Comment by preg_match

Syscalls are expensive. It's not double caching, it's layers of caching. The kernel caches inside kernel memory space, sure, but you still need a syscall to reach that. Every application has layers of caching, it's very normal. This isn't even the top layer of cache, because of course the CPU itself has multiple caches.

But, for example, in a web application you will commonly cache requests. But then the database also has a cache. And then the filesystem the database is on also has a cache.