[mythtv] curious cpu usage with mythfrontend on playback

[Andy McMullan]

I see a curious pattern when playing back recordings on my system (AMD
1700+, bt878, mpeg4, 512x576, default mpeg4 options, nVidia MX440,
6111 driver).

The pattern is as follows:

1. mythfrontend uses 25% cpu constantly for approx 1-2 minutes. Seems
to be the decoder thread.
2. cpu usage falls to < 1% over a few seconds.
3. cpu usage stays at around 1% for several seconds.
4. X process suddenly shoots up to 50% for several seconds.
5. X process falls to < 1% again.
6. mythfrontend rises to 25% again, though oddly this seems to be a
different thread (video out thread?)

This pattern repeats itself over and over, with the two mythfrontend
threads taking turns to use around 25% cpu, and the 1% cpu period and
X process 50% spike occurring between the switches.

All the time playback continues without any glitches (unless I'm
trying to record at the same time).

Before I resort to profiling, does anyone have a clue what is going on
here? Surely the 25% cpu is much too high? Recording only takes 25%
cpu, and surely playback should use much less cpu than recording? Can
anyone with a similar system tell me what I should be seeing in terms
of cpu on playback?

Thanks

Andy Mc