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physical resources. Besides, undesired interactions between VMs are sometimes
referred to as cross-talks. Hypervisors can incorporate sophisticated resource sched-
ulers and allocators to circumvent cross-talks. Finally, hypervisors offer no shar-
ing among OS distributions. The only code/data shared among VMs is indeed the
hypervisor itself.
Nonetheless, the unique benefits offered by virtualization have some side effects.
For instance, the degree of isolation comes at the cost of efficiency. Efficiency can
be measured in terms of overall execution time. In general, VMs provide inferior
performance as opposed to equivalent physical machines. This is mainly due to:
(1) the overhead of context switching between VMs and the hypervisor and (2) the
duplication of efforts by the hypervisor and the OSs running in VMs (e.g., all might
be running schedulers, managing virtual memories, and interpreting I/O requests),
among others.
16.3 MANAGING SYSTEM COMPLEXITY
Modern computers are among the most advanced human-engineered structures.
These structures are typically very complex. Such complexity stems from incor-
porating various silicon chips embodied as processors, memories, disks, displays,
keyboards, mice, network interfaces, and others, upon which programs are operated
and services are offered. The key to managing complexity in computer systems is
by dividing system components into levels of abstractions separated by well-defined
interfaces.
16.3.1 l
levels
oF
a
bstraCtions
The first aspect of managing computer complexity is by abstracting computer com-
ponents. For instance, gates are built on electronic circuits, binary on gates, machine
languages on binary, programming languages on machine languages, and operat-
ing systems along with associated applications on programming languages. Another
abstraction that almost every computer user understands and uses is the file. Files are
abstractions of disks. As demonstrated in Figure 16.4, the details of a hard disk are
File
A file system abstraction
provided by OS
File
Disk
FIGURE 16.4
Files are abstractions of disks.
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