Re: tvtime and the Linux 2.6 scheduler

[Redeeman <lkml@metanurb.dk>]

using nicksched solves this, now tvtime only uses 1% again! yay!

On Sun, 2004-05-23 at 17:48, Billy Biggs wrote:
>   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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-- 
Regards, Redeeman
redeeman@metanurb.dk