Introduction to Virtualization - Oracle Solaris 10 System Virtualization Essentials - page 4

shows the CPU consumption of the same two workloads after consolidation onto
one system. The amount of wasted investment has decreased by an entire CPU. 1
Of course, these examples use average values, and real-world computers do not
run well at 100% CPU utilization. You must also avoid being overly aggressive
when consolidating workloads.

Figure 1.2 A Consolidated System

In the early days of the computer industry, all computers were so expensive that
a corporation might be able to own only one. Given the precious nature of this
resource, it was important for an organization to make the most of a computer by
keeping it as busy as possible doing useful work.
At first, computers could run only one program at a time. This arrangement
was unwieldy when a group of users sought to use the same computer, creating a
need for a multiuser operating system. Software engineers designed such operat-
ing systems with features that prevented one program from affecting another
program in a harmful way and prevented one user from causing harm to the other
users' programs and data. Other features were designed to prevent one program
from consuming more system resources—CPU cycles, physical memory (RAM), or
network bandwidth—than it should.
Later, computer manufacturers developed inexpensive microcomputers, which
were easier to afford than relatively expensive minicomputers. Unfortunately,
these microcomputers and their early operating systems were not well suited to
run multiple production applications. This led to a common practice of running
one application per computer.
More recently, progress in computer performance has led to a desirable prob-
lem: too much compute capacity. Many servers in data centers run at an average

1. The OS will spend some compute cycles managing resource usage of the two workloads and reducing

the impact that one workload has on the other. This mediation increases CPU utilization, thereby
reducing available CPU capacity, but we will ignore this effect for now.