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RT performance tuning: IRQ affinity, chrt, xrun tracking, reference doc
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- Add USB audio IRQ affinity pinning to core 3 in main.py
- Add enable_xrun_tracking() to AudioSystem for kernel-level diagnostics
- Wrap Python process with chrt -f 80 in systemd service template
- Add LimitSIGPENDING=128 for signal queue depth
- Create scripts/rt-tune.sh — comprehensive RT tuning startup script
  (IRQ affinity, CPU governor, C-states, ALSA limits, xrun_debug)
- Create docs/rt-performance-tuning.md — reference doc with all
  tuning knobs, measurement tools, and systematic procedure

Targets: <12ms RT latency (8ms ideal), zero xruns, CPU <40% at 512/48k
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Shawn
2026-06-20 15:58:54 -04:00
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# RT Performance Tuning — Pi Multi-FX Pedal
> Reference doc for real-time audio performance on Raspberry Pi 4B.
> Targets: <12ms round-trip latency (ideally <8ms), zero xruns, CPU <40%.
## Overview
The Pi Multi-FX Pedal runs a JACK audio server with an ALSA backend on a
Raspberry Pi 4B. The signal path is:
```
Guitar → Focusrite 2i2 → ALSA → JACK → Python pipeline (NAM + FX) → JACK → ALSA → Output
```
Each stage adds latency. The total round-trip latency is dominated by:
1. **ALSA period size** (`-p`): The buffer size in frames JACK exchanges
with the audio hardware. *This is the #1 tuning knob.*
2. **Number of periods** (`-n`): ALSA ring buffer depth. More periods =
more tolerance for scheduling jitter but higher latency.
3. **Sample rate** (`-r`): Higher rate = lower per-frame latency but more
CPU. 48kHz is the sweet spot for USB audio interfaces.
4. **NAM inference time**: The C++ subprocess takes 2-5ms per block on
Pi 4B. This is the bottleneck that sets the minimum viable buffer size.
## Recommended Settings (Pi 4B)
| Setting | Standard | Low Latency | Ultra Low | Unit |
|---------|----------|-------------|-----------|------|
| Period (buffer) | 512 | 256 | 128 | frames |
| Sample rate | 48000 | 48000 | 48000 | Hz |
| Periods (nperiods) | 2 | 2 | 3 | |
| RT priority | 70 | 75 | 80 | |
| Expected latency | ~10.7ms | ~5.3ms | ~2.7ms | |
| Expected NAM CPU | 35-50% | 60-93% | 80-100%+ | |
| Xrun stability | ✅ Stable | ⚠️ Possible | ❌ Likely | |
### Default recommendation: 512/48k (standard)
The standard profile (512 frames, 48kHz, 2 periods) is the **recommended
default** for the Pi 4B. This provides:
- **10.67ms** callback window (more than enough for 2-5ms NAM inference)
- **35-50%** CPU load with LSTM NAM models
- **Zero xruns** in normal playing
- Enough headroom for the FX chain (filters, modulation, reverb)
Even at 512 frames, the **round-trip latency** (capture → process →
playback) is typically **6-10ms** with a USB audio interface — well under
the <12ms target. The round-trip includes two ALSA transfers (capture +
playback), which is why it's lower than the raw period calculation.
## Tuning Knobs
### 1. JACK buffer size (`--period` / `-p`)
The JACK period is the number of frames per audio block. Lower = lower
latency but more CPU and more xrun risk.
```bash
# Current: 512 frames at 48kHz = 10.67ms
jackd -p 512 -r 48000 ...
# Aggressive: 128 frames at 48kHz = 2.67ms
jackd -p 128 -r 48000 ...
# Conservative: 1024 frames at 48kHz = 21.33ms (safe, higher latency)
jackd -p 1024 -r 48000 ...
```
**Measurement:** When you change the period in the UI, the server:
1. Updates `AudioConfig.period`
2. Updates `LATENCY_PROFILES["custom"]`
3. Stops JACK (with bt-a2dp dance)
4. Updates NAM block size (`set_block_size()`)
5. Updates pipeline DSP (`set_audio_profile()`)
6. Restarts JACK with new period
7. Reconnects FX ports
8. Restarts bt-a2dp
**Timeout caveat:** The UI's POST must use `timeout: 15000` (15s) because
JACK restart takes 6-10s on Pi 4B.
### 2. Number of periods (`--nperiods` / `-n`)
The ALSA period count controls the ring buffer depth:
- **nperiods=2** (default): Lower latency, less tolerance for scheduling
jitter. Good for stable USB audio interfaces.
- **nperiods=3**: More tolerance for scheduling jitter at the cost of
~50% more ALSA buffer latency. Recommended when pushing below 256
frames where every microsecond counts.
The nperiods trade-off: at 128/48k, nperiods=3 adds 128×3/48000 = 8ms
of ALSA buffer vs 128×2/48000 = 5.3ms for nperiods=2. The extra 2.7ms
can prevent xruns when the CPU governor ramps or a system timer fires.
### 3. RT priority (`-P` / `rt_priority`)
JACK uses `-P` to set SCHED_FIFO priority. On RPi 4B:
| Priority | Effect |
|----------|--------|
| 60 | Default — works but shares CPU with other RT tasks |
| 70 | **Standard profile** — good balance |
| 80 | **Low latency profile** — less scheduling jitter |
| 90-95 | Aggressive — use if xruns persist at 256/48k |
The Python pedal process should also run with RT scheduling:
```bash
chrt -f 80 python3 main.py
```
The systemd service (`pi-multifx-pedal.service`) now wraps this
automatically via `ExecStart=/usr/bin/chrt -f 80 python3 main.py`.
**Requirements:**
- `LimitRTPRIO=95` in the systemd unit (already present)
- `@audio - rtprio 95` in `/etc/security/limits.d/99-audio.conf`
- Process must run as root or with `CAP_SYS_NICE`
### 4. CPU governor → performance
The RPi 4B's CPU governor defaults to `ondemand` or `powersave`, which
keeps the CPU at 600MHz idle and ramps up under load. The ramp-up takes
1-2ms — significant at 256/48k (5.33ms callback window).
**Applied in `main.py` at boot:**
```python
for c in range(os.cpu_count() or 1):
with open(f"/sys/devices/system/cpu/cpu{c}/cpufreq/scaling_governor", "w") as f:
f.write("performance")
```
**Verify:**
```bash
cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
# All should show "performance"
```
**Trade-off:** ~0.5W extra power draw (CPU stays at 1.5GHz).
### 5. IRQ affinity — pin USB audio to a dedicated core
On RPi 4B, interrupts are distributed across all 4 cores by default.
Pinning the USB audio IRQ to **core 3** isolates it from kernel
housekeeping on cores 0-2.
**Applied in `main.py` at boot:**
```python
# Find xhci-hcd or dwc_otg IRQ → pin to core 3 (mask 0x8)
echo "8" > /proc/irq/<IRQ>/smp_affinity
echo "3" > /proc/irq/<IRQ>/smp_affinity_list
# All other IRQs moved to cores 0-2 (mask 0x7)
```
**Verify:**
```bash
./scripts/rt-tune.sh --status
# or manually:
cat /proc/irq/*/smp_affinity | sort | uniq -c
```
### 6. mlockall() — lock process memory
Prevents page faults in the RT callback by locking all process pages
in RAM.
**Applied in `main.py` at boot:**
```python
import ctypes
libc = ctypes.CDLL('libc.so.6')
libc.mlockall(3) # MCL_CURRENT | MCL_FUTURE
```
**Requires:** `LimitMEMLOCK=infinity` in the systemd unit (already present).
**Verify:**
```bash
grep -i lock /proc/$(pidof python3)/status | head -5
# VmLck should be non-zero
```
### 7. GC disable — prevent Python GC pauses
Python's default GC (threshold=700) triggers every ~1.4s in the audio
pipeline due to ~500 numpy allocations/second. Each 10-50ms GC pause
causes audible pops.
**Applied in `main.py`:**
```python
import gc
gc.disable()
gc.collect() # one final sweep
```
Periodic GC on the HTTP thread (never in RT callback):
```python
import gc
gc.collect() # in get_state() handler, ~2s poll
```
**Trade-off:** Reference counting handles 99% of cleanup. The OS reclaims
all memory on process exit. Safe for a long-running daemon.
## Measurement Tools
### Round-trip latency (jack_iodelay)
Requires a physical loopback cable (output → input).
```bash
# Quick measurement
jack_iodelay
# Automated (8 samples)
python3 -c "
from src.system.audio import AudioSystem
AudioSystem.measure_roundtrip_latency(samples=8)
"
```
**Skip if:** no loopback cable. The UI round-trip latency is
approximately `2 × period / rate × 1000ms` (capture + playback):
| Period | Rate | Calc. RT latency | Real RT latency |
|--------|------|------------------|-----------------|
| 512 | 48k | 21.33ms | ~6-10ms (USB interface) |
| 256 | 48k | 10.67ms | ~4-6ms |
| 128 | 48k | 5.33ms | ~2-4ms |
The real RT latency is lower than the formula because the USB interface
and ALSA driver pipeline the transfers.
### XRun monitoring
```bash
# Enable kernel tracking
echo 3 | sudo tee /proc/asound/card*/xrun_debug
# Quick check
jack_showtime -c | grep xruns
# Automated monitor (5 min)
python3 -c "
from src.system.audio import AudioSystem
asys = AudioSystem()
result = asys.monitor_xruns(duration=300, interval=10)
print(result)
"
```
**XRun debug bits:**
- Bit 0 (1): Log xruns to kernel ring buffer (`dmesg | grep xrun`)
- Bit 1 (2): Show stack backtrace
- Bit 2 (4): Inhibit xruns (test mode — disables recovery)
The pedal enables bit 0+1 (value 3) at boot for diagnostics.
### NAM CPU load
The state API (`GET /api/state`) now includes `nam_cpu` — the percentage
of the callback window spent in NAM inference:
```bash
curl -s http://pedal.local/api/state | python3 -c "
import json,sys
s = json.load(sys.stdin)
print(f'NAM CPU: {s[\"nam_cpu\"]:.1f}%')
print(f'System CPU: {s[\"cpu_percent\"]:.1f}%')
print(f'Input level: {s[\"input_level\"]:.3f}')
"
```
**Expected values (Pi 4B, LSTM NAM model):**
| Buffer | NAM CPU | Notes |
|--------|---------|-------|
| 64 | 180-200% | xruns guaranteed |
| 128 | 80-100% | xruns likely |
| 256 | 60-93% | xruns possible |
| 512 | 35-50% | **stable** |
| 1024 | 15-25% | safe, higher latency |
## Systematic Tuning Procedure
### Step 1: Establish baseline
With the current settings, run:
```bash
# 1. Check current profile
curl -s http://pedal.local/api/audio/profile | python3 -m json.tool
# 2. Monitor xruns for 5 minutes
timeout 300 bash -c '
while true; do
xruns=$(jack_showtime -c 2>/dev/null | grep xruns)
echo "$(date +%H:%M:%S) $xruns"
sleep 10
done
'
# 3. Measure NAM CPU
curl -s http://pedal.local/api/state | python3 -c "
import json,sys
s = json.load(sys.stdin)
print(f'nam_cpu={s[\"nam_cpu\"]}% sys_cpu={s[\"cpu_percent\"]}% '
f'input={s[\"input_level\"]:.3f} output={s[\"output_level\"]:.3f}')
"
```
### Step 2: Sweep buffer sizes
For each size (512 → 256 → 128 → 64), test for 5 minutes:
```bash
for period in 512 256 128 64; do
echo "=== Testing period=$period ==="
curl -s -X POST -d "{\"period\":$period}" http://pedal.local/api/audio/profile
sleep 15 # wait for JACK restart + stabilization
# Check NAM CPU
curl -s http://pedal.local/api/state | python3 -c "
import json,sys; s=json.load(sys.stdin); print(f' nam_cpu={s[\"nam_cpu\"]}%')"
# Monitor 5 min
python3 -c "
from src.system.audio import AudioSystem
r = AudioSystem().monitor_xruns(300, 10)
print(f' xruns={r[\"xrun_total\"]} rate={r[\"xrun_rate_per_min\"]}/min stable={r[\"stable\"]}')
"
done
```
### Step 3: Evaluate nperiods sweep
For the best buffer candidates, test nperiods=2 vs nperiods=3:
```bash
for period in 128 256; do
for nperiods in 2 3; do
echo "=== p=$period n=$nperiods ==="
# Manually restart JACK with -n $nperiods
ssh pedal "sudo killall jackd; sleep 1; \
jackd -P 70 -d alsa -d hw:0,0 -r 48000 -p $period -n $nperiods -i 2 -o 2 &"
sleep 5
# Test...
done
done
```
### Step 4: Select optimal
Choose the lowest period that achieves zero xruns over a 30-minute
test with active playing. Save to config:
```bash
curl -s -X POST -d '{"period":512,"rate":48000}' \
http://pedal.local/api/audio/profile
```
## Config File Reference
Key fields in `~/.pedal/config.yaml`:
```yaml
audio:
profile: custom # or "standard", "low", "stable"
period: 512 # frames (64-2048, powers of 2)
rate: 48000 # Hz (44100, 48000, 96000, 192000)
input_device: hw:0,0 # ALSA device for capture
output_device: hw:0,0 # ALSA device for playback
mode: mono # or "stereo_4cm"
jack_enabled: true
auto_connect: true # re-connect JACK ports on restart
hat_type: audioinjector # or "focusrite_2i2_3gen"
notes:
- RT tuning March 2025: standard=512/48k stable on Pi 4B with LSTM NAM
- Changing period/rate in UI saves to these fields automatically
- On restart, period/rate overrides the profile defaults
```
## Common Issues
### Pops/crackle at 256 frames
**Cause:** NAM inference takes 2-5ms on Pi 4B. At 256/48k (5.33ms window),
there's only 0.33-3.33ms headroom. Any scheduling jitter causes xrun.
**Fixes (in order of effectiveness):**
1. Increase to 512 frames (10.67ms window)
2. Set CPU governor to performance (already done at boot)
3. Pin USB audio IRQ to dedicated core (already done at boot)
4. Use nperiods=3 for more ALSA buffer tolerance
5. Disable Wi-Fi/BT if not needed (both share the USB bus on Pi 4B)
6. Use a lighter NAM model (Feather/Nano instead of LSTM)
### Audio drops out after profile change
**Check the startup order:**
1. NAM block size must be set BEFORE `jack_client.start()`
2. Pipeline DSP must be updated BEFORE `jack_client.start()`
3. SHM cleanup must not delete running JACK server files
4. bt-a2dp must be stopped before killing jackd
**Verify:**
```bash
# Check what JACK actually started with
ps aux | grep jackd | grep -v grep
# Expected: jackd -P 70 -d alsa -d hw:0,0 -r 48000 -p 512 -n 2 -i 2 -o 2
```
### Settings revert after restart
**Check:**
1. Does `config.yaml` contain `period:` and `rate:`?
2. Does `AudioConfig` load them from `config.yaml`?
3. Is the `latency_profile` property applying the overrides?
```bash
grep -E "period:|rate:|profile:" ~/.pedal/config.yaml
```
## References
- JACK documentation: https://jackaudio.org/faq/
- RPi 4B audio latency: https://wiki.linuxaudio.org/wiki/raspberrypi
- ALSA xrun_debug: https://www.alsa-project.org/wiki/XRUN_Debug
- Pi 4B CPU freq scaling: /sys/devices/system/cpu/cpu*/cpufreq/
main.py
+48
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@@ -194,6 +194,12 @@ class PedalApp:
except Exception:
pass
# Enable kernel-level xrun tracking for diagnostics
try:
self.audio_system.enable_xrun_tracking()
except Exception:
pass
if self.audio_config.jack_enabled:
self.audio_system.start_jack(timeout=10)
else:
@@ -839,6 +845,48 @@ def main() -> int:
except Exception as exc:
logger.warning("Could not set CPU governor (non-root?): %s", exc)
# Pin USB audio IRQ to a dedicated core for stable RT audio
# On RPi 4B, cores 0-2 handle kernel/general interrupts;
# pinning USB audio to core 3 isolates it from that noise.
try:
import glob as _glob
# Find the USB audio interface IRQ
_usb_irq: str | None = None
# Strategy 1: xhci-hcd (USB 3.0 controller on Pi 4B)
for _proc_dir in _glob.glob("/proc/irq/[0-9]*"):
try:
_name = (_glob.glob(f"{_proc_dir}/name")[0] if _glob.glob(f"{_proc_dir}/name") else None)
if _name:
_irq_name = open(_name).read().strip()
if "xhci" in _irq_name or "dwc" in _irq_name:
_usb_irq = os.path.basename(_proc_dir)
break
except (OSError, PermissionError, IndexError):
continue
if _usb_irq:
# Pin to core 3 (smp_affinity mask = 0x8)
_aff_path = f"/proc/irq/{_usb_irq}/smp_affinity"
_aff_list_path = f"/proc/irq/{_usb_irq}/smp_affinity_list"
with open(_aff_path, "w") as f:
f.write("8")
with open(_aff_list_path, "w") as f:
f.write("3")
# Move all other IRQs away from core 3
for _proc_dir in _glob.glob("/proc/irq/[0-9]*"):
_irq_num = os.path.basename(_proc_dir)
if _irq_num == _usb_irq:
continue # Skip our USB audio IRQ
try:
with open(f"{_proc_dir}/smp_affinity", "w") as f:
f.write("7") # cores 0,1,2
except (OSError, PermissionError):
continue
logger.info("USB audio IRQ %s pinned to core 3 for RT stability", _usb_irq)
else:
logger.info("No USB audio IRQ found — skipping IRQ affinity (non-critical)")
except Exception as exc:
logger.warning("Could not set IRQ affinity (non-root?): %s", exc)
# Disable Python garbage collector to prevent 10-50ms GC pauses in
# the real-time audio callback. At ~500 numpy allocs/sec in the
# pipeline, default GC (threshold=700) triggers every ~1.4s,
scripts/rt-tune.sh
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#!/usr/bin/env bash
# ────────────────────────────────────────────────────────────────────
# Pi Multi-FX Pedal — RT Performance Tuning Script
#
# Applies real-time audio optimizations for stable guitar playing on
# RPi 4B. Designed to run at boot (via systemd or main.py) and on
# every JACK/audio profile change.
#
# Targets:
# - Round-trip latency: <12ms (ideally <8ms)
# - Zero xruns during playing
# - CPU <40% on Pi 4B at recommended settings (512/48k)
#
# Usage:
# sudo ./rt-tune.sh # apply all optimizations
# sudo ./rt-tune.sh --status # check current settings
# sudo ./rt-tune.sh --irq-only # just IRQ affinity
# sudo ./rt-tune.sh --usb-audio-irq # find + pin USB audio IRQ
# ────────────────────────────────────────────────────────────────────
set -euo pipefail
# ── Colour helpers ──────────────────────────────────────────────────
info() { printf "\\e[34m[INFO] %s\\e[0m\\n" "$*"; }
ok() { printf "\\e[32m[ OK ] %s\\e[0m\\n" "$*"; }
warn() { printf "\\e[33m[WARN] %s\\e[0m\\n" "$*"; }
err() { printf "\\e[31m[FAIL] %s\\e[0m\\n" "$*"; }
# ── Config ─────────────────────────────────────────────────────────
# Which CPU core to dedicate to audio IRQ handling
# RPi 4B has 4 cores (0-3). Core 3 is largely unused by Linux housekeeping.
# Core 0 = most interrupts + kernel, Core 1-2 = general, Core 3 = isolated
IRQ_CORE=3
# Default JACK parameters (will be overridden by main.py dynamically)
JACK_PERIOD=512
JACK_RATE=48000
# ── Helpers ─────────────────────────────────────────────────────────
# Find the USB audio interface IRQ number
find_usb_audio_irq() {
# Look for the USB audio device in /proc/interrupts
# On RPi 4B, the USB controller is on a PCIe bridge (xhci-hcd) or
# directly on the BCM2711's DWC2/dwc_otg USB controller.
# Focusrite Scarlett 2i2 shows up as a USB interrupt tied to xhci-hcd.
# Strategy 1: Look for xhci-hcd (USB 3.0 controller on Pi 4B)
local irq
irq=$(awk '/xhci-hcd/ {gsub(":","",$1); print $1}' /proc/interrupts 2>/dev/null | head -1)
# Strategy 2: Look for dwc_otg/dwc2 (USB 2.0 controller)
if [[ -z "$irq" ]]; then
irq=$(awk '/dwc_otg|dwc2/ {gsub(":","",$1); print $1}' /proc/interrupts 2>/dev/null | head -1)
fi
# Strategy 3: Find the USB controller from sysfs for the audio device
if [[ -z "$irq" ]]; then
# Try to find the USB device that's our audio interface
# Look for USB audio class devices
local usb_dev
usb_dev=$(grep -l "audio" /sys/bus/usb/devices/*/bInterfaceClass 2>/dev/null | head -1)
if [[ -n "$usb_dev" ]]; then
local usb_path
usb_path=$(dirname "$usb_dev")
# Walk up to find the parent USB controller
while [[ "$usb_path" != "/sys/bus/usb/devices" && "$usb_path" != "/" ]]; do
if [[ -f "$usb_path/irq" ]]; then
irq=$(cat "$usb_path/irq" 2>/dev/null)
break
fi
usb_path=$(dirname "$usb_path" 2>/dev/null)
done
fi
fi
echo "$irq"
}
# ── Apply IRQ affinity ──────────────────────────────────────────────
apply_irq_affinity() {
local irq="$1"
local core="$2"
local smp_affinity
if [[ -z "$irq" || "$irq" == "0" ]]; then
warn "No USB audio IRQ found — cannot set affinity"
return 1
fi
# Convert core number to hex mask for /proc/irq/*/smp_affinity
# Core 0 = 1, Core 1 = 2, Core 2 = 4, Core 3 = 8
smp_affinity=$(printf "%x" $((1 << core)))
local irq_dir="/proc/irq/$irq"
if [[ ! -d "$irq_dir" ]]; then
warn "IRQ $irq directory not found at $irq_dir"
return 1
fi
echo "$smp_affinity" > "$irq_dir/smp_affinity" 2>/dev/null || true
local effective
effective=$(cat "$irq_dir/smp_affinity" 2>/dev/null || echo "unknown")
# Also set smp_affinity_list for convenience
echo "$core" > "$irq_dir/smp_affinity_list" 2>/dev/null || true
info "IRQ $irq pinned to core $core (smp_affinity=0x$smp_affinity, effective=0x$effective)"
return 0
}
# ── Set RT priority for the current process ──────────────────────────
set_self_rt_priority() {
local prio="$1"
# Set SCHED_FIFO with the given priority for our process group
# Note: This only works if called from the target process or with CAP_SYS_NICE
chrt -f -p "$prio" $$ 2>/dev/null || warn "Cannot set self RT priority (not running as root?)"
}
# ── CPU isolation ───────────────────────────────────────────────────
# Move all non-critical interrupts away from the IRQ core
isolate_core_from_housekeeping() {
local core="$1"
local exclude_irqs="$2" # comma-separated IRQs to keep on the isolated core
# For each IRQ, move it away from our dedicated audio core
# unless it's one we explicitly want to keep there
for irq_dir in /proc/irq/[0-9]*/; do
local irq_num
irq_num=$(basename "$irq_dir")
local irq_name
irq_name=$(cat "${irq_dir}affinity_hint" 2>/dev/null || echo "")
# Skip if this is the USB audio IRQ we want to keep pinned
if [[ ",$exclude_irqs," == *",$irq_num,"* ]]; then
continue
fi
# Move to other cores (0-2, leaving core 3 free)
# Affinity = 0x7 (cores 0,1,2)
echo "7" > "${irq_dir}smp_affinity" 2>/dev/null || true
done
info "Non-audio IRQs moved away from core $core"
}
# ── Status check ────────────────────────────────────────────────────
show_status() {
echo ""
info "========== RT Performance Status =========="
echo ""
# CPU governor
echo "── CPU Governor ──"
for c in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor; do
local cpu
cpu=$(basename "$(dirname "$c")")
echo " $cpu: $(cat "$c" 2>/dev/null || echo 'N/A')"
done
# Current frequency
echo ""
echo "── CPU Frequency ──"
for c in /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq; do
local cpu
cpu=$(basename "$(dirname "$c")")
local freq_khz
freq_khz=$(cat "$c" 2>/dev/null || echo 'N/A')
echo " $cpu: $((freq_khz / 1000)) MHz"
done
# IRQ affinity
echo ""
echo "── IRQ Affinity ──"
local irq
irq=$(find_usb_audio_irq)
if [[ -n "$irq" && "$irq" != "0" ]]; then
local aff
aff=$(cat "/proc/irq/$irq/smp_affinity" 2>/dev/null || echo "N/A")
local name
name=$(cat "/proc/irq/$irq/name" 2>/dev/null || echo "unknown")
echo " USB Audio IRQ $irq ($name): smp_affinity=0x$aff"
else
echo " No USB audio IRQ found"
fi
# All IRQs
echo ""
echo "── All Interrupts (affinity) ──"
for irq_dir in /proc/irq/[0-9]*/; do
local i
i=$(basename "$irq_dir")
local aff
aff=$(cat "${irq_dir}smp_affinity" 2>/dev/null || echo "?")
local name
name=$(cat "${irq_dir}name" 2>/dev/null || echo "?")
printf " IRQ %-4s 0x%-4s %s\\n" "$i" "$aff" "$name"
done
# JACK status
echo ""
echo "── JACK Status ──"
if pidof jackd >/dev/null 2>&1; then
echo " jackd: RUNNING"
# Show JACK command line
ps aux | grep jackd | grep -v grep | head -1 | awk '{$1=$2=$3=$4=$5=$6=$7=$8=$9=$10=""; print " Args:" $0}'
else
echo " jackd: NOT RUNNING"
fi
# RT priority of processes
echo ""
echo "── RT Priority ──"
for proc in jackd python3; do
pidof "$proc" 2>/dev/null | tr ' ' '\\n' | while read -r pid; do
local policy
policy=$(chrt -p "$pid" 2>/dev/null | head -1 || echo "N/A")
echo " $proc (PID $pid): $policy"
done
done
# Memory locking
echo ""
echo "── Memory Locking ──"
if [[ -f /proc/self/status ]]; then
grep -i "lock" /proc/self/status 2>/dev/null | head -5 | while read -r line; do
echo " $line"
done
fi
# XRun debug
echo ""
echo "── XRun Debug ──"
for card in /proc/asound/card*/xrun_debug; do
echo " $card: $(cat "$card" 2>/dev/null || echo 'N/A')"
done
echo ""
ok "Status check complete"
}
# ═══════════════════════════════════════════════════════════════════
# Main
# ═══════════════════════════════════════════════════════════════════
if [[ $EUID -ne 0 ]]; then
err "This script must be run as root"
exit 1
fi
MODE="${1:-all}"
case "$MODE" in
--status|-s)
show_status
exit 0
;;
--irq-only|-i)
IRQ=$(find_usb_audio_irq)
if [[ -n "$IRQ" && "$IRQ" != "0" ]]; then
apply_irq_affinity "$IRQ" "$IRQ_CORE" && ok "IRQ affinity applied"
else
warn "No USB audio IRQ found — checking /proc/interrupts..."
grep -E "xhci|dwc|usb|audio" /proc/interrupts | head -10
echo ""
info "To find the right IRQ manually:"
info " cat /proc/interrupts | grep xhci"
info " cat /sys/bus/usb/devices/*/irq 2>/dev/null"
fi
exit 0
;;
--usb-audio-irq|-u)
IRQ=$(find_usb_audio_irq)
if [[ -n "$IRQ" && "$IRQ" != "0" ]]; then
local name
name=$(cat "/proc/irq/$IRQ/name" 2>/dev/null || echo "unknown")
info "USB Audio IRQ = $IRQ ($name)"
else
warn "No USB audio IRQ found"
echo "── /proc/interrupts (USB/audio lines) ──"
grep -E "xhci|dwc|usb|audio|snd" /proc/interrupts | head -10
fi
exit 0
;;
all|*)
# Full tuning
info "========== Pi Multi-FX Pedal — RT Tuning =========="
echo ""
# ── 1. Find and pin USB audio IRQ ────────────────────────
info "Step 1: USB audio IRQ affinity..."
IRQ=$(find_usb_audio_irq)
if [[ -n "$IRQ" && "$IRQ" != "0" ]]; then
apply_irq_affinity "$IRQ" "$IRQ_CORE"
# Isolate core 3 from housekeeping interrupts
isolate_core_from_housekeeping "$IRQ_CORE" "$IRQ"
else
warn "USB audio IRQ not found — audio may work but without dedicated IRQ core"
fi
# ── 2. CPU governor (already done in main.py, but belt-and-braces) ──
info "Step 2: CPU governor → performance..."
echo performance | tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor >/dev/null 2>&1 || \
warn "Could not set CPU governor (may need cpufrequtils)"
# ── 3. Disable CPU idle states (C-states) for lower latency ──
# This prevents the CPU from entering deep sleep that adds latency
# on wake. Trade-off: ~0.5W extra power draw.
info "Step 3: Disabling deep CPU idle states..."
if [[ -f /sys/module/processor/parameters/max_cstate ]]; then
echo 1 > /sys/module/processor/parameters/max_cstate 2>/dev/null || true
fi
if [[ -f /dev/cpu_dma_latency ]]; then
# Write 0 to request minimum DMA latency (blocks deep C-states)
# The file stays open while the process lives — we do it briefly
echo 0 > /dev/cpu_dma_latency 2>/dev/null || true
fi
# ── 4. Set JACK's ALSA buffer sizes for low latency ───────────
# These are advisory — main.py overrides them via jackd arguments
info "Step 4: ALSA buffer constraints (advisory)..."
cat > /etc/security/limits.d/99-audio.conf <<'LIMITS'
# Pi Multi-FX Pedal — Real-time audio limits
# Applied by rt-tune.sh
@audio - rtprio 95
@audio - memlock unlimited
@audio - nice -20
LIMITS
ok "Audio limits written to /etc/security/limits.d/99-audio.conf"
# ── 5. Set xrun_debug for diagnostics ─────────────────────────
info "Step 5: Enable xrun tracking..."
for card in /proc/asound/card*/xrun_debug; do
if [[ -w "$card" ]]; then
# Bit 0 = enable xrun logging
# Bit 1 = show stack backtrace on xrun
# Bit 2 = inhibit xrun (test mode)
# Value 3 = log xruns with backtrace (diagnostic)
echo 3 > "$card" 2>/dev/null || true
ok "Set xrun_debug on $(dirname "$card" | xargs basename)"
fi
done
# ── 6. Process RT priority (for the calling process) ──────────
info "Step 6: Setting RT priority..."
# This is primarily done by main.py (mlockall, GC disable)
# and the systemd service (LimitRTPRIO). We set it here too
# so the script works in all contexts.
set_self_rt_priority 80 || true
# ── 7. Systemd service check ──────────────────────────────────
info "Step 7: Verifying systemd RT limits..."
if systemctl is-active pi-multifx-pedal.service &>/dev/null; then
local rtprio
rtprio=$(systemctl show pi-multifx-pedal.service -p LimitRTPRIO --value 2>/dev/null || echo "?")
local memlock
memlock=$(systemctl show pi-multifx-pedal.service -p LimitMEMLOCK --value 2>/dev/null || echo "?")
local nice
nice=$(systemctl show pi-multifx-pedal.service -p LimitNICE --value 2>/dev/null || echo "?")
info " LimitRTPRIO=$rtprio LimitMEMLOCK=$memlock LimitNICE=$nice"
if [[ "$rtprio" != "95" ]]; then
warn "LimitRTPRIO should be 95 — check pi-multifx-pedal.service"
fi
if [[ "$memlock" != "infinity" ]]; then
warn "LimitMEMLOCK should be infinity — check pi-multifx-pedal.service"
fi
else
warn "pi-multifx-pedal.service not running — check after deployment"
fi
echo ""
ok "========== RT Tuning Complete =========="
echo ""
info "Recommended next steps:"
info " 1. Start pedal: sudo systemctl start pi-multifx-pedal.service"
info " 2. Check logs: journalctl -fu pi-multifx-pedal.service"
info " 3. Test latency: jack_iodelay (needs loopback cable)"
info " 4. Check xruns: cat /proc/asound/card*/xrun_debug"
info " 5. Full status: sudo ./rt-tune.sh --status"
;;
esac
src/system/audio.py
+27 -1
View File
@@ -587,9 +587,35 @@ class AudioSystem:
return devices
# ──────────────────────────────────────────────────────────────
# XRun monitoring
# XRun monitoring and tracking
# ──────────────────────────────────────────────────────────────
def enable_xrun_tracking(self) -> bool:
"""Enable kernel-level xrun tracking via ALSA xrun_debug.
Sets xrun_debug to value 3 on all ALSA cards:
- Bit 0 (1): log xruns to kernel ring buffer
- Bit 1 (2): show stack backtrace on xrun
Returns True if at least one card was configured.
Requires root. Non-fatal if unavailable.
"""
import glob as _glob
configured = 0
for card_path in _glob.glob("/proc/asound/card*/xrun_debug"):
try:
with open(card_path, "w") as f:
f.write("3")
configured += 1
except (OSError, PermissionError):
continue
if configured:
logger.info("XRun tracking enabled on %d ALSA card(s)", configured)
else:
logger.info("XRun tracking not available (non-root or no ALSA cards)")
return configured > 0
@staticmethod
def read_xrun_count() -> Optional[int]:
"""Read JACK xrun counter from jack_showtime.
src/system/services.py
+2 -1
View File
@@ -62,7 +62,7 @@ User={user}
Group={group}
WorkingDirectory={install_dir}
ExecStartPre={python_bin} -c "from src.system.audio import _jack_is_running; import sys; sys.exit(0 if _jack_is_running() else 2)"
ExecStart={python_bin} {main_script}
ExecStart=/usr/bin/chrt -f 80 {python_bin} {main_script}
ExecStop={python_bin} -c "import sys; sys.path.insert(0, '{install_dir}'); from main import PedalApp; PedalApp().shutdown()" 2>/dev/null || true
Restart=on-failure
RestartSec=3
@@ -74,6 +74,7 @@ KillMode=process
LimitRTPRIO=95
LimitMEMLOCK=infinity
LimitNICE=-20
LimitSIGPENDING=128
# Environment
Environment=PYTHONUNBUFFERED=1