Variable refresh rate
Variable refresh rate (VRR), also referred to as adaptive sync, allows the monitor to adjust its refresh rate to the output signal. This allows for games to eliminate screen tearing with less of the usual downsides of Vsync (such as stuttering). For a comprehensive look at VRR see PC Gaming Wiki.
There are multiple implementations of VRR:
- FreeSync is AMD's implementation of VESA's VRR standard, and the phrases are often used interchangeably. FreeSync branded monitors should be compatible with all VESA compatible drivers.
- G-SYNC is NVIDIA's proprietary hardware and software implementation of VRR.
- Intel plans on implementing VESA's standard in their upcoming 11th Gen and dedicated GPUs.
For setup purposes, it is necessary to differentiate between "native" G-SYNC monitors that licenses Nvidia's own chip, and G-SYNC Compatible monitors, FreeSync monitors which support a subset of G-SYNC's functionality.  Within the category of G-SYNC Compatible monitors, the monitor may or may not be validated by NVIDIA.   Even if a VRR monitor has not passed NVIDIA's validation (and thus would not be called G-SYNC Compatible in marketing material), you may still be capable of using it with G-SYNC.
Enable on AMDGPU
FreeSync is only available if your monitor is compatible with FreeSync, as well as if your GPU is compatible with FreeSync.
If you are using a laptop, you can check if your laptop is compatible with FreeSync.
Using an Xorg conf file
Add the line to your AMDGPU .conf file in the
Section "Device" block:
Option "VariableRefresh" "true"
Verify vrr_capable is set to 1 using xrandr:
$ xrandr --props
vrr_capable: 1 range: (0, 1)
xrandr will show the properties for all video output ports; make sure to look at the one that's actually connected to your monitor - the other outputs will report vrr_capable: 0.
Suppose you have a new 144Hz FreeSync capable monitor and want to use it as your primary monitor and connect your old 60Hz monitor as secondary monitor. In that case you may want to enable the AsyncFlipSecondaries option in your AMDGPU .conf file in the
Section "Device" block:
Option "AsyncFlipSecondaries" "true"
This option is available since xorg-server release 21.1.0 and will allow synchronized page flips up to the highest refresh rate your primary monitor supports. Your secondary monitor(s) may exhibit tearing however.
Enable on NVIDIA
Using a Xorg conf file
"Native" G-SYNC and validated G-SYNC Compatible monitors should automatically have VRR enabled. To check the state of your monitors, open nvidia-settings and navigate to your GPU (e.g. GPU 0), under which there will be an node for each active monitor (e.g. DP-0, HDMI-0). In each of those monitor frames, the G-SYNC Mode Available indicates the status of G-SYNC support, either:  
- G-SYNC, indicating that this is a "native" G-SYNC monitor. Full G-SYNC support is enabled by default.
- G-SYNC Compatible, indicating that this is a validated G-SYNC Compatible monitor. G-SYNC support is enabled by default.
- G-SYNC Unvalidated, indicating that this is an unvalidated G-SYNC Compatible monitor. G-SYNC support is not enabled by default. 
- None, indicating that this monitor does not seem to support G-SYNC. Note that there are some FreeSync VRR monitors which are not G-SYNC compatible at all. 
If you have an unvalidated G-SYNC Compatible monitor, you can override NVIDIA's default and enable VRR with it:
- In nvidia-settings go to the "X Server Display Configuration" page, then under the Advanced button is the option to "Allow G-SYNC on monitor not validated as G-SYNC Compatible". Click apply.
- Now, under OpenGL settings, check "Allow G-SYNC/G-SYNC Compatible."
Install mutter-vrrAUR and gnome-control-center-vrrAUR to use VRR in GNOME. VRR needs to be enabled for each supported monitor in the Displays settings. When running on a supported and enabled monitor, GNOME automatically enables VRR for all full screen applications.
KDE Wayland should automatically enable VRR for full screen applications .
Sway supports variable refresh rate. To enable it for all of your outputs you can add the following to the sway configuration, or apply the setting to on a per output basis:
output * adaptive_sync on
You can verify that your display supports adaptive sync with swaymsg:
$ swaymsg -t get_outputs
Output DP-1 'Display Name' Adaptive sync: enabled
VRRTest is a simple testing tool which should work for FreeSync and G-Sync. Install vrrtest-gitAUR or, manually install love package, clone repository, then run
$ love /path/to/cloned/repository
With VRR off, if the application's FPS is less than the monitor's native refresh rate then the bars will stutter a lot since frames are being skipped. With VRR active, the bars will always move smoothly across the screen since the screen's refresh rate will match the application's refresh rate. Even with VRR functional you may experience tearing in which case you can also enable the TearFree option for AMDGPU; with both enabled there should be neither stuttering nor tearing (what is the nvidia equivalent?).
If you are using a Nvidia GPU, you can test G-SYNC compatibility with gl-gsync-demoAUR. This program will allow you to test VRR and Vsync so you can observe resulting effects. See project's Readme for more information.
According to this page: "gl-gsync-demo is made with G-SYNC but that does not matter, it will test AMD adaptive sync just fine". However, it may still not work as expected for FreeSync testing.
Change VRR range of a FreeSync monitor
Freesync monitors usually have a limited range for VRR that are much lower than their max refresh rate. It should be possible to overclock the monitor to change the Freesync range.
Editing the EDID File
External Display Identification Data (EDID) stores driver information about your monitor. By default, this file is sent by your monitor and read on connect. You will need to extract this file using something like read-edid or nvidia-settings.
You can edit this file with wxedidAUR.
You may follow one of the guides of people changing the freesync range on Windows: 
Process of overclocking on Linux (works only on NVidia GPUs): 
Make a Xorg .conf file for your monitor and add a path to the custom EDID file you have edited. See xrandr to find find out the other information about your monitor.
Section "Screen" Identifier "Screen0" Device "nvidia" # e.g. Radeon, nvidia Monitor "DP1" Option “CustomEDID” “MONITOR:/home/USER/Desktop/modified-edid.bin” EndSection
Tips and Tricks
Remove applications from Blacklist
Mesa has a list of blacklisted applications to avoid unexpected behavior, you can edit this blacklist here:
- Wayland is supported as of KDE 5.22 (any compatible GPU) and Sway (no Nvidia)  . GNOME Wayland is not yet supported .
- The monitor must be plugged in via DisplayPort. Some displays which implement (part of) the HDMI 2.1 specification also support VRR over HDMI. This is supported by the Nvidia driver and is supported by the AMD driver (pre HDMI 2.1) in Kernel 5.13 and later .
- If you want to use G-SYNC and possibly Freesync on X11, only the G-SYNC/Freesync monitor must be connected.
- On X11, multiple monitors in a single X display will break G-SYNC/Freesync, however, this problem does not exist on Wayland.
- A secondary monitor set as copy/duplicate of the primary monitor does not break G-SYNC/Freesync on the primary monitor.
- If on X11, if you want to use multiple monitors and still use G-SYNC/Freesync, you can create a new X display which only covers the G-SYNC/Freesync monitor and run games there.
- Compositors will most likely need to be disabled before the OpenGl/Vulkan program is started (disabling compositors is not relevant or necessary on Wayland ).
- On X11, Mesa blacklists many applications including video players.
- Although tearing is much less noticeable at higher refresh rates, FreeSync monitors often have a limited range for their VRR of 90Hz, which can be much lower than their max refresh rate. See Change VRR Range of a FreeSync Monitor.