIe 4 x8 - which is the same as PCIe 3 x16 in terms of speed. So, for most common tasks, a 4090 on a PCIe 5.0 x8 slot would be within 2-3% of real-world max.