jellyfin using cpu when watching movie with NVENC

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Based on what I can garner quickly from your transcode and general logs, NVENC appears to be functioning as expected (it's being used as HWA for the transcoding and there are no errors), plus I'd be impressed if you could hit 15x+ with just your CPU doing actual transcoding of video and audio streams simultaneously.

If you look at the only other ffmpeg calls in your general log, there are 95 instances of "trickplay" which utilizes ffmpeg to generate mjpeg/bif files for scrubbing (as far as I can tell...there's no documentation yet). So you just happened to spot an operation happening while you were transcoding as well. It's actually pretty nice that you're able to utilize your GPU for transcoding while your CPU handles "administrative" tasks. That's how HWA should work: as a force multiplier for your server.

If you want to "fix" this, go to Dashboard > Scheduled Tasks and change the triggers for any of these that you notice degrade performance. Set the trickplay file generation to 3 AM and you'll never see this particular "issue" again. Most of this is inferences since 10.9 is so new and I'm learning as I test as well.