wattaouille

watt + ratatouille + ouille! (ouch!) — a French pun for the moment you find out what's been eating your laptop's battery.

A live, friendly CPU and energy monitor for Linux laptops, written in Rust with zero dependencies — just std and /proc//sys.

It started as "what's making my ThinkPad's fan spin?" and grew into something that answers, in plain language:

• What's eating my CPU right now?
• What has been eating it across this whole session?
• How many watts is the package pulling, and which process is to blame?
• Is the battery seeing more draw than the CPU explains? By how much?
• How long until the battery dies at the current rate?

If you've ever stared at htop and wished it would just tell you that the spinning thing was Librewolf and not "300 lines of process tree", this is for you.

wattaouille — 1500 ms · 12 CPU(s) · 715 procs · ~42s tracked
  · ⚡ RAPL 12.5 W avg · 0.146 Wh (525 J)
  · 🔋 BAT 18.0 W avg · 0.210 Wh (756 J) · 87% · 4h 23m left
  · Δ non-CPU +5.5 W avg · 0.064 Wh (231 J · +31%)

SESSION TOP CONSUMERS  (cumulative since program start)
    PID     AVG%     NOW%    NOW W      TOTAL W (J)   COMMAND
  18105    47.4%    48.5%   1.25W    0.65W (27J)     Claude Code (Happy: wattaouille)
  24040    44.1%    49.9%   1.10W    0.61W (26J)     Claude Code (Happy: action-pull-request-merge)
   4785    21.9%    25.3%   0.55W    0.30W (13J)     Guake terminal
   4560    17.6%    17.6%   0.45W    0.24W (10J)     [librewolf, 50 procs] librewolf …
   3653     8.6%    10.6%   0.21W    0.12W (5J)      Xorg :0 -seat seat0 …

LIVE TREE  (this 1500 ms sample)
    PID     %CPU    %CORE  TREE / COMMAND
   4785     2.1%    25.3%  Guake terminal
  18063     0.4%     4.7%  ├─ Happy (wattaouille)
  18105     4.0%    48.5%  │  └─ Claude Code (Happy: wattaouille)
  24040     4.2%    49.9%  ├─ Claude Code (Happy: dotfiles)
  17274     0.4%     5.3%  └─ Claude Code (Happy: action-pull-request-merge)
   3653     0.9%    10.6%  Xorg :0 …
   4560     0.2%     2.7%  [librewolf, 50 procs] librewolf …
  28421     0.1%     1.0%  [opera, 42 procs] opera
  29173     0.1%     1.0%  mysqld (projA)
  29286     0.1%     1.0%  mysqld (projB)

Quick start

git clone <this repo> wattaouille && cd wattaouille
cargo build --release
./target/release/wattaouille

That's it. No npm, no pip, no apt install, no daemon. One static binary.

cargo test --release   # 27 unit tests, no /proc dependence

What makes it different

It hides the boring stuff

init → lightdm → lightdm-session-child → xfce4-session → bash → bash → bash → node → node → node → claude.exe — eight rows of context, one busy process. Most monitors print all of it.

wattaouille flattens any 0%-CPU "middleman" out of the tree and surfaces the actual busy leaf at the top level. The result is a tree that fits on one screen and tells you who, not via what chain.

It collapses browser worker farms

Modern browsers spawn 40+ helper processes. They make the tree unreadable.

wattaouille knows about Firefox/Librewolf, Chrome/Chromium, Opera, Brave and Vivaldi. The whole subtree folds into a single line:

4560   17.6%   2.7%   [librewolf, 50 procs]   librewolf --sm-client-id …

CPU and watts shown are the aggregate across the whole subtree.

It speaks human about the things you actually run

• claude.exe --session-id 83df5645-… --append-system-prompt ALWAYS when you start a new chat … (370 chars) becomes Claude Code (Happy: wattaouille). The folder name is the session, taken straight from /proc/[pid]/cwd.
• node /home/you/.config/yarn/global/node_modules/happy/dist/index.mjs claude becomes Happy (wattaouille). Daemon variants become Happy daemon.
• /usr/share/smartgit/jre/bin/java -XX:+UseG1GC … -jar /usr/share/smartgit/lib/bootloader.jar becomes SmartGit. (Yes, that's all it ever needed to say.)
• python3 /usr/bin/guake becomes Guake terminal.
• mysqld --sql_mode= running with cwd /var/lib/mysql/projA becomes mysqld (projA) — perfect when you're juggling multiple instances.

It remembers

CPU readings refresh every 1.5 s by default — without memory, the busiest process keeps moving up and down the list. Hard to spot.

wattaouille keeps a session-cumulative leaderboard at the top of every frame: AVG% is your real heavy hitter, NOW% is the live spike, and the row sticks even when the process briefly idles.

It tells you watts

If the kernel lets it (mode 0400 on RAPL since the Platypus side-channel disclosure — see Setup), wattaouille reads Intel RAPL package energy and computes:

• Total package draw in W avg, Wh and J for the session
• Per-process W and J by scaling the package total by each process's CPU share (same trick Scaphandre uses — silicon doesn't actually report per-PID wattage)

If the kernel doesn't let it, wattaouille still works: a single warning line, then everything-but-watts.

It cross-checks against your battery

Unplug the laptop and wattaouille reads /sys/class/power_supply/BAT*/power_now and shows it next to RAPL, including:

• Average wattage and Wh consumed while discharging
• Current state of charge (e.g. 87%)
• Time-to-empty at the current draw (e.g. 4h 23m left)
• Drift — Δ non-CPU +5.5 W avg · 0.064 Wh (231 J · +31%) — the battery draw the CPU package doesn't account for: screen, RAM, NVMe, Wi-Fi, USB. The drift is shown as a rate (W avg) and as totals (Wh/J), so you can read it as "the rest of the system is currently pulling about 5 W."

Plug back in and BAT switches to 🔌 on AC · 87% and stops accumulating drift data.

Usage

wattaouille [OPTIONS]

  -i, --interval <MS>   Sampling interval in milliseconds [default: 1500]
  -n, --rows <N>        Total rows budget per frame       [default: 50]
      --no-power        Force wattage off (preview new-user fallback path)
  -h, --help            Show full help and exit

Hit Ctrl+C to quit. The terminal goes back to whatever was on screen before — wattaouille runs in the alternate screen buffer (the same trick htop, vim and less use), so your scrollback isn't polluted.

Setup (wattage, optional)

Modern kernels make RAPL counters root-readable only. To enable per-process wattage as your user:

One-shot for this boot:

sudo chmod a+r /sys/class/powercap/intel-rapl:0/energy_uj \
     /sys/class/powercap/intel-rapl/intel-rapl:0/intel-rapl:0:*/energy_uj

Persist across reboots (udev rule):

echo 'SUBSYSTEM=="powercap", ACTION=="add", RUN+="/bin/chmod a+r /sys%p/energy_uj"' \
  | sudo tee /etc/udev/rules.d/60-rapl-readable.rules
sudo udevadm control --reload && sudo udevadm trigger --subsystem-match=powercap

Or just run with sudo:

sudo ./target/release/wattaouille

If RAPL isn't readable at startup, wattaouille prints these instructions to your scrollback and waits for Enter before running. Try wattaouille --no-power to preview that path even on a working setup.

Reading the columns

Header:

Field	Meaning
1500 ms	Sampling interval
12 CPU(s)	Logical cores in /proc/stat
715 procs	Live processes this frame
~42s tracked	Cumulative wall time of the session
⚡ RAPL X W avg · Y Wh (Z J)	Average package wattage and total energy since start
🔋 BAT X W avg · Y Wh (Z J) · N% · Hh Mm left	Battery side: avg draw, total energy used, state of charge, time-to-empty
🔌 on AC · N%	Same field while plugged in (state of charge only)
Δ non-CPU +X W avg · Y Wh (Z J · ±N%)	Battery draw NOT accounted for by RAPL (display + RAM + …)

Session leaderboard:

Column	Meaning
PID	Process ID (or the collapse-root PID for browser subtrees)
AVG%	Cumulative CPU since start, as % of one core
NOW%	This frame's CPU, as % of one core
NOW W	Estimated watts this frame
TOTAL W (J)	Average wattage and total joules since start

Live tree:

Column	Meaning
PID	Process ID
%CPU	Share of total system CPU during the sample
%CORE	top-style percent of one core
TREE	Process tree. [<browser>, N procs] = whole browser subtree folded.

How does it work?

Per frame:

1. Read /proc/[pid]/{stat,cmdline,cwd} for every PID. ~1 ms.
2. Diff against the previous snapshot to get jiffies-per-process and /proc/stat's aggregate to get total system jiffies.
3. Read /sys/class/powercap/intel-rapl:0/energy_uj (twice, before and after the sleep). Difference / interval = package watts.
4. Read /sys/class/power_supply/BAT*/{power_now, energy_now, energy_full} and the AC online flag. Compute battery wattage, state of charge, and time-to-empty.
5. Distribute package watts to processes by CPU share.
6. Walk the tree, hide 0% middlemen, collapse browser subtrees, sort each level by subtree CPU desc.
7. Update session-cumulative jiffies and joules per PID.
8. Render leaderboard + tree, cap at --rows, flush.

Total external dependencies: zero. Cargo.toml lists none.

Tests

cargo test --release

Twenty-seven unit tests cover the things that don't need /proc: process labelling (Claude Code, Happy, SmartGit, Guake, mysqld), browser collapse detection, RAPL counter wraparound, the flatten_visible tree-promotion rules, the XhYm time formatter, and cwd_basename edge cases. Run them in CI; they don't touch real sysfs.

Limitations

• Linux only. It's all /proc and /sys.
• Per-process watts are estimates. No silicon reports per-PID power. We attribute by CPU share; if your process pegs a core, it gets the watt credit. iGPU work shows up under the iGPU domain (see /sys/class/powercap/intel-rapl:0:1/uncore) which we don't currently break out.
• AMD/ARM: the kernel exposes RAPL only on Intel. AMD's amd_energy driver lives elsewhere; not yet supported.

Changelog

See CHANGELOG.md for the per-release history.

License

MPL-2.0 — Mozilla Public License v2.0. File-level copyleft: you can use wattaouille in proprietary projects, but modifications to existing wattaouille files must be shared back under MPL-2.0.

Special thanks

• Scaphandre — open-source energy-consumption metrology agent. wattaouille reuses its core insight (per-process watts = package watts × CPU share) and would have been far more painful to figure out without it.
• The Linux kernel intel_rapl_common and intel_rapl_msr modules (alongside the powercap subsystem) for exposing the RAPL counters at /sys/class/powercap/intel-rapl:0/. Without them this whole project would have to talk to MSRs directly and live in kernel space.
• The Linux kernel power_supply subsystem maintainers for the BAT* and AC sysfs entries that the battery cross-check reads from.

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Written while listening to Indila's "Mini World":

mini world mini..mini world mini