With the Big Pages and Huge Pages feature you specify how many physically contiguous large memory pages should be allocated and pinned in RAM for shared memory like Oracle SGA. For example, if you have three Oracle instances running on a single system with 2 GB SGA each, then at least 6 GB of large pages should be allocated. This will ensure that all three SGAs use large pages and remain in main physical memory. Furthermore, if you use ASM on the same system, then it is prudent to add an additional 200MB. I have seen ASM instances creating between 70 MB and 150 MB shared memory segments. And there might be other non-Oracle processes that allocate shared memory segments as well.
It is, however, not recommended to allocate too many Big or Huge Pages. These pre-allocated pages can only be used for shared memory. This means that unused Big or Huge Pages will not be available for other use than for shared memory allocations even if the system runs out of memory and starts swapping. Also take note that Huge Pages are not used for the
shared memory file system, see Section 14.8, “Huge Pages and Shared Memory File System in Red Hat Enterprise Linux 3”
, but Big Pages can be used for the
file system in Red Hat Enterprise Linux 2.1.