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8.3.3. Huge Pages and Transparent Huge Pages (THP)
x86 CPUs usually address memory in 4kB pages, but they are capable of using larger 2 MB or 1 GB 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).
A kernel feature enabled by default in Red Hat Enterprise Linux 6, 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.
Red Hat Enterprise Linux 6.7 systems support both 2 MB and 1 GB huge pages, which can be allocated at boot or at runtime. See Section 8.3.3.3, “Enabling 1 GB huge pages for guests at boot or runtime” for instructions on enabling multiple huge page sizes.
8.3.3.1. Configuring Transparent Huge Pages
Transparent huge pages (THP) automatically optimize system settings for performance. By allowing all free memory to be used as cache, performance is increased. As KSM can reduce the occurence of transparent huge pages, you may want to disable it before enabling THP. If you want to disable KSM, refer to Section 8.4.4, “Deactivating KSM”.
Transparent huge pages are enabled by default. To check the current status, run:
# cat /sys/kernel/mm/transparent_hugepage/enabled
To enable transparent huge pages to be used by default, run:
# echo always > /sys/kernel/mm/transparent_hugepage/enabled/sys/kernel/mm/transparent_hugepage/enabled to always.
To disable transparent huge pages:
# echo never > /sys/kernel/mm/transparent_hugepage/enabled
Transparent Huge Page support does not prevent the use of static huge pages. However, when static huge pages are not used, KVM will use transparent huge pages instead of the regular 4kB page size.
