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8.2. Huge Page Support

This section provides information about huge page support.
Introduction

x86 CPUs usually address memory in 4kB pages, but they are capable of using larger pages known as huge pages. KVM guests can be deployed with huge page memory support in order to improve performance by increasing CPU cache hits against the Transaction Lookaside Buffer (TLB). Huge pages can significantly increase performance, particularly for large memory and memory-intensive workloads. Red Hat Enterprise Linux 6 is able to more effectively manage large amounts of memory by increasing the page size through the use of huge pages.

By using huge pages for a KVM guest, less memory is used for page tables and TLB misses are reduced, thereby significantly increasing performance, especially for memory-intensive situations.
Transparent Huge Pages

Transparent huge pages (THP) is a kernel feature that reduces TLB entries needed for an application. By also allowing all free memory to be used as cache, performance is increased.

To use transparent huge pages, no special configuration in the qemu.conf file is required. Huge pages are used by default if /sys/kernel/mm/redhat_transparent_hugepage/enabled is set to always.
Transparent huge pages do not prevent the use of the hugetlbfs feature. However, when hugetlbfs is not used, KVM will use transparent huge pages instead of the regular 4kB page size.

Note

See the Red Hat Enterprise Linux 7 Virtualization Tuning and Optimization Guide for instructions on tuning memory performance with huge pages.