Limit ffmpeg CPU usage (no Hardware decoding/encoding)

[I-G-1-1]

Normally I'm using hardware (VAAPI) transcoding but there is a device (iPhone) that always trigger CPU conversion from my h.265 mkv media:

Code:

[/color][color=#000000]Stream #0:0 -> #0:0 (hevc (native) -> h264 (h264_vaapi))[/color]
[color=#000000]Stream #0:1 -> #0:1 (ac3 (native) -> aac (libfdk_aac))[/color][color=#000000]

I tought that wasn't a problem as is a single user and the CPU is an i5-6300U capable to perform the job. Instead after a while the CPU transcoding completely lock the system and the server become unresponsive and I can't even ssh to the server to kill the process.

So I'm trying to limit ffmpeg CPU usage during transcoding.

I stopped Jellyfin (sudo systemctl stop jellyfin)

edited /etc/jellyfin/encoding.xml from

Code:

<EncoderAppPathDisplay>/usr/lib/jellyfin-ffmpeg/ffmpeg</EncoderAppPathDisplay>

to

Code:

<EncoderAppPathDisplay>/usr/bin/nice -n 19 /usr/lib/jellyfin-ffmpeg/ffmpeg</EncoderAppPathDisplay>

and restarted jellyfin.

This isn't working as the line revert back to automatically <EncoderAppPathDisplay>/usr/lib/jellyfin-ffmpeg/ffmpeg</EncoderAppPathDisplay>.

is there any way I could limit ffmpeg CPU usage to 80% to avoid system become unresponsive? 

Thanks for the help