Archive:It IT/Releases/12/Features/KVM Huge Page Backed Memory

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KVM Huge Page Backed Memory


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


Current status

  • Targeted release: Fedora 12
  • Last updated: 2009-09-24
  • Percentage of completion: 100%

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


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

Detailed Description

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:

  1. Mount hugetlbfs to /dev/hugepages - mount -t hugetlbfs hugetlbfs /dev/hugepages
  2. Reserve some memory for huge pages - sysctl vm.nr_hugepages=516
  3. 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.

Benefit to Fedora

Enables Fedora KVM hosts to achieve better performance.

Altre informazioni


  • Obbiettivi
  • Test Plan
  • Esperienza Utente
  • Dipendenze
  • Progetto corrente
  • Documentazione
  • Note di rilascio
  • Commenti e Discussioni

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