The Linux disk cache is very unobtrusive. It uses spare memory to greatly increase disk access speeds, and without taking any memory away from applications.
It is a bad practice to drop the cache Unless you’re trying to address a performance issue, there’s no need to prevent any cache use. When you drop the cache (or caches), you will see the CPU load go up (sometimes way up) because the cache is gone. Available RAM goes up, but it does not matter because performance is slower because the cache is empty.
Then over time, if you leave the caches off, the performance will suffer because you are not taking advantage of the cache.
Linux tries to use all available RAM, so the caches will fill over time if you don’t instruct Linux to drop the caches, and this is a good thing. You will see available RAM go down, but don’t worry, it is available for applications when needed because applications take higher priority than cache
If Asterisk is not involved on this issue ,next questions must be addressed outside this forum,