Scheduling behavior when using idprio

[Geoff Fritz]

I recently noticed something odd.

Whenever I have a cpu-bound process (say, piping a lot of data through
"gzip -9c") that's been given an idle priority using "idprio 31 -<PID>", I
notice that my system's overall interactive responsiveness feels peppier.
Apps seems to respond faster, screens redraw faster, web pages render
quicker, etc..

It's reminiscent of enabling the interactive real-time options in the Linux
kernel (at least, as I remember it years ago from the 2.4 days).

My system is a little long in the tooth (2GHz Athlon64, single core), so
any speed improvements are quickly noticed around here.

I was wondering if there was an explanation for my observations, or if
anyone else has seen -- or could repeat -- this.  If this is a
repeatable/known behavior, would this be a scheduler bug?  Why would
things run more smoothly only when there was a single ulta-low priority
process running?

Thanks.

-- Geoff