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:

```ini
# ~/.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:

```bash
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:

```bash
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:

```bash
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`) stores 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.

Upscaling alone adds no detail. Each original pixel becomes an N×N block of the
same colour, and the GPU's filtering already blends a small texture that much
when it draws it. The extra size only shows when a replacement PNG in
`~/.config/mocktail/textures` has more detail than the original.

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. In Blade Ball the
memory share is small: decoded textures came to 5.1 times their ETC2 size both
with the default and with upscaling off. The `emulated texture (MB)` and
`compressed texture (MB)` counters in a `--profile` trace give the figures for
any game.

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:

```bash
MOCKTAIL_SMALL_TEXTURE_UPSCALE=1 mocktail
```

## Native code in the menus

The home screen and avatar editor run Roblox's Lua app in the Luau interpreter,
and its long frames come from that thread. PC profiles set
`FFlagDebugLuauCodegenAll`, which compiles every script to native code. That
makes the menus' Lua about 2.5 times cheaper for about 56 MB of compiled code.

In a game the same flag compiles every game script too: Blade Ball grew from 7
MB to about 300 MB and hit Roblox's native code limit, which shows warnings in
F9. So Nightcap turns the flag off when a game starts joining and back on when
you return to the menus. Roblox reads the flag each time it compiles a script,
and a game's compiled code is freed when you leave it. Games run with Roblox's
own per-script choice, the same as without the flag.

The log shows each switch:

```text
[luau-codegen] menus-only switch at +0x<offset in libroblox.so>
[luau-codegen] native code for all scripts off
[luau-codegen] native code for all scripts on
```

Setting `FFlagDebugLuauCodegenAll` in `fflags.json` turns the switching off
and your value applies everywhere. To keep the interpreter in the menus too:

```json
{
  "FFlagDebugLuauCodegenAll": false
}
```

## 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:

```bash
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:

```bash
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](BENCHMARKING.md).