4 Monitors into 1 GPU bandwidth issues?

rafbb · 2025-12-18 05:08:11

Hello,

I've been trying to troubleshoot this for a while now without success. I'm trying to hook up 4 monitors (3x-1920x1080@60Hz + 5120x2160@165Hz) to a single GPU but no matter what I do the 5K monitor is limited to 100Hz. The 165Hz option is available, but if I try to apply it I get the "Couldn’t apply display configuration: The driver rejected the output configuration" message.

Here's the thing - it works just fine at 165Hz with a 3 monitor configuration or less (2x1080 + 5K) but not 4.

My specs:
OS: Arch Linux x86_64 (Linux 6.18.1-arch1-2), KDE (Wayland)
Mobo: MSI Z690 ACE
CPU: i9 14900K
GPUs: RTX 5090 + RX 9070 (dual GPU config with x8+x8 PCIe 5.0 - I've tried single GPU x16 without success)
RAM: 96GB DDR5

I use RX 9070 to run Linux, all 4 monitors are connected to it. The 5090 is passed through to various VMs, or used with nvidia prime with native linux apps when needed. I've been using this setup for a while and works great. Both GPUs are capable of DP 2.1 and HDMI 2.1, both are running on PCIe gen 5. The 5K monitor is 45GX950A-B (LG 5K2K), the other 3 are portable OLED monitors with HDMI and USB-C inputs. LG 5K2K is connected to the GPU with a DP2.1 capable cable, 2 monitors with HDMI cables + active DP->HDMI converters, and one directly with HDMI.

Here's what I tried:
- different cables (DP)
- running the whole setup on one GPU (x16)
- running the whole setup on different GPUs (5090, RX 7600, Radeon Pro W7500 and RX 9060 XT - it only has 3 outputs but works fine with 5K@165 and 2x1080@60)
- running different Linux distros and WMs (X11 and Wayland) and even trying Windows 11 - no joy
- running 2 1080 monitors via USB-C/Thunderbolt - my mobo can pass-trough DP signal to TB4
- running 5K monitor via USB-C/Thunderbolt
- trying to lower resolution and refresh rate of the 1080p monitors
- trying different color resolutions - I really only need 8 bits anyway
- trying to run the 5K monitor off the iGPU - this "works", but I'm really trying to avoid using the iGPU
[edit] - I also tried using MST hubs, and connecting 3x1080 monitors to one DP port - no joy

No matter what combination of inputs I use, if I hook up 4 monitors to the single GPU the 5K monitor is limited to 100Hz. If I disconnect even one - I can go up to 165Hz no problem. This behavior appears to be independent of OS/driver, so I'm leaning towards hardware limitation.

Could someone please explain to me where's is the bottleneck? Is it the GPU? But then - it's hard to believe a 5090 isn't capable of driving 4 monitors. 5120x2160@165Hz 8bit RGB is about 48.14 Gbps - well within DP2.1 specs - the 5090 should be able to drive 3 displays like that at once with no problem... am I wrong thinking that?

Last edited by rafbb (2025-12-18 05:49:04)

sfn · 2025-12-24 10:41:25

Sounds like it could be a limitation on display heads. The only documentation NVIDIA provides says it supports "4 independent displays at 4K 165Hz using DP or HDMI". That could reasonably be interpreted to mean that 5k@165Hz requires two display heads, leaving you with only two left over for other displays (for a total of three, not four displays).

Instead of putting the 5k monitor on the iGPU, you could put one of the secondary monitors on it (preferably one you would normally use only for e.g. browsing, text editing, other non-graphically-intensive tasks). This leaves you the full power of the 5090 for your main display.

Also, check your cables, what standards they support, and what sort of quality they are.

x-ref: https://www.reddit.com/r/nvidia/comment … _monitors/.

Lone_Wolf · 2025-12-24 11:13:23

I use RX 9070 to run Linux, all 4 monitors are connected to it.

So no monitors are directly connected to the nvidia card ?

Have you tried using DP MST daisy chaining for the 2x 4k DP monitors ?

rafbb · 2025-12-30 08:55:25

Lone_Wolf wrote:

I use RX 9070 to run Linux, all 4 monitors are connected to it.
So no monitors are directly connected to the nvidia card ?
Have you tried using DP MST daisy chaining for the 2x 4k DP monitors ?

The 5K monitor is connected to Nvidia 5090 just to get a proper EDID for the VMs I'm running. I'm blocking that output via kernel parameter so the KDE doesn't try to use that when nvidia modules are loaded (I have a script to load and unload nvidia when I need it).

The problem with MST is, even if I daisy-chain them the GPU still sees them as separate monitors.

rafbb · 2025-12-30 09:07:58

sfn wrote:

Sounds like it could be a limitation on display heads. The only documentation NVIDIA provides says it supports "4 independent displays at 4K 165Hz using DP or HDMI". That could reasonably be interpreted to mean that 5k@165Hz requires two display heads, leaving you with only two left over for other displays (for a total of three, not four displays).

I did some more testing and it seems this is the reason why. One "head" can do max 4K@165Hz, to run 5K I'd need more than one. Any monitor connected regardless of resolution will take at least one head, so even if 1920x1080@60 is way way lower than 4K@165 it still requires a full "head". 3x1080p@60Hz monitors + 1x5K@165Hz requires a total of 5 heads. It is kind of disappointing GPUs like a 5090 and 9070 have limitations like that to be honest.

For now my solution is to use my old TripleHead2Go DP edition. I hooked up 3x1080p monitors to it. It basically creates one large 5760x1080@50Hz display. They key here is - my GPU sees it as a single monitor - so it uses only one head. And 5K monitor now runs at 165Hz. Another upside is I still have 1 extra head for one more monitor if I need it.

Anyway, thanks for your help guys!

Last edited by rafbb (2025-12-30 09:14:59)

Arch Linux

#1 2025-12-18 05:08:11

4 Monitors into 1 GPU bandwidth issues?

#2 2025-12-24 10:41:25

Re: 4 Monitors into 1 GPU bandwidth issues?

#3 2025-12-24 11:13:23

Re: 4 Monitors into 1 GPU bandwidth issues?

#4 2025-12-30 08:55:25

Re: 4 Monitors into 1 GPU bandwidth issues?

#5 2025-12-30 09:07:58

Re: 4 Monitors into 1 GPU bandwidth issues?

Board footer