KVM Huge Page Backed Memory

Sommario

Enable KVM guests to use huge page backed memory in order to reduce memory consumption and improve performance by reducing CPU cache pressure.

An SELinux issue in the kernel still remains, preventing full sVirt protection being applied to huge pages.

KVM support for huge pages
libvirt patch posted for running guests with -mem-path /dev/hugepages
libvirt patch-v2 posted which detects the host's hugetlbfs mount point automatically.
Identify SELinux issue in the kernel
Get the libvirt patch committed to libvirt-0.7.1

x86 CPUs usually address memory in 4kB pages, but they also have the ability to use huge pages (4MB on x86_32, 2MB on x86_64 and x86_32 PAE).

By using huge pages for a KVM guest's memory, less memory is used for page tables and TLB misses are reduced, thereby increasing performance.

Using huge pages for guest memory does have a downside, however - you can no longer swap nor balloon guest memory.

In order to use huge pages with KVM, one must do the following:

Mount hugetlbfs to /dev/hugepages - mount -t hugetlbfs hugetlbfs /dev/hugepages
Reserve some memory for huge pages - sysctl vm.nr_hugepages=516
Pass the hugetlbfs path to qemu-kvm - qemu-kvm -mem-path /dev/hugepages

This feature aims to improve allow huge pages to be used with libvirt managed guests.

Enables Fedora KVM hosts to achieve better performance.

Per:

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