[LAD] ? jack/qjackctl XRUN info

Greetings, what does it mean when I get messages like this in the qjackctl window? (or where should I look to find out) 14:21:16.546 XRUN callback (2 skipped). 14:21:23.026 XRUN callback (908). 14:21:24.594 XRUN callback (1 skipped). 14:21:27.767 XRUN callback (910). 14:21:28.617 XRUN callback (7 skipped). 14:21:30.632 XRUN callback (7 skipped). AFAIK I am not getting actual xruns - I don't get any message like this, and my audio sounds OK. **** alsa_pcm: xrun of at least 2721.798 msecs The XRUN callbacks only happen when I use an alsa multi device (2 stereo substreams linked and synched together), not when I use a single stereo substream. thanks Eliot