Bug#330366: kded activity disrupts video grabbing despite realtime sched

[Andreas Mohr <andi@rhlx01.fht-esslingen.de>]

Package: kdelibs-bin
Version: 4:3.4.2-3

Hi,

I've been having huge trouble doing frame grabbing from my video camera via
firewire recently. Every ~ 20 seconds a frame got dropped.
And this all despite running a -ck kernel with dvgrab running in SCHED_ISO,
which should normally avoid any and all frame drops quite easily.

Environment: P3/700 with 2.6.13-ck* kernels, CONFIG_PREEMPT and CONFIG_PREEMPT_BKL.

I then changed to IceWM which immediately worked.
So it had to be something within the KDE environment.
A kill -STOP kded then fixed it.

Possibilities:
a) kded is doing way too much and thus stealing too much CPU. Since this should still
   never cause frame drops since dvgrab is SCHED_ISO, it would indicate a scheduling
   weakness. Highly unlikely.

b) kded calling one huge non-preemptive system call, causing huge latency during
   grabbing. --> kernel issue.

c) kded calling non-preemptive system call with too high latency too often.
    kded issue / kernel issue.

Note that top shows kded sometimes having 3% CPU spikes, and strace illustrates
this activity.

dvgrab has been verified to use SCHED_ISO, via schedtool.

Is there a way to improve kded to have less activity within one burst?

Other ideas?

Thanks!

Andreas Mohr