A headless server is a system without a monitor attached. This is usually how servers are managed when up and running due to power savings. You don't need a monitor, mouse, and keyboard always attached to them, but this could cause problems with the system not booting up when a GPU is present. But even if your server fires up okay, you may find some features of the GPU are disabled because no monitor is connected. That's where a $3 can save the day.
What is an HDMI dummy plug?
It's smarter than at first glance
An HDMI dummy plug is a compact dongle with a simple resistor network. The goal of these tiny devices is to mimic a display's Extended Display Identification Data (EDID), which essentially tells the GPU what the monitor can handle, such as resolutions, refresh rates, and synchronization technologies. Without a monitor or dummy plug, the port remains vacant, and the GPU assumes that no screen will be connected and running, so it may even drop into a low-power mode.
The driver running the GPU may see no screen present and disable hardware acceleration altogether. Depending on what you plan to use the server for, this could render it useless for AI inferencing and working with video. Suppose you've ever attempted to get GPU pass-through to work within Proxmox or some other hypervisor and failed miserably. In that case, it may be due to the GPU not correctly initializing because no display is connected to it.
Enter a simple HDMI adapter, which can cost as little as $3, depending on where you prefer to shop. This compact device makes the GPU (and wider system) believe a monitor is always connected. It's great for removing to need to quickly fetch a small monitor and plug it into the headless server at boot to make it through to the OS and disconnect everything. This method can sometimes fail as pass-throughs can hit a wall.
Modern GPUs waste power, and you probably never even noticed
Your GPU is probably drawing more power than it needs, all the time
It's useful for home labs
Make your headless life easier
So, how does this all tie together with a home lab? It all comes down to using a GPU and board combo that doesn't play well without a monitor tethered to your server. Depending on your configuration, this likely won't be an issue since I've found most of my board and CPU combinations do work well as a headless system, but there are instances when the GPU would act strangely or simply cause the system to not boot at all. That's where the $3 HDMI dummy plug can solve everything.
The best part about using an HDMI dummy plug is how easy it is to use. Simply attach the adapter to the rear of your GPU, using one of the available HDMI ports, and you're done! I used it with a couple of older GPUs that I had lying around collecting dust, so they could be repurposed for handling some heavier work that bogged the CPU down. They required HDMI dummy plugs when used in a server environment, which isn't an issue for remote management via Proxmox.
With the cheap dongle installed, the GPUs would be fully functional, offering full hardware transcoding capabilities and advertised clock speeds. That's not to say they were completely unusable without this "fix," but having better stability, quicker boots, and better performance as a whole is a no-brainer. There's literally no reason to run with hardware that's unable to perform as designed, especially when it's as simple as purchasing something that takes a few seconds to install (and remove).
One caveat with a multi-GPU setup is that all graphics cards may require a dummy plug to fully "activate." To begin with, I added the dummy plug to the primary GPU, and everything for that card loaded up fine, but the secondary card was left in a cold state and failed to ramp up its clock speeds. Picking up a second dummy plug and adding it to the other card solved this issue. It's something to look out for when running multiple GPUs.
Not all dummy adapters work the same
Interestingly, when shopping around for one, I found out that not all dummy adapters support high resolutions. But this made perfect sense since they're spoofing EDID. So you could consider each dummy adapter as a monitor, each with its own specs. Some may support only 1080p or "2K," while others will come rocking full support for resolutions up to 4K. For servers, this really isn't an issue since Full HD or UHD makes no difference to GPU performance.
The only time you'd want to buy a 4K plug is if it's about the same price as lesser options, or you absolutely need to remote desktop into the server at higher resolutions. There's also the case for streaming, though one would usually have a physical monitor attached to a secondary system when used in a multi-PC setup. Overall, 1080p is just fine, though 4K will provide all the higher-res benefits should you eventually need them.