Chapter 14. Large Memory Optimization, Big Pages, and Huge Pages
As explained in detail in this section, enabling big pages helps reduce TLB misses. However, this performance benefit is realized primarily when using large SGA sizes. Once a portion of memory is locked down for big pages, applications that use normal pages cannot access that portion of the memory. It is very important to make sure that there is enough memory for normal pages for applications and users to avoid excessive swapping. So, it is recommended that big pages be used only on systems that have large amounts of physical memory and for SGA sizes of 16GB or greater.
Big Pages in Red Hat Enterprise Linux 2.1 and Huge Pages in Red Hat Enterprise Linux 3, 4 and 5 are very useful for large Oracle SGA sizes and in general for systems with large amount of physical memory. It optimizes the use of Translation Lookaside Buffers (TLB), locks these larger pages in RAM, and the system has less bookkeeping work to do for that part of virtual memory due to larger page sizes. This is a useful feature that should be used on x86 and x86-64 platforms. The default page size in Linux for x86 is 4KB.
Physical memory is partitioned into pages which are the basic unit of memory management. When a Linux process accesses a virtual address, the CPU must translate it into a physical address. Therefore, for each Linux process the kernel maintains a page table which is used by the CPU to translate virtual addresses into physical addresses. But before the CPU can do the translation it has to perform several physical memory reads to retrieve page table information. To speed up this translation process for future references to the same virtual address, the CPU saves information for recently accessed virtual addresses in its Translation Lookaside Buffers (TLB) which is a small but very fast cache in the CPU. The use of this cache makes virtual memory access very fast. Since TLB misses are expensive, TLB hits can be improved by mapping large contiguous physical memory regions by a small number of pages. So fewer TLB entries are required to cover larger virtual address ranges. A reduced page table size also means a reduction in memory management overhead. To use larger page sizes for shared memory, Big Pages (Red Hat Enterprise Linux 2.1) or Huge Pages (Red Hat Enterprise Linux 3, 4 and 5) must be enabled which also locks these pages in physical memory.
14.1. Big Pages in Red Hat Enterprise Linux 2.1 and Huge Pages in Red Hat Enterprise Linux 3
In Red Hat Enterprise Linux 2.1 large memory pages can be configured using the Big Pages,
bigpages, feature. In Red Hat Enterprise Linux 3 or 4 Red Hat replaced Big Pages with a feature called Huge Pages,
hugetlb, which behaves a little bit different. The Huge Pages feature in Red Hat Enterprise Linux 3 or 4 allows you to dynamically allocate large memory pages without a reboot. Allocating and changing Big Pages in Red Hat Enterprise Linux 2.1 always required a reboot. However, if memory gets too fragmented in Red Hat Enterprise Linux 3 or 4 allocation of physically contiguous memory pages can fail and a reboot may become necessary.
The advantages of Big Pages and Huge Pages for database performance are:
- increased performance through increased Translation Lookaside Buffer (TLB) hits.
- pages are locked in memory and are never swapped out which guarantees that shared memory such as SGA remains in RAM.
- contiguous pages are pre-allocated and cannot be used for anything else but for System V shared memory, for example SGA.
- less bookkeeping work for the kernel in that part of virtual memory due to larger page sizes.