Memory in the computers is managed in terms of pages by the operating systems. In 32-bit systems, the size of one page is 4KB.
In 64-bit systems, there are also HugePages. HugePages allow us to have page size of 2MB.
Its always better to use HugePages for the Oracle database, because more SGA can be crammed into fewer pages.
But the Automatic Memory Management Feature of Oracle 11g is incompatible with the HugePages.
If it comes to deciding between using HugePages and AMM, I always opt for HugePages, because its efficient in memory management.
The default of OS is to mount tmpfs on /dev/shm. /dev/shm is used by SGA.
For HugePages, ramfs is mounted over /dev/shm, which is not supported for the AMM for now.
A very fine discussion is here.