[PD] CPU usage with multiple cameras and different operating systems. (Gem)

Martin Schied crinimal at gmx.net
Wed Jul 22 19:51:48 CEST 2009

Hi all!

I'm developing an installation which currently uses six webcams 
(Logitech Vision Pro, but tried also lots of other cameras). I'm using 
Gem patches on Linux and OS X and realized that OS-X's CPU consumption 
is much less than on linux. I made tests with a single webcam on Linux 
and single (and multiple webcams) on OS X. Both computers are not 
directly comparable, because of different hardware, but maybe this is 
interesting anyways.

The Linux computer: AMD Athlon64, 2.2 GHz, GeForce 7600 GS, nvidia 
proprietary driver, Kernel 2.6.30-ARCH (and also other kernels from 
debian and ubuntu), webcam @ 640x480, 30 FPS. Measurements of CPU 
consumption are done using htop, on the above described computer.

The OS-X 10.5 Computer: Macbook pro 2x2.2 GHz, Geforce M 8xxx (don't 
know exactly, it's not my computer, but I can look more accurate if 
someone wants to know.) Measurements are made by using top (and OS-X 
activity monitor, which seem to be the same)

when using one webcam in cheese or luvcview it's using approximately 40% 
CPU usage.
when using the same webcam in Gem's pix_video help Patch it's 
approximately 80% CPU usage

didn't test simple application like cheese, but:
one webcam in pix_video help patch uses 15% cpu.
Gem, single webcam: approx. 15% usage
Gem, three webcams: approx 20%

For comparison in other programs on Linux and with other video content 
in Gem:
Gem, pix_movie (352x288, 25 fps, mpg) 10% cpu usage
Gem pix_movie (720x480, 24 fps, quicktime) 59% CPU
mplayer - vo gl: (720x480, 24 fps, quicktime) 62% cpu  
mplayer -vo null: (720x480, 24 fps, quicktime) 40% cpu    (40% for 
decoding? or is there already some system overhead?)

so it seems "normal" to have such huge cpu consumptions for video replay 
and cameras on linux?
I didn't measure it, but I also had extremely high cpu usage when using 
firewire dv cameras some time ago.

where does this big difference come from? does OS-X use video decoding 
on graphics card or something similar?

any ideas for making video faster on Linux?


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