In
computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, e ...
, kernel same-page merging (KSM), also known as kernel shared memory, memory merging, memory deduplication, and page deduplication is a
kernel feature that makes it possible for a
hypervisor system to share
memory pages that have identical contents between multiple
processes or
virtualized guests. While not directly linked,
Kernel-based Virtual Machine (KVM) can use KSM to merge memory pages occupied by
virtual machine
In computing, a virtual machine (VM) is the virtualization/emulation of a computer system. Virtual machines are based on computer architectures and provide functionality of a physical computer. Their implementations may involve specialized hardw ...
s.
Deduplication
KSM performs memory deduplication by scanning through
main memory
Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers.
The central processing unit (CPU) of a computer ...
for physical pages that have identical content, and identifies the virtual pages that are mapped to those physical pages. It leaves one page unchanged, and re-maps each duplicate page to point to the same physical page, after which it releases the extra physical pages for re-use. It also marks both virtual pages as "
copy-on-write" (COW), so that kernel will automatically remap a virtual page back to having its own separate physical page as soon as any process begins to write to it.
KSM was originally intended to run more virtual machines on one host by
sharing memory between processes as well as virtual machines. Upon its
implementation, users found KSM to also be useful for non-virtualized environments in which memory is at a premium.
An experimental implementation of KSM by Red Hat found that 52 virtual instances of Windows XP with 1 GB of memory, could run on a host computer that had only 16 GB of RAM.
KSM was merged into the
Linux kernel
The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ope ...
mainline in version 2.6.32, which was released on December 3, 2009.
To be effective, the operating system kernel must find identical memory pages held by different processes. The kernel also needs to predict whether the pages are likely to update infrequently enough that the merging would be an efficient use of processor resources.
A concern is that although memory usage is reduced, CPU usage is increased, thus negating potential increases in performance.
Security risks
Security is also a concern:
* Allows circumvention of
address space layout randomization (ASLR)
* Exposes information via
timing attacks
* Allows contamination of cryptographic resources in other virtualized guests via the memory
row hammer attack
See also
*
Data deduplication
In computing, data deduplication is a technique for eliminating duplicate copies of repeating data. Successful implementation of the technique can improve storage utilization, which may in turn lower capital expenditure by reducing the overall amou ...
– Reduction of storage requirement by sharing duplicate files
*
Single-instance storage
References
External links
Linux kernel documentationFedora KSM pageKernel-based virtual machine site - KSM
{{DEFAULTSORT:Kernel samepage merging
Computer memory
Linux kernel features