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8.4. Kernel Same-page Merging (KSM)

Kernel same-page Merging (KSM), used by the KVM hypervisor, allows KVM guests to share identical memory pages. These shared pages are usually common libraries or other identical, high-use data. KSM allows for greater guest density of identical or similar guest operating systems by avoiding memory duplication.
The concept of shared memory is common in modern operating systems. For example, when a program is first started, it shares all of its memory with the parent program. When either the child or parent program tries to modify this memory, the kernel allocates a new memory region, copies the original contents and allows the program to modify this new region. This is known as copy on write.
KSM is a Linux feature which uses this concept in reverse. KSM enables the kernel to examine two or more already running programs and compare their memory. If any memory regions or pages are identical, KSM reduces multiple identical memory pages to a single page. This page is then marked copy on write. If the contents of the page is modified by a guest virtual machine, a new page is created for that guest.
This is useful for virtualization with KVM. When a guest virtual machine is started, it only inherits the memory from the host qemu-kvm process. Once the guest is running, the contents of the guest operating system image can be shared when guests are running the same operating system or applications. KSM allows KVM to request that these identical guest memory regions be shared.
KSM provides enhanced memory speed and utilization. With KSM, common process data is stored in cache or in main memory. This reduces cache misses for the KVM guests, which can improve performance for some applications and operating systems. Secondly, sharing memory reduces the overall memory usage of guests, which allows for higher densities and greater utilization of resources.

Note

In Red Hat Enterprise Linux 6.7 and later, KSM is NUMA aware. This allows it to take NUMA locality into account while coalescing pages, thus preventing performance drops related to pages being moved to a remote node. Red Hat recommends avoiding cross-node memory merging when KSM is in use. If KSM is in use, change the /sys/kernel/mm/ksm/merge_across_nodes tunable to 0 to avoid merging pages across NUMA nodes. This can be done with the virsh node-memory-tune --shm-merge-across-nodes 0 command. Kernel memory accounting statistics can eventually contradict each other after large amounts of cross-node merging. As such, numad can become confused after the KSM daemon merges large amounts of memory. If your system has a large amount of free memory, you may achieve higher performance by turning off and disabling the KSM daemon. Refer to Chapter 9, NUMA" for more information on NUMA.

Important

Ensure the swap size is sufficient for the committed RAM even without taking KSM into account. KSM reduces the RAM usage of identical or similar guests. Overcommitting guests with KSM without sufficient swap space may be possible, but is not recommended because guest virtual machine memory use can result in pages becoming unshared.
Red Hat Enterprise Linux uses two separate methods for controlling KSM:
  • The ksm service starts and stops the KSM kernel thread.
  • The ksmtuned service controls and tunes the ksm service, dynamically managing same-page merging. ksmtuned starts the ksm service and stops the ksm service if memory sharing is not necessary. When new guests are created or destroyed, ksmtuned must be instructed with the retune parameter to run.
Both of these services are controlled with the standard service management tools.

Note

KSM is off by default on Red Hat Enterprise Linux 6.7.

8.4.1. The KSM Service

  • The ksm service is included in the qemu-kvm package.
  • When the ksm service is not started, Kernel same-page merging (KSM) shares only 2000 pages. This default value provides limited memory-saving benefits.
  • When the ksm service is started, KSM will share up to half of the host system's main memory. Start the ksm service to enable KSM to share more memory. 
# systemctl start ksm
Starting ksm:                                              [  OK  ]
The ksm service can be added to the default startup sequence. Make the ksm service persistent with the systemctl command. 
# systemctl enable ksm