[gst-devel] Reduce latency for a MJPEG over UDP multicast pipe

[Arnout Vandecappelle]

 Here's a bag of tricks:

- Make sure your framegrabber is configured at a high framerate.

- If it's an analog camera that is digitized by your framegrabber, you need to 
make sure it doesn't do deinterleaving or other multi-frame tricks.  I've had 
a framegrabber that introduces a 80ms latency for that...

- If the framegrabber can do jpeg compression by itself, use that.  It's not 
really faster than doing it on the processor, but you get your compressed data 
as you go along (otherwise you have a full frame before the encoder and a full 
frame after the encoder, so a minimum delay of two frames).

- Make sure your gstreamer pipeline isn't doing any copies.

- Make sure you use v4l2src, which avoids a kernel-to-user copy in the source 
element.

- Configure your kernel to use jumbo packets on the gigabit link.  This way 
you can put a full video frame in each ethernet frame and you avoid 
fragmentation.

- If all that isn't enough yet, you're in for some serious hacking :-).  In 
the v4l2 driver, instead of passing a buffer to user space when you have a 
full frame, pass it when you have enough data.  For already compressed data, 
it's the size of your UDP packets.  For raw data, that means 8 lines (all 
that's needed for jpeg compression).  You'll also need to hack into jpegenc to 
make it work that way.  Therefore, you're probably better off dropping 
GStreamer altogether and directly ioctl'ing v4l2 data into jpeg buffers.

- On the receiver side, the story is more or less the same.

 Regards,
 Arnout

On Thursday 30 September 2010 09:18:00, STJME wrote:
> I am trying to reduce the latency for a live video feed from a camera
> connected to a video grabber board, compressing using MJPEG and distribute
> it over UDP to a computer uncompressing the video and imediately render the
> video to the display. Today, using Dual core Inte CPU's and Gbit network,
> the total latency for the above scenario is 200 ms (from the light hits the
> camera until the pixel illuminates).
> 
> Q: How can I tweek the latency to get it as short as possible.
> 
> Basically, I want to remove all buffers, compress/decompress with such
> codecs and rates so it is as quick as possible. Less focus on packet loss
> etc..

-- 
Arnout Vandecappelle                               arnout at mind be
Senior Embedded Software Architect                 +32-16-286540
Essensium/Mind                                     http://www.mind.be
G.Geenslaan 9, 3001 Leuven, Belgium                BE 872 984 063 RPR Leuven
LinkedIn profile: http://www.linkedin.com/in/arnoutvandecappelle
GPG fingerprint:  31BB CF53 8660 6F88 345D  54CC A836 5879 20D7 CF43
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.freedesktop.org/archives/gstreamer-devel/attachments/20101008/d88080e7/attachment.htm>