Why can't OS use entire 64-bits for addressing? Why only the 48-bits?

Question

I\'m reading \"Understanding Linux Kernel\".

Paging for 64-bit Architectures

As we have seen in the previous sections,

Answer 1

Its just hard to believe for me that 32-bit addressing ie.. 4GB (2GB/3GB to be more correct) address space per process is a limit. If you really encountered this limit. Please give me example.

It's more efficient (quicker) to get data from RAM than to get it from disk.

The speed of SQL server depends partly on how much data (e.g. how many of its index and data pages) it's able to keep in RAM instead of on disk.

So, SQL databases (for example) may be faster on machines with more than 4GB of RAM.

The same is true for other types of server (e.g. file servers, HTTP proxies, etc.), which can be faster if they can have larger RAM caches.