tvtime and the Linux 2.6 scheduler

[Billy Biggs]

  I am the author of tvtime, a TV application with advanced image
processing algorithms.  Some users are complaining about poor
performance under Linux 2.6, and I would like more information about how
tvtime will be treated by the scheduler.  Here is an example of the
intended usage:

  - Program running as root and SCHED_FIFO
  - NTSC, input ~30 fps, each field processed for an output of ~60 fps
  - CPU intensive processing, say 9 ms per field on my P3-733
  - with a typical AGP card, the X driver takes 4 ms to draw
  - Wait using /dev/rtc set to 1024 Hz

  for(;;)
      9 ms : process frame
      4 ms : draw frame
      3 ms : wait until next field time using /dev/rtc
      9 ms : process frame
      4 ms : draw frame
      3 ms : block on /dev/video0 for next frame
     -----
     33 ms : time per NTSC frame

  The theory is that Linux classifies this as a CPU hog regardless of
its priority, and preempts tvtime with other processes.  Oswald
Buddenhagen describes the effect as this:

  "[...] it starts up fine, but after a few seconds (when the scheduler
gathered some stats) ... well, it looks funny: the scene goes roughly
exponentially into slow motion, then there is a frame drop and the
process starts over.  this behaviour can be observed at any priority,
which is clearly against the claim "no normally priorized interactive
process will preempt a highly priorized cpu-hog" that i've read
somewhere.  the xserver priority does not change anything, either;"

  Avoiding root/SCHED_FIFO and using usleep() instead of /dev/rtc seems
to exhibit the same behavior.

  Thoughts?

  -Billy

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