I am implementing some data structure in which I need to invalidate some entries after some time, so for each entry I need to maintain its insertion timestamp. When I get an entry I need to get a timestamp again and calculate the elapsed time from the insertion (if it's too old, I can't use it).

This data structure is highly contented by many threads, so I must get this timestamp (on insert and find) in the most efficient way possible. Efficiency is extremely important here.

If it matters, I am working on a linux machine, developing in C++. What is the most efficient way to retrieve a timestamp?

BTW, in some old project I was working on, I remember I saw some assembly command which gets a timestamp directly from the CPU (can't remember the command).