Nvidia Reflex should limit FPS within VRR range similar to its behaviour on windows

[EstoricBah]

In windows, when reflex and gsync are enabled, nvidia driver automatically limits the FPS to just below the maximum refresh rate to prevent vsync latency.

Currently there is no good solution to this in linux, as external FPS limiters unfortunately break reflex immediately upon enabling the limit (ie mangohud in linux and rtss in windows). And without a reliable FPS limit that is compatible with reflex, one has to choose between mailbox judder, vsync latency, or tearing when FPS exceeds the upper limit of VRR.

Sorry if this is not the correct place for this issue.

[Saancreed]

Reflex already has a mechanism for frame rate limiting, in NVAPI:

NvU32 minimumIntervalUs; //!< (IN) Minimum frame interval in microseconds. 0 = no frame rate limit.

And in VK_NV_low_latency2:

uint32_t minimumIntervalUs is the microseconds between vkQueuePresentKHR calls for a given swapchain that vkLatencySleepNV will enforce.

Now, I'm not sure how all this interacts with gsync, or how it works under the hood on Windows, but my laptop's VRR display generally doesn't appear to suffer from any kind of juddering when this is set to limit FPS at a value no higher than my display's upper limit.

On the other hand, wouldn't something like present timing help with this when no FPS limit is set? You'd probably have to run a very specific setup, because I'm not sure how's the compositor and xwayland support for it these days (kwin will get it in 6.7 I believe), but with them, the FIFO_LATEST_READY present mode starts being interesting.

[wcampbell-nv]

We do have plans to improve this functionality in the NVIDIA driver, but it will be some time before we release that.

If you're interested in a short term solution, it would be easy enough to have a Vulkan layer (such as MangoHud) intercept vkSetLatencySleepModeNV and override minimumIntervalUs to a value slightly below your display's refresh rate. You can query the display's refresh rate with VK_EXT_present_timing's vkGetSwapchainTimingPropertiesEXT (if you're on X11 or native wayland - the refresh rate query is not implemented on XWayland).

[AlawamiAZ]

Shouldn't commit-timing-v1 with VK_EXT_present_timing solve this without needing FPS limiter?