x264enc bitrate adjustment latency

[Samuel Hurst]

Hello Nicolas,

On 11/05/2022 19:18, Nicolas Dufresne via gstreamer-devel wrote:
> Le mercredi 11 mai 2022 à 14:43 +0100, Samuel Hurst via gstreamer-devel a
> écrit :
>> 
>> [snip]
>>
>> videotestsrc horizontal-speed=10 ! \
>>     video/x-raw,format=NV12,width=1920,height=1080,framerate=25/1 ! \
>>     x264enc bitrate=10000 speed-preset=ultrafast \
>>       tune=zerolatency+fastdecode ! \
>>     queue ! \
> 
> This queue will fill in, causing high latency as you observe. Sets is-live=1 on
> videotestsrc, configure that queue to be smaller.
> 
>>     avdec_h264 ! \
>>     queue ! \
>>     fpsdisplaysink

Thanks for offering the suggestion. I've retested with videotestsrc 
is-live=1, as well as configuring the queue to have a hard limit of a 
single buffer, as well as removing it entirely, but it doesn't seem to 
have made a substantial difference, as there's still a definite lag to 
the setting of the new bitrate parameter and the buffer sizes trending 
towards a smaller size.

https://www.dropbox.com/s/mfwnnwpu05awkid/x264-buffer-bitrate-test-is-live-no-queues-10mb-to-8mb.png?dl=0

Surely the queue size shouldn't have made much difference, as I'm 
measuring the bitrate at the x264enc src pad with gst-shark, so I'd have 
seen the instantaneous output of the encoder, before the queue starts to 
fill up with buffers?

Best regards,
-Sam