5.2. Huge Pages and Transparent Huge Pages

Memory is managed in blocks known as pages. A page is 4096 bytes. 1MB of memory is equal to 256 pages; 1GB of memory is equal to 256,000 pages, etc. CPUs have a built-in memory management unit that contains a list of these pages, with each page referenced through a page table entry.
There are two ways to enable the system to manage large amounts of memory:
  • Increase the number of page table entries in the hardware memory management unit
  • Increase the page size
The first method is expensive, since the hardware memory management unit in a modern processor only supports hundreds or thousands of page table entries. Additionally, hardware and memory management algorithms that work well with thousands of pages (megabytes of memory) may have difficulty performing well with millions (or even billions) of pages. This results in performance issues: when an application needs to use more memory pages than the memory management unit supports, the system falls back to slower, software-based memory management, which causes the entire system to run more slowly.
Red Hat Enterprise Linux 6 implements the second method via the use of huge pages.
Simply put, huge pages are blocks of memory that come in 2MB and 1GB sizes. The page tables used by the 2MB pages are suitable for managing multiple gigabytes of memory, whereas the page tables of 1GB pages are best for scaling to terabytes of memory. For details on configuring huge pages, see Section 5.2.1, “Configure Huge Pages”
Huge pages can be difficult to manage manually, and often require significant changes to code in order to be used effectively. As such, Red Hat Enterprise Linux 6 also implemented the use of transparent huge pages (THP). THP is an abstraction layer that automates most aspects of creating, managing, and using huge pages.
THP hides much of the complexity in using huge pages from system administrators and developers. As the goal of THP is improving performance, its developers (both from the community and Red Hat) have tested and optimized THP across a wide range of systems, configurations, applications, and workloads. This allows the default settings of THP to improve the performance of most system configurations. However, THP is not recommended for database workloads.
THP can currently only map anonymous memory regions such as heap and stack space.

5.2.1. Configure Huge Pages

Huge pages require contiguous areas of memory, so allocating them at boot is the most reliable method since memory has not yet become fragmented. To do so, add the following parameters to the kernel boot command line:

Huge pages kernel options

hugepages
Defines the number of persistent huge pages configured in the kernel at boot time. The default value is 0. It is only possible to allocate (or deallocate) huge pages if there are sufficient physically contiguous free pages in the system. Pages reserved by this parameter cannot be used for other purposes.
Default size huge pages can be dynamically allocated or deallocated by changing the value of the /proc/sys/vm/nr_hugepages file.
In a NUMA system, huge pages assigned with this parameter are divided equally between nodes. You can assign huge pages to specific nodes at runtime by changing the value of the node's /sys/devices/system/node/node_id/hugepages/hugepages-1048576kB/nr_hugepages file.
For more information, read the relevant kernel documentation, which is installed in /usr/share/doc/kernel-doc-kernel_version/Documentation/vm/hugetlbpage.txt by default. This documentation is available only if the kernel-doc package is installed.
hugepagesz
Defines the size of persistent huge pages configured in the kernel at boot time. Valid values are 2 MB and 1 GB. The default value is 2 MB.
default_hugepagesz
Defines the default size of persistent huge pages configured in the kernel at boot time. Valid values are 2 MB and 1 GB. The default value is 2 MB.