High CPU usage in hardy

[Steinar Bang]

Platform: Intel Pentium M, ATI r300-based Radeon Mobility screen card
	  BroadCom bcm43xx-based WLAN,
	  Ubuntu 8.04,
	  linux-image-2.6.24-16-generic 2.6.24-16.30
	  linux-restricted-modules-2.6.24-16-generic 2.6.24.12-16.34
	  xserver-xorg 1:7.3+10ubuntu10
	  xserver-xorg-video-ati 1:6.8.0-1
	  pulseaudio 0.9.10-1ubuntu1
	  rhythmbox 0.11.5-0ubuntu6

After upgrading from 7.10 (which had no problems to hardy), everything
seemed to work fine.  But then sound suddenly started to become choppy,
and when I looked at top, three processes used a lot of CPU (Xorg 30-40%
CPU, rhythmbox 25-30%CPU and pulseaudio 15% CPU).

If I stop the rhythmbox playing sound, then Xorg stabilizes at around 5%,
but moving the mouse around makes the Xorg CPU usage creep up to around
20%. 

Normal CPU usage is 0-3% for Xorg (regardless of operations), 0-1% for
rhythmbox, and 2-3% for pulseaudio.

Initially I suspected pulseaudio, but later I've seen this high CPU
usage of Xorg and other programs, without rhythmbox or pulseaudio being
involved (other high CPU usage culprits have been maven builds, opera
(especially opera+flash), VMWare workstation, and evolution).

So I don't know if what I see are the symptoms or the cause...?  If
there is _something_ that tips the system over the edge so that some
processes start burning CPU?

The problem is to find that elusive "something"... I've googled, and
seen lots of complaints about high CPU usage caused by display drivers
(both ATI and NVIDIA), pulseaudio, and evolution, but nothing about
generic high CPU usage.

The cure for the high CPU usage is to shut down the high CPU usage
processes and leave the system alone for a while.  After that, it
behaves normally (ie. sound plays back without problems, and window
system operations have no noticable effect on CPU usage).

Has anyone else seen similar behaviour?

Thanx!


- Steinar