A question about "Software rendering for gnome-shell"

[Adam Jackson]

On Thu, 2012-02-23 at 08:44 -0500, mwesten wrote:

> Fedora-17-Alpha-i686-Live-Desktop.iso (RC2) on USB
> 
> 2.13GHz AthlonXP 2600+ / RV200 (Radeon 7500) / 1280x1024
> 
> Fallback mode is fine, but once the shell starts, it's a battle just to 
> get System Monitor open and to the Resources tab.  Once there, the CPU 
> sits at 100% until I get off that tab.  Very high latency, screen 
> flickering, artifacts, incomplete rendering...

"Incomplete" is worrisome, I've not seen that.

There's a bit of a quantum effect here where the act of drawing the CPU
monitor necessarily raises CPU load.  If you ssh into the machine and
run top from there, gnome-shell and Xorg shouldn't be taking any undue
load at idle.  If they are, that means there's rendering happening at
idle, which is a legitimate bug that would affect even accelerated
drivers by keeping the GPU from going to sleep to save power.

In the non-steady-state, though, the current implementation is known to
be incredibly memcpy-heavy.  Fixes coming soon, beta should be much
better I hope.  If you want to collect some data about where CPU time is
being spent, 'perf record' against the X server or gnome-shell (with
debuginfo installed) and then 'perf report' should be enlightening.  I
suspect you'll find the vast majority of the time spent in memcpy in one
form or another (pixman or fb blit, copying data into and out of the
kernel across the unix socket, etc).

> For comparison:
> 
> 3,2GHz Pentium 4 (HT Disabled) / Gallium 0.4 on ATI RV370 / 1680x1050
> 
> Fine.  System Monitor CPU at 19%.

That's still way higher than I'd like, but it's at least an r300g bug
not an llvmpipe bug.

- ajax
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