Abstract

In this talk I will present a memory Copy-on-Write mechanism developed for Xen Virtual Machine Monitor. Memory CoW is widely recognised to be an effective method of reducing memory pressure, by eliminating duplicate memory pages. While CoW is extensively used in general purpose operating systems, it’s acceptance in virtualisation environments has been hindered by the semantic gap between hypervisor and the guest OSes. The advent of paravirtualisation, however, allows to exploit extra information directly available from the guests.

The discussion of the CoW mechanism will focus on the following challenges:

1. How to detect sharing opportunities efficiently?

2. How to reduce the cost of CoW faults?

3. How to manage the excess memory obtained when sharing pages, and how to asynchronously reclaim memory from the guests?

No detailed knowledge of Xen/Xenolinux memory managed is required. The talk will be summarised with a status report.