Clue to X using 100% cpu problem

[Matt Patterson]

I have noticed more than a few posts recently about X being very slow 
and using 100% cpu on some systems. I had never experienced the problem 
until a few days ago, and it coincided with a hardware change, so maybe 
this will be of use to someone.

System Specs:
Ubuntu Hoary 5.10
Stock KDE (kubuntu-desktop)
Dual Athlon 1700+ (Tyan Thunder K7)
Onboard IDE + 3 drives + 1 DVDRom
Nvidia Geforce 5200 driving two monitors using ubuntu included binary 
driver, glx disabled

The Problem:
I purchased some more hard disks, so I needed to add a hard disk 
controller. I pulled an old Ultra66 (Promise) out and slapped it into 
the machine. I put 4 disks on the onboard IDE and one disk and one 
DVDRom on the Ultra. The machine would not start X after that. It kept 
giving errors about the nvidia card not receiving interrupts. So I tried 
the "noapic nolapic" kernel options and low and behold X came right up. 
BUT, performance was hideous. Things paused all the time, and X would 
consume 100% cpu whenever a window was moved. I determined that the 
Ultra66 and the nvidia were sharing an interrupt (17). So I moved the 
Ultra66 to a different slot and tried again. This time it booted without 
the kernel options and everything worked well, except that I couldnt 
turn on dma for the cdrom without freezing up the cdrom. So I swapped 
thed cdrom onto the onboard ide, and now everything works as expected. 
The Ultra66 cards are known to have bus master and dma issues, but I had 
never seen this particular behavior before, or a dislike for cdroms.

Matt