Performance

FlickOS has to feel quick on a ten-year-old laptop with 4 GB of RAM and a slow disk. These rules turn light and fast (principle 2) into numbers and habits.

Budgets

PERF-1 Must: Meet the time budgets

Measured on the dev host in the headless labwc harness (see Measuring), where an old laptop is about three to five times slower:

Moment	Budget	Today
Start a window → first frame	≤ 1 s, content may still be loading	Settings 0.6 s, of which 0.25 s is Python and GTK starting
Click or key → a running daemon’s surface drawn	≤ 100 ms	Start menu 34 ms the first time, then 3–5 ms
Keystroke → search results	≤ 16 ms (one frame)	Start menu 1.5–3 ms
A command’s list/current	≤ 200 ms	about 130 ms, mostly Python starting
Login → panel on screen	Nothing FlickOS adds may delay it	Autostart starts everything at once, in the background

A change that makes one of these slower says so and why, with the numbers before and after.

PERF-2 Must: Never block the main loop

The window keeps drawing and reacting while work happens:

• Reading (running a command, reading many files) happens in a thread: Ui.background(work, done, key) calls done back in the main loop, and only the newest call per key is shown, so a slow old answer can’t overwrite a newer one.
• Changing runs the owner’s command with Gio.Subprocess and communicate_utf8_async (Ui.run_async()).
• Never in a signal handler or a constructor: subprocess.run(), time.sleep(), network access, or walking a large folder.

PERF-3 Must: Build first, read later

• A window’s constructors only build widgets. A page reads its values in shown(), when it is first displayed, with a spinner until the answer comes back (WIN-11).
• GTK is imported only when a window opens, so the program’s command-line commands start quickly and work without a display (list over ssh, in the boot test, in unit tests).
• When a page needs many values from one owner, the owner gets a command that returns them all at once (flickos-layout state: one run instead of 16).

Costing nothing

PERF-4 Must: Nothing runs while a feature is off

Turned off means no process, no timer and no file in the overlay (INT-9). A feature that is installed but off costs only its disk space (the start menu: about 50 KB).

PERF-5 Must: Idle means asleep

A process that runs for the session wakes only when something happens: a Wayland event, a signal, a D-Bus message, a file change. No polling loops and no timers that fire while nothing is on screen. A clock that ticks only while its page is visible is fine. A loop that checks a file every second is not.

PERF-6 Must: A daemon has to earn its memory

A GTK 3 Python process holds about 19 MB of private memory before it shows anything. Make a program a daemon only when starting it per use would miss the time budget, and write the measurement down (the start menu is a daemon because a click must draw it within 100 ms. Python alone takes about 30 ms to start).

• Signal a running daemon with pkill -SIGNAL -x NAME (about 1 ms), not by starting Python to talk to it.
• A daemon that is only needed in some layouts or while a feature is on starts and stops with it (flickos-layout panel and set).

Size

PERF-7 Must: State the cost of what you add

Before a new program, daemon, dependency or feature is merged, its costs are measured and written in its design doc (or the commit message for a small change):

• memory: private or PSS of each new process (tools/measure.py PID, which reads /proc/PID/smaps_rollup), with and without its window shown,
• disk: installed size including every new dependency, compared with the image rather than the build host (tools/package-cost.py PKG, an apt install --simulate without Recommends),
• time: its effect on the PERF-1 budgets and on login,
• CPU while idle: zero, or why not (tools/measure.py also counts wakeups a minute).

PERF-8 Must: No heavy dependencies

• No web engines (WebKitGTK, Chromium, Electron), no second toolkit (Qt, GTK 4) and no JavaScript runtimes, directly or through a dependency. gufw was rejected for pulling in 134 MB of WebKitGTK.
• FlickOS’s Python programs use the standard library plus python3-gi. A new Python dependency needs a reason in debian/control.
• A new dependency of more than about 10 MB installed is a maintainer’s decision, recorded with its size.
• Everything in flickos-desktop’s Depends is installed everywhere, forever, so it is the most expensive place to add something.

Measuring

PERF-9 Should: Measure, don’t guess

• Time real starts in the headless labwc harness (labwc 0.8.3 from trixie debs), not on the developer’s own desktop.
• A program with a hot path gets a bench command that times it (flickos-menu bench: the app index and search).
• Record results with the date, the machine and the package version, next to the decision they support (docs/design/flickos-menu.md has a table).

PERF-10 Should: Do slow work before it is needed, off the critical path

• Build a daemon’s window hidden and call realize() and get_preferred_size() ahead of time: the start menu’s first show dropped from 51–105 ms to 9–23 ms.
• Build indexes in a low-priority idle callback after the window is up (GLib.idle_add(..., priority=GLib.PRIORITY_LOW)).
• Warm caches the first interaction would otherwise fill (render the icons the first search will show).