High CPU usage with radeon driver.

[Ken Moffat]

 I have two desktop machines with radeons - one, a phenom, has 8GB
and an RS780L - that works fine, although I note that with the
radeon driver Xorg.bin is taking up to 13% of (one) CPU (according to
'top') when it starts, but mostly less than 10%, often only 2-3%
when the box is idle and the CPU frequency has dropped off, but if
I change to the modesetting driver the usage is normally under 1% (I
did once see 3% after I closed firefox).

 But the other machine is an A4 with the Aruba graphics, and it was
here that I first noticed the problem.  This box is underpowered
(its main use is for when I have to run windows (tax, one music
store, satnav updates), although I have some linux systems there.
My desktop of choice is icewm (lean, but with a network window to
remind me that a backup is running, and a CPU window).

 Some time last year on the A4, I noticed that icewm's CPU window
showed a lot of red (possibly, that is memory pressure) in its lower
part after Xorg started.  At the time, I assumed that irregular fcron
jobs such as updatedb were running (the individual linux systems do
not get booted often, so running a weekly job would not be
surprising).  But today I wanted to take a look at how the
modesetting driver compared on the A4 (this is the 1.16.3 server,
with released drivers.  I already knew that the modesetting driver
reduced the apparent Xorg.bin CPU usage on the phenom.

 The A4 only has 4GB (less 256MB for graphics), and Xorg.bin is not
using a lot - only 0.8% of memory - and the box was not swapping.
Testing was with linux-3.19.0-rc5.

 After running startx, the CPU% for Xorg.bin was initially around
85% (icewm, two urxvt terminals, one running top, and me typing
notes in the other).  Left it, when I came back and moused to stop
the screensaver it was again using 85-90% (this is presumably with
the processor frequency at minimum), but after a couple of minutes
it had fallen below 30%.

 I then did some minor compiles, including the modesetting driver,
and watched the Xorg.bin CPU% running between 12% and 20%.  During
the compiles, I noticed from my power meter that the total system
power consumption (monitor, net switch, kvm switch, this box) went
up for a while - presumably, both cores running at full speed - and
the Xorg.bin CPU% varied between 9% and 20%.

 Left it again, came back, ran some screensavers from
xscreensaver-demo (Morph3D, MoebiusGears, GLHanoi fullscreen, CPU%
seemed to be 2.0 to 2.7% immediately after each of them).  When I
closed the small window from xscreensaver-demo the CPU% again shot
up to 15 to 20% briefly.

 At that point I closed X, then reran startx.  As soon as I could
start 'top', I saw that the CPU% was around 45%, but after a few
seconds it dropped into a range between 30% and 42% : much better
than the first time, but not wonderful.

 I then renamed the ati and radeon drivers so that they would not be
found, and restarted X with the modesetting driver.  Top showed
Xorg.bin at 6.4% on the first reading, then 4%, but after that 2.0
to 2.3% and the system was idling with at 103-104W instead of the
(initial) 125W and then 113-114W.

 With xscreensaver-demo (i.e. just a *small* window showing a random
saver), Xorg.bin CPU% was typically 5%.

 Unfortunately, I could not run any of the interesting screensavers
mentioned above, nor glxgears.  That is my problem, if I care about
it enough.  FWIW it's a Mesa/LLVM problem - I had something similar
in qemu which was solved by rebuilding Mesa.

 I've just started top again (modesetting driver) - Xorg.bin again
6.3% on first sample, but then 1.0 to 1.3%.

 Anyway, I'd rather have a lower power usage, or more CPU devoted to
what I want to run, but it would be _nice_ to have the correct
driver using less CPU ;)  Any suggestions, please ?

ĸen
-- 
Nanny Ogg usually went to bed early. After all, she was an old lady.
Sometimes she went to bed as early as 6 a.m.