InstanceBrick--nightcap/docs/PERFORMANCE.md

Performance and CPU load

What Nightcap does to your CPU and GPU, which knobs change it, and how to measure rather than guess.

GameMode core pinning

Nightcap asks Feral GameMode for a performance session at startup (performance.gamemode, default auto). GameMode raises the CPU governor and the process priority, which helps.

On a CPU with a wide spread of per-core maximum frequencies — Intel P-core and E-core parts, AMD parts with preferred cores — GameMode also pins the process to the fastest cores. On a 32-thread host that can mean 4 usable threads out of 32. Roblox spreads its work across a worker pool, so the pin concentrates that pool onto a couple of physical cores instead of spreading it. Those cores sit near their boost ceiling and run hot while the rest of the chip idles.

GameMode re-applies the pin roughly every 5 seconds, so widening the mask from inside the process does not hold. Releasing the request does release the pin.

Under auto, Nightcap therefore checks its CPU affinity immediately after the request. If GameMode narrowed it, Nightcap ends the request and prints:

  [gamemode] released: it pinned this process to a subset of the CPUs, ...

The trade is that GameMode's governor and priority boost go with it. To keep those and lose only the pinning, turn pinning off for every game on the host.

These are the settings to run with:

# ~/.config/gamemode.ini
[general]
desiredgov=performance

[cpu]
pin_cores=no
park_cores=no

desiredgov is the one to check first. The packaged default in /usr/share/gamemode/gamemode.ini is already performance, but a value in your own file replaces it, and desiredgov=powersave there means GameMode clocks the CPU down for the whole session while still reporting [gamemode] performance request active. The symptoms are a low CPU clock under load, a GPU that never leaves its lower SM clocks, and the engine pump falling back to tpause — which costs about 70% of a core instead of 3%, because sleep is only chosen when the governor reads performance. On one 13900K host, correcting powersave to performance took the process from 177% CPU to 42% and the package from 70°C to 53-63°C, at the same frame rate.

GameMode reads $XDG_CONFIG_HOME, /usr/share/gamemode and /etc, in that order. It does not read anything under Nightcap's own config directory, so this file has to live in one of those three places. The daemon reads it when it starts, so restart it after editing: systemctl --user restart gamemoded.

Restarting the daemon drops the session of any game already running; relaunch the game for new settings to apply to it.

gamemoded is D-Bus activated through com.feralinteractive.GameMode.service, so systemctl --user stop gamemoded does not hold — the next client request starts it again within seconds. To measure with GameMode genuinely off, mask it:

systemctl --user mask gamemoded     # and --unmask afterwards

The governor switch runs through polkit. Debian and Ubuntu only allow it for members of the gamemode group; without that, the daemon's journal shows cpugovctl set performance: Not authorized and the governor stays put while the session is otherwise active. Add yourself and log in again:

sudo usermod -aG gamemode "$USER"

Fedora and Arch allow any logged-in local user.

Setting performance.gamemode: on accepts the pinning instead: on is an explicit request for GameMode, and Nightcap keeps the session whatever it does to the affinity. off never contacts the daemon.

To see which CPUs the process may use:

pid=$(pgrep -x Main)
for t in /proc/$pid/task/*; do taskset -pc "${t##*/}"; done

Two traps here. The engine process reports its name as Main, not mocktail; pgrep -x mocktail finds only the launcher, which sits near 0% CPU and tells you nothing. And taskset -p reads one thread, not the whole process, so a single call reports the main thread's mask and says nothing about the workers — which is where the game's load actually is.

For the same reason, ps -L -o psr= shows which core each thread is running on right now, not which cores it is allowed on. On a hybrid CPU the scheduler preferring P-cores for runnable work is normal and is not a mask.

The main-thread pump

Roblox posts its per-frame main-thread step at a time of its own choosing, and nothing signals the host when it does. The host main thread therefore polls nativeCallMessagesFromMainThread, and a post that waits more than a fraction of a millisecond costs a whole frame. Between polls the thread rests for 100 µs, and how it rests decides the CPU clock:

Mode	When	What it costs
sleep	The governor is performance, which GameMode sets	The thread idles at about 3% of a core. The governor holds the clock.
tpause	Otherwise, on a CPU with WAITPKG (Intel 12th gen and newer)	The core stays awake in a light C0 state. Frame rate holds; the OS books the time as busy, so the thread shows around 70% of a core, at lower power than a spin.
spin	Otherwise	The core stays awake at full power.

Under a power-saving governor the CPU clocks a bursty core down unless some core stays awake, and Roblox's render thread then runs slower and drops frames. That is why sleep is not the default there. Startup prints the chosen mode:

  [main] engine pump rest: tpause (MOCKTAIL_ENGINE_PUMP_REST=sleep|tpause|spin)

MOCKTAIL_ENGINE_PUMP_REST forces a mode. sleep without the governor's help runs cooler for a few frames per second less; measure before keeping it.

Small texture upscaling

MOCKTAIL_SMALL_TEXTURE_UPSCALE (default 4) redraws ETC2 textures of 64px or smaller at N times their width and height. At the default that is 16 times the pixels per affected texture.

The cost lands in two places. Decoding and resampling is CPU work on up to 8 threads while textures load, which shows up as a burst rather than a steady load. The enlarged textures then stay in GPU memory for the session and cost extra sampling bandwidth on every frame that uses them.

That decode does not run on the thread that submits the frame. On a device with a timeline semaphore, uploads decode on a background dispatcher and the submit carries a semaphore wait, so the GPU waits for the texture instead of the render thread. In the trace this is the etc2 decode slice on its own thread, with only a negligible etc2 gather left on the submitting one. A device without that feature decodes inline as before.

Set it to 1 to turn upscaling off:

MOCKTAIL_SMALL_TEXTURE_UPSCALE=1 mocktail

Frame rate and present mode

graphics.frame_rate_limit (default -1) leaves Roblox's own in-game framerate cap in charge; Nightcap sets no DFIntTaskSchedulerTargetFps override. A fixed value forwards that number to the scheduler.

In VR, vr.frame_rate_limit (MOCKTAIL_VR_FRAME_RATE_LIMIT) replaces graphics.frame_rate_limit and takes the same values. Its default is unlimited: the scheduler target goes to 240 and xrWaitFrame holds each frame to the headset's refresh rate. Set -1 to restore Roblox's own cap, or a number such as 90 to pin it.

MOCKTAIL_VR_RENDER_SCALE (default 1.0, range 0.25–2.0) scales each axis of the OpenXR runtime's recommended eye size. SteamVR's recommendation already includes its own supersampling, so on a Quest 2 at 90 Hz it is about 2244x2352 per eye. If a frame misses the 11.1 ms budget, the runtime halves the rate to 45 FPS; 0.75 renders about 56% of the pixels.

MOCKTAIL_VR_UNPACED=1 is a benchmark mode. Roblox no longer waits for the headset between frames; each XR frame stays open across several Roblox frames and is submitted with the newest eyes once per display period. Pair it with a fixed scheduler target above 240 and MOCKTAIL_VSYNC=off to find the maximum rate:

MOCKTAIL_VR_UNPACED=1 MOCKTAIL_VR_FRAME_RATE_LIMIT=9999 MOCKTAIL_VSYNC=off mocktail --vr

Frames beyond the headset refresh are never displayed, and they all reuse the open frame's head pose, so expect judder. Do not play in this mode.

graphics.vsync (default auto) selects the presentation mode. auto and on both pick the lowest-latency synchronized mode the driver offers, in order: FIFO_LATEST_READY, MAILBOX, FIFO_RELAXED, FIFO. The first two do not block the render thread on the display refresh, so the frame rate is governed by the in-game cap rather than by presentation. off selects IMMEDIATE and does not synchronize at all.

If the frame rate is higher than your refresh rate, the extra frames are work you cannot see. Lower the in-game cap to your refresh rate first.

Measuring

Shift+F4 in game shows Roblox's own scheduler jobs, with a millisecond cost and a percentage for each. It does not show Nightcap's threads — the Vulkan adapter, the ETC2 decode workers, the libc shim — so it is a reading on the game, not on the process.

For the whole process:

top -H -p $(pgrep -x mocktail)      # per-thread CPU, sorted
ps -L -o psr=,pcpu=,comm= -p $(pgrep -x mocktail)   # which core each thread is on
nvidia-smi --query-gpu=utilization.gpu,temperature.gpu,power.draw --format=csv

A thread pinned near 100% while the frame rate is steady is a busy-wait. The one exception is the host main thread in tpause mode, which the OS reports as busy while the core is parked (see above). Load spread across many RBX Worker threads is the game's own scheduler, and its cost tracks the place you are in — part count, moving parts and joints all show in the Shift+F4 World section.

--profile <file> writes a Chrome trace of the Vulkan adapter's work for ui.perfetto.dev. See BENCHMARKING.md.