Performance Tuning

The Raspberry Pi 5 in the v3 model has a ton of CPU headroom; the 3/4 models in v1/v2 builds are much less capable. No matter which device you have, pi-Stomp maximizes its performance by shipping with a real-time enabled linux kernel, prioritizing audio processing and I/O.

Monitoring for XRUNs

XRUNs (buffer over/under-runs) happen when the CPU can't deliver audio data in time. A few during system start-up / pedalboard loading are normal. If you start seeing hundreds during normal play, the CPU is overworked.

In MOD-UI, the bottom bar shows the XRUN count. Click it to reset the counter.

CPU-intensive plugins

Some plugin types demand more CPU than others:

• Generators (synths, samplers, drum machines) — heaviest
• Simulators (amp models, cabinet IRs) — heavy
• Pitch shifters — moderate to heavy
• Reverbs — moderate
• Delays, modulations, EQs — light to moderate

If you're hitting CPU limits, replace heavy plugins with lighter alternatives, reduce the number of active plugins on the pedalboard, or try loosening some of the JACK audio server settings.

Buffer size and sample rate

The JACK audio server runs at 48 kHz. The period defaults to 128 frames on a Pi 5 and 256 frames on older hardware. JACK uses two periods by default (-n 2), so actual round-trip I/O latency is roughly double:

Larger buffers reduce CPU load and XRUNs at the cost of higher latency. If you hear glitches, go up one step.

Changing the period size

You can find the current period in the bottom-right of the MOD-UI pedalboard application at http://pistomp.local. It's right beside the XRUN counter.

Edit JACK_PERIOD in /etc/default/jack, substituting 128 for your current value and 256 for your desired value, and reboot:

sudo sed -i 's/JACK_PERIOD="128"/JACK_PERIOD="256"/' /etc/default/jack
sudo reboot

Changes to this file persist across system updates.

Very large pedalboards

The way MOD works is that every plugin on your pedalboard is a JACK client and registers its own port: roughly three per plugin; more for stereo plugins with MIDI.

You would need something like 100 plugins on a pi-Stomp v2 to reach the cap. If you do hit it, plugins will load, but their audio connections won't, so the pedalboard goes partially silent rather than failing outright. Check the logs if you suspect this:

sudo journalctl -u jack -b | grep -i "cannot register"

To resolve, raise JACK_PORT_MAX in /etc/default/jack and reboot. Each additional port costs RAM that JACK locks and never releases — JACK's own 2048-port default would reserve 102 MB, so budget roughly 50 KB per port and raise the cap only as far as you need:

sudo sed -i '/^JACK_PORT_MAX=/d' /etc/default/jack
echo 'JACK_PORT_MAX="512"' | sudo tee -a /etc/default/jack
sudo reboot

See Configuration Guide for how the allocation works.

Thermal management

The Raspberry Pi 5 throttles at 85°C. The pi-Stomp v3 includes an Active Cooler. To check your temperature:

vcgencmd measure_temp

To check if throttling has occurred:

vcgencmd get_throttled

• 0x0 — no throttling
• 50000 — has throttled since boot
• 50005 — currently throttling

If you see throttling, improve ventilation or reduce CPU load. Typical operating temperature is 45–60°C.

Power supply

The pi-Stomp v3 needs at least 15W (5V / 3A); the factory 27W (5V / 5A) adapter is recommended and required for full-power USB peripherals. Insufficient power can cause throttling and sudden shutdowns. Check for undervoltage warnings:

dmesg | grep -i voltage

To suppress voltage warnings (not recommended if you actually have a power problem):

echo avoid_warnings=2 | sudo tee -a /boot/config.txt