4K HDR transcoding Question

Plex Players Streaming Devices
1

Server Version#: 1.25.6.5577
Player Version#: Any that cannot play 4k Native

I run my Plex server as a VM. The VM’s OS is Server 2019 and I have a nVidia Quadra P4000 direct assigned to the VM. Everything works great until something tries to transcode 4K with HDR. h264 and h265 up to FHD transcode without issue. I can even get multiple streams of them going.

What I see happening when a 4K HDR transcode session start’s is the GPU Memory and CPU kick up a bit. about the same as when FHD h265 transcodes. And almost immediately after the main CPU starts running at 80% and up. Plex does report as HW transcode, and I don’t see any errors in logs stating that it failed to start hw transcode due to hardware compatibility as I used to see before replacing my K2200 with the P4000 with general h265 files.

From what I understand the P4000 does support HDR with a compatible monitor. Which isn’t detected since I do not have a monitor plugged into the card.

My Questions

  • The High CPU usage I see when transcoding 4K HDR, Is the transcoder working on the HDR part of the video using the CPU while the main resolution is being handled by the GPU?
  • If I were to connect a Monitor or Dummy plug with HDR capabilities will the plex transcoder transcode the HDR without taxing the CPU?
2

On Windows based systems, Plex does not support HDR → SDR tonemapping with Nvidia GPUs.

The transcode uses the GPU, but tonemapping uses the CPU.

In Settings → Transcoder, disable HDR tonemapping, play a 4K HDR movie, force a video transcode, and check the CPU utilization.

It won’t help the situation, as the colors will looked washed out, but it will let you know if that is the problem.

3

That’s too bad that HDR to SDR isn’t supported on Windows with an nVidia GPU. It is better with HDR tone mapping disabled. CPU is 30 to 40% now. playback still does continue to buffer constantly, just plays more between buffering

Looks like I’ll stick with the Optimized versions at 1080 for systems that don’t support 4k on direct play.

Thanks for the info.

4

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