[usamec]

Ehm. But sometimes stupid linux kernel prefers moving something usefull to swap and caching some shit instead of keeping stuff in ram and not doing caching.

So linux sometimes eats your RAM (but in different way).

[Amadou]

For situations where you have a time-critical app, you can pin the app's memory into RAM with the mlock() family of functions.

There may be a command-line utility that lets you pin an arbitrary running a process too, I haven't looked for it on linux.