One of the reasons why Linux is not suitable for real-time use is virtual memory. It's not even about swapping. The Translation Lookaside Buffer introduces too

One of the reasons why Linux is not suitable for real-time use is virtual memory. It's not even about swapping. The Translation Lookaside Buffer introduces too much variance into timing. Same thing with instruction and data caches and even instruction pipelining. It's not just Linux, but most general-purpose CPUs that are not suited for real-time control applications.