What are Huge Pages

• Hugepages is a feature that allows the Linux kernel to utilize the multiple page size capabilities of modern hardware architectures. Linux creates multiple pages of virtual memory, mapped from both physical RAM and swap. A page is the basic unit of virtual memory, with the default page size being 4096 Bytes in the x86 architecture.

• Linux uses a mechanism in the CPU architecture called "Translation Lookaside Buffers" (TLB) to manage the mapping of virtual memory pages to actual physical memory addresses. The TLB is a limited hardware resource, so utilizing a huge amount of physical memory with the default page size consumes the TLB and adds processing overhead - many pages of size 4096 Bytes equates to many TLB resources consumed. By utilizing Huge Pages, we are able to create pages of much larger sizes, each page consuming a single resource in the TLB. A side effect of creating Huge Pages is that the physical memory that is mapped to a Huge Page is no longer subject to normal memory allocations or managed by the kernel virtual memory manager, so Huge Pages are essentially 'protected' and are available only to applications that request them. Huge Pages are 'pinned' to physical RAM and cannot be swapped/paged out.

• A typical purpose for allocating Huge Pages is for an application that has characteristic high memory use, and you wish to ensure that the pages it uses are never swapped out when the system is under memory pressure. Another purpose is to manage memory usage on a 32bit system - Creating Huge Pages and configuring applications to use them will reduce the kernel's memory management overhead since it will be managing fewer pages. The kernel virtual memory manager utilizes low memory - fewer pages to manage means it will consume less low memory.

• In the Linux 2.6 series of kernels, hugepages is enabled using the CONFIG_HUGETLB_PAGE feature when the kernel is built. All kernels supplied by Red Hat for the Red Hat Enterprise Linux 4 release and later releases have the feature enabled.

• Systems with large amount of memory can be configured to utilize the memory more efficiently by setting aside a portion dedicated for hugepages. The actual size of the page is dependent on the system architecture. A typical x86 system will have a Huge Page Size of 2048 kBytes. The huge page size may be found by looking at /proc/meminfo

# cat /proc/meminfo |grep Hugepagesize
Hugepagesize: 2048 kB

• In RHEL 6 or later, the hugepagesize can be displayed using the following command (this value is in bytes):

# hugeadm --page-sizes-all
2097152

Note: Enabling hugepages requires the kernel to find contiguous, aligned unallocated regions of memory. For most systems, this means that a reboot will be required to allocate the hugepages.