i encountered, that 'ondemand' cpu scaling doesn't work correctly when running pd with realtime priority. even if pd is using 80% or more resources (of one core), the cpu frequency is not raised and staying at 800MHz. i can force it to go up by running an instance of 'burnMMX' (or something else, that burns as many cycles as possible). when running pd with '-nrt', automatic scaling works fine. also, as a workaround, i can set the scaling_governor to 'userspace' and then set the cpu frequency manually.
I don't remember being hit by this, but my memory may trick me. I'd use cpufreq-set(1) as root to switch the scaling governor to "performance" when performing.
i usually set the cpu speed to the lowest feasible frequency ... the slower the cpu, the cooler the computer, so less audible noise is produced by the cpu fan during the performance ... using gnome's cpufreq-applet, one can change the scaling governor/ frequency with two mouse clicks ...
t