Our realtime app is troubled by a realtime problem related to X on RHEL4U4, you 
can reproduce the problem like below
 
 * run RHEL4U4
 * boot into runlevel 5
 * killall -9 X
 * system scheduling latency (+ interrupt latency) jumps to 6382ms!! 

I'm using realfeel to measure scheduling latency, you can find the tool below
(PS. before compiling the tool, you should change SAMPLES to larger value, eg. 
10000000)
 
  http://www.captain.at/howto-captest.tar.gz
  http://www.captain.at/howto-linux-real-time-patch.php

The problem was reproduced on 3 test machines, all have configuration below
 
 * /etc/redhat-release
     Red Hat Enterprise Linux AS release 4 (Nahant Update 4)
 * uname -a
     Linux localhost.localdomain 2.6.9-42.EL #1 Wed Jul 12 23:16:43 EDT 2006 
i686 i686 i386 GNU/Linux
 * lspci | grep VGA
     00:02.0 VGA compatible controller: Intel Corporation 945G/GZ Express 
Integrated Graphics Controller (rev 02)

Test result is below

  0.0 15214
  0.1 3
  0.2 373
  0.3 3
  0.4 91
  0.6 1
  3.1 1
  4.1 1
  6382.4 1
 
If using smp kernel, scheduling latency jumps to 100ms