I remember sysadms in the company I was working for 20+ years ago. They've been managing HPUX and Solaris systems forever and we suddenly added a bunch of Linux

I remember sysadms in the company I was working for 20+ years ago. They've been managing HPUX and Solaris systems forever and we suddenly added a bunch of Linux servers for an application delivered by my team. They gave me a call one day. "Those Linux machines were at 100% RAM so we doubled it. They are still at 100% RAM. What's your software doing?" I guessed that the system management software on HPUX and Solaris was clearer about the distinction between memory used by applications and memory used by the file system cache, because I remember that file system cache was a thing even in HPUX and all the other UNIX flavors I used at university ten years before.