WP-2013-01E: ESX Memory Resource Management: Transparent page Sharing
Author(s): Ishan Banerjee, Philip Moltmann, Kiran Tati, Rajesh Venkatasubramanian
Abstract: Data centers strive to provide maximum compute and storage resources while minimizing cost of operation. To this end, hardware virtualization has become an integral part of data centers. Virtualization provides a tool to effectively and efficiently manage data center resources.
Virtualization software attempts to consolidate multiple virtual machines onto one physical machine. The consolidation ratio is the number of virtual machines placed on a single physical machine. A higher consolidation ratio leads to more savings in cost of operation of the data center.
VMware® ESX® Server (ESX) is a hypervisor providing efficient consolidation of virtual machines on physical machines. It implements overcommitment techniques to efficiently consolidate virtual machines with more virtual RAM and virtual CPUs than are available on the physical machine.
ESX is said to be memory overcommitted when virtual machines with a combined virtual RAM greater than that
available to ESX are powered on. ESX employs different memory reclamation techniques and policies to effectively distribute available physical memory to virtual machines with the goal of maximizing performance. Transparent page sharing (TPS) is a method using ESX that collapses multiple memory pages with the same content into a single memory page. The remaining pages become available for using elsewhere. Transparent page sharing works without any modification to virtual machines. It works with all guest operating systems (OSes) supported by ESX and does not impact performance of guest workloads significantly.
This white paper describes the transparent page-sharing technique. It provides a high-level description of the reclamation technique as well as its low-level configuration.