As I understand it, a modern X server and its graphics card drivers has two choices for low-level hardware acceleration methods: the older XAA and the more recent EXA. EXA's big goal is to support the sort of fancy composition and graphics that modern desktops and programs want to use; in particular, if it works it speeds up Firefox's page scrolling for web pages with complex graphical backgrounds.

(If your card's driver supports it, you switch the X server to using EXA through an xorg.conf change. I am not going to try to summarize the details here; see the manpage for your card driver, such as radeon(4), and the xorg.conf manpage. XAA is the default acceleration method.)

When I read about this I was running into some web pages that Firefox was scrolling slowly, so I promptly leapt to try it out. However, I ran into some problems on my ATI-using machines:

• on my dual-head setup at work with both an ATI X300 and an ATI X750, solid window moves promptly got achingly slow. This is apparently a known issue with some EXA configurations, but in any case it meant that I turned off EXA at work.

• more seriously, I started to have erratic total system lockups on my home system (identical to my work machine except that it has an ATI X800GTO in a single-headed configuration). They seemed to be most often provoked by watching YouTube Flash videos, although I believe some happened just from using xine and perhaps other Xv-using applications.

  (I am not completely sure EXA is at the root of these lockups, but I haven't had any in a week since I reverted to XAA, and EXA is said to be 'possibly unstable'.)

Unfortunately, this may well matter in the future because of Cairo. Cairo is is more or less Gnome's 2D graphics library (it used to draw SVG graphics in Firefox, for example). Cairo makes heavy use of the XRender extension and thus assumes it will be reasonably fast, but the XRender extension is exactly what EXA is designed to speed up. No EXA means no fast XRender which means Cairo-based programs are probably going to perform relatively badly.

For now I have my fingers crossed that Fedora 7 has an updated ATI driver and X server that fix these issues, although I am not sure that I will have the courage to try it out immediately. (Having my home machine lock up erratically was really, really irritating.)

(Just to be clear, I using the open-source radeon driver, not ATI's binary fglrx driver.)