[Mesa-dev] Very low framerate when recording desktop content in Weston using mesa git on Radeon 5770 (glReadPixels slow path)

[Bengt Richter]

On 03/26/2013 06:26 PM Rune Kjær Svendsen wrote:
> (I'm re-sending this message because the attachment was too large for
> mesa-dev, and because I want to add wayland-devel CC. The valgrind output
> can be found here: http://runeks.dk/files/callgrind.out.11362).
>
> Seems like you are right Pekka.
>
> I just ran weston through valgrind, and got some interesting results. I ran
> it like so:
>
> valgrind --tool=callgrind --dump-instr=yes --trace-jump=yes weston
>
> Which allows me to get the time spent on a per-instruction level. Now, this
> is running inside a virtual machine, so it won't be the same as running it
> natively, but it agrees with the other benchmarks in the sense that it
> suggests it is the simple calculations inside screenshooter.c that take up
> most of the CPU time, not calls to outside functions (like it was before
> with the slow glReadPixels() path).
>
> The callgrind output can be found at the following URL:
> http://runeks.dk/files/callgrind.out.11362
> Open it with KCachegrind, select the function
> weston_recorder_frame_notify(), and go to the "Machine Code" tab in the
> lower right corner to see the interesting stuff.
>
> According to callgrind, a total of 54.39% CPU time is used in the four
> lines 251, 252, 253 and 255 in screenshooter.c. That's the function
> component_delta():
>
> dr = (next>>  16) - (prev>>  16);
>   dg = (next>>   8) - (prev>>   8);
> db = (next>>   0) - (prev>>   0);
>
>   return (dr<<  16) | (dg<<  8) | (db<<  0);

Better double check (since I did this from old memories of bit-diddling ;-)
but I think this does the same, just a bit faster (with -O2 reduces time
vs the above by about 44% in my QND test):

uint32_t
component_delta2(uint32_t next, uint32_t prev)
{
     return ((((next&0xff00ff)-(prev&0xff00ff)+0x100)&0xff00ff)+
         (((next&0xff00)-(prev&0xff00))&0xff00));
}

Regards,
Bengt Richter
(Note that I appended '2' to the name for testing).