Improvements for Low-Power Intel CPU/GPU

[mazzz1y]

Since both requests are relatively small, I am creating a single issue for them:

1. Is there any reason to hardcode low_power=0 on the Intel QSV encoder?
2. After starting a transcoding session, it continues working for about a minute, causing a heavy load when channels are changed frequently.

I have currently fixed this with a local dirty patch. It works for me by setting low_power and reducing the session cleanup delay.

So my request is: can we make these options configurable, or are there any disadvantages or issues with doing that?

[chrisbenincasa]

re: low_power mode:

low_power is hard-coded to reduce risk of compat issues and thus support burden (streaming issues are the number 1 issue I handle in general). It's worth noting that ffmpeg 7.1 labels the option as experimental:

  -low_power         <boolean>    E..V....... enable low power mode(experimental: many limitations by mfx version, BRC modes, 
etc.) (default auto)     

Theoretically we should be able to detect at least some compatibility using our current method of querying vainfo (of course, this will not work on Windows...which is not ideal)

vainfo --display drm --device /dev/dri/renderD128 -a 2>&1 | grep LP      
VAProfileH264Main/VAEntrypointEncSliceLP
VAProfileH264High/VAEntrypointEncSliceLP
VAProfileH264ConstrainedBaseline/VAEntrypointEncSliceLP

I'm somewhat open to adding the option to the UI with a disclaimer that the option is experimental, but I'm wary of adding things that will cause more confusion.

re: startup load

there's probably 2 things happening here. Tunarr works ahead at the beginning of the stream (transcoding faster than real time) to attempt to create an ~8 second buffer of content for smoother playback. depending on your pipeline, some of the steps may be happening on the CPU, which of course would increase load. pure GPU pipelines are, in general, extremely fast, and should cause negligible system load.

re: cleaup

I would be open to considering making the grace period of cleanup after disconnect configurable.