XRUNs on meter change during playback/record

I have a session in an early stage that changes from 4/4 to 17/8 and back. It’s just midi tracks at the moment with Helm, Noize Mak3r and ZynAddSubFX.

During playback (and record) I get XRUNs at the first time change or a split second into the first bar of 17/8. Interestingly, I have exported the session a few times and don’t get XRUNs or clicks in the exported audio. So it’s not a problem at the moment, but if I add audio tracks it will be when I go to record. Has anyone experienced something similar?

Sorry to say but I’m on a distro build - v6.3 via backport on Kubuntu 20.04, low-latency kernel (not that it matters anything in this case).

What is the settings for your JACK server - how many periods per buffer, how many frames per period? And what are you using for hardware?

By default, Ardour does not perform a realtime export, but decouples from the soundcard. Export is as fast as possible, as slow as necessary. x-runs cannot happen during a freewheeling export.

So just to clarify, they CAN happen during the actual realtime export function that can check in the export options? Or when the audio engine crossruns during that, do I not need to worry about it?

Yes, realtime export plays the session normally. – It is intended for some rare cases where you want to include live external (analog) gear via send/return. (normally one would bounce that).

x-run = short for audio buffer under/over run. aka. buffer overflow or underflow.

The system cannot keep up, so the soundcard’s input (capture) buffer overflows, and the output (playback) buffer has insufficient data and underruns.

I have no explanation why this can happen on meter changes though. Ardour does inform synths about the current tempo and meter, so perhaps something goes amiss there.
Could you try if this happens when you use different synths with the same session? (Session > Save snapshot. then replace Helm, Noisemaker, and Zyn with, say, general-midi-synth).

What is the DSP load (top right in Ardour’s status-bar)?

Sorry to reply so late, I’ve been away. Sample rate 48k, Buffer 256, Periods/Buffer 3. Scarlet 2i2 second gen, i5 9600K, 16GB RAM.

ok - I removed all the synths. No XRUNs. Added a reasonable synth for all midi tracks. None. Replaced a reasonable synth with Noize Mak3r. XRUNs. It’s hovering around 10-20% load. I’ll test with just Helm and Zyn later - gotta run.