JACK falls back to SCHED_OTHER in KDE + Ardour I/O priority warning

#1

I’m running JACK2 (jackdmp 1.9.21) on KDE Neon (Ubuntu-based) using ALSA (no PipeWire).

Issue 1: JACK real-time priority

JACK starts correctly with:

jackd -R -P 80 -dalsa -r48000 -p64 -n2

but it does not run in real-time priority. It falls back to:

SCHED_OTHER (TS)

Verified with:

chrt -p $(pgrep jackd)

System status

  • RT limits OK (ulimit -r 95)
  • user in audio group
  • rtkit-daemon running
  • JACK has cap_sys_nice
  • standard kernel (not lowlatency)

If forced manually:

chrt -f 80 jackd ...

JACK correctly runs as:

SCHED_FIFO (FF)

Issue 2: Ardour disk I/O warning

Ardour also reports:

“cannot set disk I/O priority”

which suggests it cannot apply I/O scheduling (ionice), likely due to system or cgroup restrictions.

Question

Why would JACK fail to acquire SCHED_FIFO in a normal KDE user session despite correct RT limits, and why would Ardour fail to set disk I/O priority?

Is this expected behavior under modern KDE/systemd/RTKit setups, or is there a known configuration fix?

#2

jackd itself should not run with SCHED_FIFO, only realtime I/O threads created by jack should do so.

That will interfere with actual realtime I/O. The main jack thread which does housekeeping is not realtime safe.

#3

Okey, just leave only with jackd. And what about this warning?

2026-05-20T18:42:21 [INFO]: Cannot set I/O Priority for disk read/write thread.

#4

What version of Ardour are you using?

There was a similar disk I/O priority issue here and which seems to have been due to Ardour 8.7 and was supposedly fixed in 8.10

#5

Coming from 9.1 and today installed 9.5, but the same warning.

#6

This isn’t a warning, just an informational message. You probably don’t need to do anything about it. For more details, see this thread: