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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchThe biggest CPU saving to check first is whether FFmpeg is unnecessarily re-encoding your stream. If the source codecs and output container are acceptable to the destination and you do not need to alter the media, try streamcopy with -c copy. Streamcopy avoids decoding and encoding; transcoding does both and can be computationally expensive. Whether copying works depends on your input and delivery setup, so verify playback at the destination before leaving it running.
First find out what FFmpeg is doing
FFmpeg can copy encoded audio and video packets, or decode them and encode them again. The FFmpeg manual defines transcoding as decoding a stream and then encoding it again, and recommends streamcopy when re-encoding is unnecessary. See the FFmpeg documentation.
- Streamcopy: the encoded streams are copied without re-encoding. This is the first approach to test when the existing codecs and output container suit the destination and no media transformation is required.
- Transcoding: FFmpeg decodes and re-encodes the affected streams. This may be necessary when the destination requires different codec settings or when you need to resize, overlay, deinterlace, resample audio, or otherwise filter the media.
Do not infer a CPU reduction from the command alone. Measure the FFmpeg process on the machine and stream you actually plan to use.
Test streamcopy when the source is already suitable
This schematic command copies streams without re-encoding:
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ffmpeg -i INPUT -c copy OUTPUT
It is not a complete live-streaming command. Replace the placeholders and choose the right input, output muxer and destination, stream mappings, and audio/video handling for your setup. Copying cannot perform a required resize, overlay, or codec conversion. Test the resulting output at the intended destination and confirm both audio and video behave as expected before relying on it continuously.
Keep stream selection deliberate
Review the streams in the input and the command’s mappings. Select only the audio and video streams the output needs, rather than processing unneeded streams. The right mapping depends on the source and destination; there is no universal mapping command for every ambient-stream setup.
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Remove transformations you do not need
Inspect the full command for filters and other stream changes. Resizing, deinterlacing, overlays, audio resampling, and mixing can require decoding and re-encoding. Remove transformations only if the output does not need them; otherwise, preserve the intended result and accept that the affected stream may need processing.
A bitstream filter is not the same as a decode-filter-re-encode pipeline. It works on encoded stream data without decoding, and can be appropriate for some specific codec or container changes when the chosen filter supports the input and output. Consult the FFmpeg bitstream filters documentation for the relevant filter and its limits.
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If you must encode, tune for the actual output
Check which encoder FFmpeg is using and whether your installed build includes it and the options you intend to use. For libx264, FFmpeg documents controls such as preset and CRF. Preset choice involves a trade-off between encoding speed and resource use, compression efficiency, and quality; there is no universally best preset for every 24/7 ambient stream. The FFmpeg codec options documentation describes available controls.
Choose settings against your actual quality target, bitrate or bandwidth limit, latency needs, and destination compatibility. Then validate the output and observe CPU use during the real workload. The available evidence does not establish a particular preset, hardware encoder, or percentage reduction as best for your machine. Hardware encoding, where considered, depends on the hardware, drivers, and FFmpeg build and must be tested on that setup.
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Do not treat -re as a CPU optimization
FFmpeg documents -re as reading file input at its native frame rate, for example to simulate a capture device or live input. It is not documented as a CPU-saving switch. The manual cautions that limiting the read rate of an actual capture or live input can cause packet loss. Use it only when its input-timing behavior fits your source and workflow, not as a general fix for high CPU use. See the FFmpeg manual.
Measure the result on the machine that will run it
- Record the current command, input, output requirements, FFmpeg version and build, and observed CPU use while the stream is running normally.
- Identify whether each stream is copied or encoded, and list filters or other transformations in the command.
- Change one unnecessary operation at a time. If the source is compatible and needs no transformation, test streamcopy; otherwise, preserve required processing and test encoder settings against your output requirements.
- Verify destination playback, including audio, video, and any required quality or latency characteristics.
- Compare CPU use under the same workload and conditions. Do not treat another person’s hardware result as a prediction for your system.
FFmpeg’s documentation portal says its current documentation is regenerated nightly and advises users of older releases to consult the documentation for those versions. Match options to the version and build installed on your machine: FFmpeg documentation portal.
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What one reported low-CPU result does—and does not—show
In a January 18, 2021 post to the FFmpeg-user mailing list, a user described relaying RTSP streams with video and audio copy options and reported low CPU use on a Raspberry Pi 3B+. That is a setup-specific anecdote, not a controlled benchmark or a current hardware recommendation. It illustrates why avoiding re-encoding can help, but it does not predict the result on another stream or machine: the mailing-list post.
Troubleshoot high CPU or a failed copy
- CPU remains high after adding
-c copy: check that the option applies to the relevant streams and that another part of the command is not encoding or filtering them. Measure the actual FFmpeg process under the live workload. - The output is rejected or will not play: the destination may not accept the source codec or container, or the output muxer or stream mapping may not match the delivery setup. Confirm compatibility; if a change is required, process only what needs to change.
- A filter is required: scaling, overlays, deinterlacing, or audio changes may require decoding and encoding. Removing the filter is not a valid optimization if it removes a required feature.
- An encoder option is unavailable: the installed FFmpeg build may not include that encoder or option. Check the local build and version against the documentation for it.
- Live input loses packets: do not use a low read-rate limit such as
-reon actual capture or live input as a CPU workaround; FFmpeg warns this can cause packet loss.
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