Limit FFmpeg CPU Threads on Linux

By default, FFmpeg maximizes your system’s resources by utilizing all available CPU cores to speed up video and audio processing. While this speeds up encoding, it can completely freeze your system or starve other applications of processing power. Fortunately, you can easily restrict FFmpeg’s resource consumption by using its native -threads flag, pinning it to specific cores with the taskset command, or utilizing cgroups and nice values for advanced system prioritization.

Method 1: Using the Native FFmpeg -threads Flag

The simplest and most effective way to control CPU usage is by telling FFmpeg directly how many threads to create. You do this by passing the -threads option into your command.

To limit FFmpeg to a specific number of threads, place the -threads flag after your input file but before your output file:

ffmpeg -i input.mp4 -threads 4 output.mp4

• How it works: In this example, FFmpeg is strictly limited to using 4 CPU threads.
• The Default Behavior: Setting -threads 0 tells FFmpeg to use its optimal default, which usually means filling up every available thread on your processor.

Note: Some specific video codecs (like libx264 or libx265) have their own internal threading management. If you want to ensure the codec itself obeys your limits, you can pass codec-specific arguments like this: ffmpeg -i input.mp4 -c:v libx264 -x264opts threads=4 output.mp4

Method 2: Hard-Limiting Cores with taskset

If you want to prevent FFmpeg from touching specific CPU cores entirely, you can use the Linux utility taskset. This sets the “CPU affinity” for the process, forcing Linux to run FFmpeg only on the hardware cores you permit.

To run FFmpeg on specific CPU cores (for example, cores 0, 1, 2, and 3):

taskset -c 0-3 ffmpeg -i input.mp4 output.mp4

Alternatively, if FFmpeg is already running and you want to throttle it mid-process, find its Process ID (PID) and restrict it on the fly:

taskset -cp 0-3 <PID>

Method 3: Managing Priority with nice and cpulimit

Sometimes, you don’t care how many threads FFmpeg uses, as long as it doesn’t make your desktop lag. In these scenarios, changing the process priority or capping the overall CPU percentage is a better approach.

Lowering Priority with nice

The nice command tells Linux to give FFmpeg a lower scheduling priority. Other tasks (like your web browser or code editor) will get first dibs on the CPU.

nice -n 19 ffmpeg -i input.mp4 output.mp4

A value of 19 is the “nicest” possible setting, meaning FFmpeg will quietly process in the background using only idle CPU cycles.

Capping Total CPU Percentage with cpulimit

If you want to ensure FFmpeg never exceeds a strict total percentage of your CPU capacity (e.g., maximum 200%, which equals 2 full cores), use the cpulimit utility:

cpulimit -l 200 -- ffmpeg -i input.mp4 output.mp4

Summary of Best Practices