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2.20 [10/10/10/10/10] <2.4> Virtual Machines (VMs) have the potential for adding many be-
neicial capabilities to computer systems, such as improved total cost of ownership (TCO)
for availability. Could VMs be used to provide the following capabilities? If so, how could
they facilitate this?
a. [10] <2.4> Test applications in production environments using development machines?
b. [10] <2.4> Quick redeployment of applications in case of disaster or failure?
c. [10] <2.4> Higher performance in I/O-intensive applications?
d. [10] <2.4> Fault isolation between different applications, resulting in higher availabil-
ity for services?
e. [10] <2.4> Performing software maintenance on systems while applications are run-
ning without significant interruption?
2.21 [10/10/12/12] <2.4> Virtual machines can lose performance from a number of events, such
as the execution of privileged instructions, TLB misses, traps, and I/O. These events are
usually handled in system code. Thus, one way of estimating the slowdown when run-
ning under a VM is the percentage of application execution time in system versus user
mode. For example, an application spending 10% of its execution in system mode might
slow down by 60% when running on a VM.
Figure 2.32
lists the early performance of vari-
ous system calls under native execution, pure virtualization, and paravirtualization for
LMbench using Xen on an Itanium system with times measured in microseconds (courtesy
of Mathew Chapman of the University of New South Wales).
a. [10] <2.4> What types of programs would be expected to have smaller slowdowns
when running under VMs?
b. [10] <2.4> If slowdowns were linear as a function of system time, given the slowdown
above, how much slower would a program spending 20% of its execution in system
time be expected to run?
c. [12] <2.4> What is the median slowdown of the system calls in the table above under
pure virtualization and paravirtualization?
d. [12] <2.4> Which functions in the table above have the largest slowdowns? What do
you think the cause of this could be?
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