optimization on Raspberry Pi

[Xuchen Yao]

Hi List,

I'm new to both gstreamer and Raspberry Pi, so please bear with me. Here
are a few simple questions you can probably answer from the top of your
head, and thanks in advance!

My config is Raspberry Pi B+ (single core 700MHz) and PlayStation Eye (USB
camera + microphone), running gstreamer-0.10.

Basically I ran 3 tests with the video capture with the following command
to compare CPU usage:

$ watch -d -n 2 ps -C gst-launch-0.10 -o %cpu,%mem,cmd

Test 1 (converting to RGB and sample at 30 fps):

$ gst-launch-0.10  v4l2src ! autovideoconvert ! videorate !
video/x-raw-rgb, format=RGB3, depth=24, framerate=*30/1*, width=320,
height=240 ! autovideoconvert ! fakesink

CPU was at 25%.

Then I wondered whether I can improve over this 25%, one nature thought was
to lower the sample rate from 30/1 to 5/1:

Test 2 (sampling at 5/1 instead of 30/1):

$  gst-launch-0.10  v4l2src ! autovideoconvert  ! videorate !
video/x-raw-rgb, format=RGB3, depth=24, framerate=*5/1*, width=320,
height=240 ! autovideoconvert ! fakesink

CPU usage jumped to 70%!!

Note that this camera is able to do 120 fps at 320x240 (according to
Wikipedia <http://en.wikipedia.org/wiki/PlayStation_Eye>), or 60 fps at
320x240 according to v4l2-ctl output (appended below), so I ran test 3:

Test 3 (sampling at 60/1 instead of 30/1):

$ gst-launch-0.10  v4l2src ! autovideoconvert ! videorate !
video/x-raw-rgb, format=RGB, depth=24, framerate=60/1, width=320,
height=240 ! autovideoconvert ! fakesink

CPU was around 50%.

Then here are 3 questions:

1. Could someone explain to me why this CPU usage:
5fps: 70%
30fps: 25%
60fps: 50%

2. My application needed RGB, that's why I was converting to it
(x-raw-rgb). To do computation on the *appsink* end, currently I do this:
still sample at 30 fps (because CPU is lowest at 25%), but in my
application for every 6 frames I drop 5, thus I'm in fact processing at a
speed of 5 fps. This leaves enough CPU power to my appsink. Does this make
sense?

3. Is there any other way to get lower than 25% CPU?

4. This deals with audio, that has been bugging me a bit. If I print the
duration of each audio buffer, like the following in python:

print appsink.emit('pull-buffer').duration/1000000.0

For "alsasrc" I get a constant 10ms, but for "pulsesrc" I've seen 26ms,
36ms, and 46ms in different runs! Any idea?

Thank you!

Xuchen

P.S.,

The camera looks like this:

$ v4l2-ctl --all


Driver Info (not using libv4l2):
Driver name   : ov534
Card type     : USB Camera-B4.09.24.1
Bus info      : usb-bcm2708_usb-1.2
Driver version: 3.12.22
Capabilities  : 0x85000001
Video Capture
Read/Write
Streaming
Device Capabilities
Device Caps   : 0x05000001
Video Capture
Read/Write
Streaming
Priority: 2
Video input : 0 (ov534: ok)
Format Video Capture:
Width/Height  : 640/480
Pixel Format  : 'YUYV'
Field         : None
Bytes per Line: 1280
Size Image    : 614400
Colorspace    : SRGB
Streaming Parameters Video Capture:
Capabilities     : timeperframe
Frames per second: 60.000 (60/1)
Read buffers     : 2

User Controls

                     brightness (int)    : min=0 max=255 step=1 default=0
value=0 flags=slider
                       contrast (int)    : min=0 max=255 step=1 default=32
value=32 flags=slider
                     saturation (int)    : min=0 max=255 step=1 default=64
value=64 flags=slider
                            hue (int)    : min=-90 max=90 step=1 default=0
value=0 flags=slider
        white_balance_automatic (bool)   : default=1 value=1
                       exposure (int)    : min=0 max=255 step=1 default=120
value=253 flags=inactive, volatile
                 gain_automatic (bool)   : default=1 value=1 flags=update
                           gain (int)    : min=0 max=63 step=1 default=20
value=255 flags=inactive, volatile
                horizontal_flip (bool)   : default=0 value=0
                  vertical_flip (bool)   : default=0 value=0
           power_line_frequency (menu)   : min=0 max=1 default=0 value=0
                      sharpness (int)    : min=0 max=63 step=1 default=0
value=0 flags=slider

Camera Controls

                  auto_exposure (menu)   : min=0 max=1 default=0 value=0
flags=update
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