Long story short: Disable ColorTiling in xorg.conf when using bitmap
fonts on a Radeon.
While searching for clues on this problem, I found this:
http://unix.stackexchange.com/questions/111454/what-are-the-purposes-of-the-different-types-of-xwindows-fonts
in which Gilles explains a bit of the history of font rendering in X11,
notably the move to client-side font rendering that accompanied
outline/antialiased fonts (and presumably Xft). This in turn led me to
these:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=648222
https://bugs.freedesktop.org/show_bug.cgi?id=34486
I can report that disabling the Radeon driver's ColorTiling option in
xorg.conf seems to have solved the problem on my system, at least with X
clients that use bitmap fonts. This is consistent with my observation
that older X clients such as xosview and Notion seemed particularly
badly affected. The Radeon driver turns ColorTiling on by default for
most of the newer chipsets, according to the man page.
However, GIMP (version 2.8.14) is still very slow, both in navigating
the menus and when using drawing tools, and causes large amounts of CPU
time to be spent in the Xorg process in the __memcpy_sse2_unaligned
function. But perhaps the ColorTiling solution might suggest possible
causes. I'm also interested in any comments on the slow memcpy
performance on this system (which I might take up with the GNU libc team
after doing some more testing).
Cheers all,
Chris
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