This is a very handy command in day-to-day work, actually. For example, I use to limit the total amount of memory available to an application, including page cache:

  $ systemd-run --user --scope --property=MemoryHigh=1G qbittorrent

It works just as you'd expect — if qbittorrent's working set goes above 1024 MiB, it pushes the least recently used page out of the page cache. Doesn't really have any effects on upload or download speeds, while helping to keep more useful data in memory.

Many isolation flags are not available in `systemd-run --user`, though, so if you'd like to have some protection you either have to combine `sudo systemd-run` with `su -c`, or wrap the command in firejail.

https://github.com/netblue30/firejail/