Timekeeping in VMware Virtual Machines

Because virtual machines work by time‐sharing host physical hardware, a virtual machine cannot exactly duplicate the timing behavior of a physical machine. VMware virtual machines use several techniques to minimize and conceal differences in timing behavior, but the differences can still sometimes cause timekeeping inaccuracies and other problems in software running in a virtual machine. This information guide describes how timekeeping hardware works in physical machines, how typical guest operating systems use this hardware to keep time, and how VMware products virtualize the hardware. This paper is intended for partners, resellers, and advanced system administrators who are deploying VMware products and need a deep understanding of the issues that arise in keeping accurate time in virtual machines. The VMware knowledge base contains additional and more frequently updated information, including best practices to configure specific guest operating system versions for the most accurate timekeeping, as well as recipes for diagnosing and working around known issues in specific versions of VMware products. This document includes the following topics: “Timekeeping Basics” on page 1 “Time and Frequency Units” on page 4 “PC Timer Hardware” on page 4 “VMware Timer Virtualization” on page 6 “Timekeeping in Specific Operating Systems” on page 10 “Synchronizing Virtual Machines and Hosts with Real Time” on page 14 “Time and Performance Measurements Within a Virtual Machine” on page 18 “Resource Pressure” on page 21 “Troubleshooting” on page 22 “Resources” on page 26