vaapidecode vs avdec_h264 latency

[Joona Laine]

Hi folks,

I'm doing some FHD real time streaming stuff and I'm curious about the
latency I'm experiencing using two different decoders. The pipeline I'm
using is the following:

gst-launch-1.0 rtspsrc
location=rtsp://root:root@10.128.1.88/axis-media/media.amp?videocodec=h264
latency=0 ! rtph264depay ! h264parse ! queue ! avdec_h264 ! vaapisink

That gives me a more or less solid end-to-end latency of 130 ms. However, if
I change the decoder to vaapidecode I can see a drop in CPU load (which is
what I need), but the unfortunate drawback is the added 70 ms of latency.

I'm just curious where that comes from and is there anything I can do about
it?

I know this much:
  - vaapi needs to buffer whole frame(s) before the data is transferred to
GPU memory, and this adds latency compared to sw decoding
  - in the vaapi source code there has been defined a constant latency for
the pipeline (dunno if I should just change that)

My system includes
  - i7-6700
  - Intel HD 530
  - Ubuntu 16.04 
  - GStreamer 1.8.1 + plugins
  - libva-intel-driver-1.7.0


Thanks,

Joona



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