If no one is playing content from your Plex Media Server, there are a few common reasons for it to still be consuming notable amounts of CPU. In most cases, that would specifically be the “Plex Transcoder” process.
Detection for TV “Skip Intro”
Plex Pass subscribers can set their TV library to try and determine when an “intro” occurs during episodes, to allow quickly skipping over them during playback. Detecting these intros requires processor-intensive analysis of the episode files, as well as comparing against other episodes in the season.
Related Page: Skip TV Show Intros
Sonic or Loudness Analysis for Music
When music is added to a library, Plex Pass subscribers can have their server run extra analysis on that music content. There are two main types of additional analysis:
- Loudness: The server can analyze each track for its loudness data. This allows features such as Loudness Leveling and Sweet Fades. This analysis is typically reasonably quick and might take a couple of minutes per album.
- Sonic Anaylsis: A deeper analysis of the actual sonic characteristics of each track, allowing features such as Track Radio and sonic similarity of tracks, albums, and artists. Such sonic analysis is a CPU-intensive process and can take a significant amount of time to complete. If you added a new, large music library to get analyzed, it might take days or even weeks (depending on the library size and processor power).
Related Page: Sonic Analysis for Music
Related Page: Library
Video Preview Thumbnails Generation
One of the most common reasons users will see this unexpected CPU usage is when they enable their Plex Media Server to generate video preview thumbnails.
If you have the Generate video preview thumbnails (advanced) setting enabled, then your server will generate video preview thumbnails for new content.
Related Page: Video Preview Thumbnails
This process essentially requires a transcode of the media file in order to generate a series of screengrabs, which are then used to make the media index file. The process is CPU-intensive and can take anywhere from less than a minute to several minutes for a single item depending on both your processor power/speed as well as the duration and resolution of the media file.
If you enable that setting prior to adding a large amount of library content, you can expect the process to be running and using a large amount of CPU for hours or even days.
Chapter Thumbnails Generation
When you add new content to a video library, one of the things Plex does is try to find chapter information to match that item so that you can quickly jump to a desired chapter when watching it. If we find chapter information (either embedded in the file or from an online source), then we also grab some quick screenshots around the start of every chapter. Those screenshots can be used by apps when displaying chapter information to you.
In most cases, you’ll hardly even notice this occurs. On most desktop computers, this would take just a few seconds, for instance. However, if you run your Plex Media Server on a low-powered computer such as a NAS device, then it’s possible the process could take a fair bit longer and you might notice some high CPU usage while the chapter thumbnails are generated.
Preparing Sync/Download Content
When you specify content to sync—either for Downloads or Mobile Sync—then your Plex Media Server will first prepare that content to ensure it is in a compatible format. In many cases, this will involve transcoding the content.
Content transcoding is a very CPU-intensive process and can take tens of minutes for a single item in some cases.
Note: If you share libraries with friends and you’ve enabled Allow Sync for their share, they may have set content to download or sync, too.
Related Page: Monitoring Sync Status
Media Optimizer
If you have created any “optimization” jobs for content, then Media Optimizer might be using the transcoder to process and optimize content for you. If that is happening, then it will appear in the Conversion queue.
Related Page: Media Optimizer