10.1 About HugePages

The HugePages feature enables the Linux kernel to manage large pages of memory in addition to the standard 4KB (on x86 and x86_64) or 16KB (on IA64) page size. If you have a system with more than 16GB of memory running Oracle databases with a total System Global Area (SGA) larger than 8GB, you should enable the HugePages feature to improve database performance.

Note

The Automatic Memory Management (AMM) and HugePages features are not compatible in Oracle Database 11g and later. You must disable AMM to be able to use HugePages.

The memory allocated to huge pages is pinned to primary storage, and is never paged nor swapped to secondary storage. You reserve memory for huge pages during system startup, and this memory remains allocated until you change the configuration.

In a virtual memory system, the tables store the mappings between virtual addresses and physical addresses. When the system needs to access a virtual memory location, it uses the page tables to translate the virtual address to a physical address. Using huge pages means that the system needs to load fewer such mappings into the Translation Lookaside Buffer (TLB), which is the cache of page tables on a CPU that speeds up the translation of virtual addresses to physical addresses. Enabling the HugePages feature allows the kernel to use hugetlb entries in the TLB that point to huge pages. The hugetbl entries mean that the TLB entries can cover a larger address space, requiring many fewer entries to map the SGA, and releasing entries that can map other portions of the address space.

With HugePages enabled, the system uses fewer page tables, reducing the overhead for maintaining and accessing them. Huges pages remain pinned in memory and are not replaced, so the kernel swap daemon has no work to do in managing them, and the kernel does not need to perform page table lookups for them. The smaller number of pages reduces the overhead involved in performing memory operations, and also reduces the likelihood of a bottleneck when accessing page tables.

Huge pages are 4MB in size on x86, 2MB on x86_64, and 256MB on IA64.