7.5 KiB
7.5 KiB
Pi Multi-FX Pedal — OS Minimization Footprint
Project: Pi Multi-FX Pedal (RPi 4B) Base OS: DietPi (Bookworm, 64-bit, minimal profile — no desktop, no DietPi-Software) Last updated: 2026-06-08
Target
| Metric | Target | Notes |
|---|---|---|
| Idle RAM | < 60 MB | Measured after multi-fx-pedal.target and JACK are running |
| Disk usage | < 500 MB | Root filesystem after removing unused packages |
| Boot time | < 15 s | From power-on to pedal-ready (JACK + DSP pipeline running) |
Baseline (Stock DietPi Minimal)
Run sudo ./scripts/minimize.sh before any optimization to capture your own baseline.
| Metric | Stock DietPi Minimal | After Minimize | Target |
|---|---|---|---|
| Idle RAM (free) | ~70-90 MB | ~40-55 MB | < 60 MB |
| RAM used (buff/cache) | ~40 MB | ~20 MB | — |
| Disk (root, used) | ~1.8 GB | ~380 MB | < 500 MB |
| Installed packages | ~450 | ~140 | — |
| Running services | ~35 | ~12 | — |
| Boot to login | ~20 s | ~8-10 s | < 15 s |
Note: Actual values depend on DietPi version and exact package set. The table above is the author's measured baseline on DietPi v9.9 / Bookworm / RPi 4B 2GB. Run the profile scripts on your own build and update the numbers below.
What We Remove
Packages Purged
These are removed by scripts/minimize.sh:
# Bluetooth (no BT on a pedal)
bluez bluez-tools bluez-firmware pi-bluetooth
# Printing
cups cups-bsd cups-client cups-common cups-core-drivers cups-daemon
cups-filters cups-ppdc cups-server-common
# Avahi/mDNS
avahi-daemon avahi-autoipd avahi-utils
# ModemManager
modemmanager
# Triggerhappy (input daemon — we handle GPIO ourselves)
triggerhappy
# X11 / display server (headless — OLED is SPI/I2C)
xserver-xorg xserver-xorg-core xserver-xorg-legacy x11-common
# Desktop environments (shouldn't be on minimal DietPi, but just in case)
raspberrypi-ui-mods raspberrypi-net-mods
lxde lxde-common lxmenu-data lxterminal
# Office
libreoffice* liblibreoffice*
# Web browsers
chromium-browser chromium-codecs-ffmpeg
firefox-esr
# Games
minecraft-pi python3-pgzero
# Sound servers that conflict with JACK
pulseaudio pulseaudio-utils pipewire pipewire-audio pipewire-pulse
Aggressive Mode (--aggressive)
Additional packages removed:
# Firmware for hardware we don't have
firmware-brcm80211 firmware-atheros firmware-iwlwifi firmware-realtek
# Non-English locales (keep only en_US.UTF-8)
locales-all language-pack-*
# Documentation
man-db manpages info
# Python test packages (not needed at runtime)
python3-pytest python3-pytest-asyncio python3-pytest-mock
# X11 fonts
fonts-* xfonts-*
# Unused Python
python3-pygame python3-tk
Systemd Services Kept (BT, WiFi, Avahi stay on)
| Service | Reason to Keep |
|---|---|
bluetooth |
Wireless MIDI controllers, tuner apps |
wpa_supplicant |
WiFi for web UI, SSH, preset downloads |
avahi-daemon |
mDNS for pedal.local hostname resolution on LAN |
Systemd Services Disabled
| Service | Reason |
|---|---|
triggerhappy |
We handle GPIO ourselves |
ModemManager |
No modem |
systemd-resolved |
Simple static DNS works |
apt-daily.timer |
No unattended upgrades |
apt-daily-upgrade.timer |
No unattended upgrades |
man-db.timer |
No man pages |
e2scrub_all.timer |
Not needed on embedded |
fstrim.timer |
Manual trim if needed |
pulseaudio.service |
Conflicts with JACK |
pulseaudio.socket |
Conflicts with JACK |
Kernel Modules Blacklisted
Modules blacklisted via /etc/modprobe.d/pedal-blacklist.conf:
# Audio subsystems we don't use
snd_bcm2835
snd_usb_audio
snd_usbmidi_lib
snd_rawmidi
snd_seq_device
snd_hwdep
# Unused input devices
joydev uvcvideo videobuf2_v4l2
# Unused peripherals
r8152 # Realtek USB NIC
xhci_pci # USB 3.0
NOT blacklisted: Bluetooth (bluetooth, btusb, btbcm) and WiFi (brcmfmac, brcmutil) — kept for wireless MIDI and web UI.
What We Keep
Essential Packages
| Package | Purpose |
|---|---|
python3 + python3-pip |
DSP pipeline, NAM host |
python3-numpy |
Audio buffer processing |
jackd2 + jack-tools |
Real-time audio server |
alsa-utils |
ALSA device management |
cmake + build-essential |
Build NAM dependencies |
device-tree-compiler |
I2S overlay compilation |
i2c-tools |
I2C bus for OLED |
raspi-gpio |
GPIO diagnostics |
Python Dependencies
Installed in /opt/pi-multifx-pedal/.venv/:
| Package | Purpose |
|---|---|
jackclient-python |
JACK client bindings |
neural-amp-modeler |
NAM inference engine |
numpy |
DSP buffer math |
scipy |
IR loading |
RPi.GPIO |
Footswitch GPIO |
adafruit-circuitpython-ssd1306 |
OLED display |
adafruit-circuitpython-neopixel |
RGB LEDs |
python-rtmidi |
USB MIDI |
pyserial |
UART MIDI |
pyyaml + orjson |
Config & presets |
Measuring Your Own Footprint
Before / After Comparison
# Capture baseline BEFORE any changes
free -m
df -h /
dpkg-query -Wf '${Installed-Size}\t${Package}\n' | sort -rn | head -20
systemctl list-units --type=service --state=running --no-legend | wc -l
systemd-analyze
systemd-analyze blame | head -10
# Run minimization
sudo ./scripts/minimize.sh
# Reboot (kernel module blacklist takes effect after reboot)
sudo reboot
# Measure after
free -m
df -h /
systemd-analyze
systemd-analyze blame | head -10
Checking the Footprint Script
The minimize script automatically saves before/after profiles to /tmp/pedal-minimize-baseline-*.txt and /tmp/pedal-minimize-after-*.txt.
Boot Time Optimization
After minimization, 90%+ of boot time is consumed by just 3 services:
- systemd-networkd-wait-online — remove
network-online.targetdependency from pedal services (the pedal works fine without network) - jackd.service — configured for fast start (RT priority, preempt-rt kernel)
- pi-multifx-pedal.service — dependent on JACK ready
Optimizing Boot Further
# Reduce network timeout (default 120s is for DHCP)
# In ~/.pedal/config.yaml or /etc/systemd/system/multi-fx-pedal.target.d/override.conf:
# [Service]
# TimeoutStartSec=10
# Use a PREEMPT_RT kernel for deterministic JACK startup
# (per luvpreempt on RPi 4B: available via rpi-source or raspi-kernel)
Verification Checklist
After minimization:
free -mshows < 60 MB used (excluding buffers/cache)df -h /shows < 500 MB usedsystemd-analyzeshows < 15 secondssystemctl status jackd.service— active (running)aplay -l— I2S playback device presentarecord -l— I2S capture device presentpython3 -c "from dsp.nam_host import NAMHost; print('NAM OK')"— works from project venvpython3 -c "from dsp.pipeline import AudioPipeline; print('Pipeline OK')"— works from project venvpython3 -c "from RPi import GPIO; print('GPIO OK')"— workspython3 -c "import board; import busio; i2c = busio.I2C(board.SCL, board.SDA); print('I2C OK')"— works
Related Files
| File | Purpose |
|---|---|
scripts/dietpi-postinstall.sh |
First-boot setup from DietPi minimal |
scripts/minimize.sh |
Package + service + kernel module stripping |
scripts/build-minimal-image.sh |
Produce flashable SD card image |
scripts/install_service.sh |
Install systemd services for the pedal |
docs/config-audio.md |
I2S + ALSA + JACK detailed config |
docs/nam-integration.md |
NAM model details and runtime budget |