napd — Linux App Nap Daemon

Automatically throttles background applications on Linux, inspired by macOS App Nap.

When you switch focus to another window, napd puts background apps to sleep: they get lower CPU priority, idle I/O scheduling, and relaxed timer resolution. The moment you bring an app back to the foreground, it is fully restored.

How it works

1. Monitors window focus via zwlr_foreign_toplevel_manager_v1 (Wayland)
2. Waits for a configurable grace period before throttling
3. Skips apps that are playing audio (tracked via PipeWire)
4. Applies a bundle of throttle knobs to background processes and all their children:
   • Moves to a CPU-limited cgroup (cpu.max)
   • Sets scheduling policy to SCHED_BATCH
   • Sets CPU nice to +19
   • Sets I/O priority to IOPRIO_CLASS_IDLE
   • Sets timer slack to 2 seconds
5. Fully restores all saved state on focus return or daemon shutdown

Requirements

• Linux with cgroup v2
• Wayland compositor with zwlr_foreign_toplevel_manager_v1 (Sway, Niri, wlroots-based)
• PipeWire (for audio bypass)
• Rust toolchain

Install

# Install build dependency
sudo apt install libpipewire-0.3-dev      # Debian/Ubuntu
sudo dnf install pipewire-devel           # Fedora
sudo apt-get install libpipewire-devel    # ALT Linux

# Build and install as a systemd user service
./install.sh

The script builds the binary, installs it to /usr/local/bin/napd, and enables the systemd user service.

Uninstall

./uninstall.sh

Configuration

Config file: /etc/napd/config.toml (or override with NAP_CONFIG=/path/to/config.toml)

If the file does not exist, built-in defaults are used.

# Seconds an app must stay in background before throttling begins
grace_period_secs = 5

# Timer slack for background threads (nanoseconds). Max ~4.29s (kernel u32 limit).
bg_timerslack_ns = 2_000_000_000

# Timer slack restored for foreground threads (nanoseconds)
fg_timerslack_ns = 50_000

# Cgroup path relative to /sys/fs/cgroup/
bg_cgroup = "background"

# CPU quota for background cgroup in µs per 100ms period.
# Example: 10000 = 10% of one core. Comment out for unlimited.
# bg_cpu_quota_us = 10000

# App IDs to never throttle (Wayland app_id values)
bypass_app_ids = ["spotify", "org.mozilla.firefox"]

Logging

# Follow live logs
journalctl --user -u napd -f

# Verbose output
RUST_LOG=debug napd

Limitations

• Wayland only — X11 is not supported
• Process matching uses heuristics (comm, cmdline basename) to map app_id to PIDs; some apps may not be found
• Requires cgroup v2 unified hierarchy for CPU quota enforcement