Hardware Reference
In-Depth Information
Section 1.5
.
Applications on PMDs are often Web-based and media-oriented, like the Google
Goggles example above. Energy and size requirements lead to use of Flash memory for stor-
Responsiveness and predictability are key characteristics for media applications. A
real-time
performance
requirement means a segment of the application has an absolute maximum exe-
cution time. For example, in playing a video on a PMD, the time to process each video frame
is limited, since the processor must accept and process the next frame shortly. In some ap-
plications, a more nuanced requirement exists: the average time for a particular task is con-
strained as well as the number of instances when some maximum time is exceeded. Such ap-
proaches—sometimes called
soft real-time
—arise when it is possible to occasionally miss the
time constraint on an event, as long as not too many are missed. Real-time performance tends
to be highly application dependent.
Other key characteristics in many PMD applications are the need to minimize memory and
the need to use energy eiciently. Energy eiciency is driven by both batery power and heat
dissipation. The memory can be a substantial portion of the system cost, and it is important to
optimize memory size in such cases. The importance of memory size translates to an emphasis
on code size, since data size is dictated by the application.
Desktop Computing
The first, and probably still the largest market in dollar terms, is desktop computing. Desktop
computing spans from low-end netbooks that sell for under $300 to high-end, heavily con-
igured workstations that may sell for $2500. Since 2008, more than half of the desktop com-
puters made each year have been batery operated laptop computers.
Throughout this range in price and capability, the desktop market tends to be driven to
optimize
price-performance
. This combination of performance (measured primarily in terms of
compute performance and graphics performance) and price of a system is what maters most
to customers in this market, and hence to computer designers. As a result, the newest, highest-
performance microprocessors and cost-reduced microprocessors often appear first in desktop
systems (see
Section 1.6
for a discussion of the issues affecting the cost of computers).
Desktop computing also tends to be reasonably well characterized in terms of applications
and benchmarking, though the increasing use of Web-centric, interactive applications poses
new challenges in performance evaluation.
Servers
As the shift to desktop computing occurred in the 1980s, the role of servers grew to provide
larger-scale and more reliable file and computing services. Such servers have become the back-
bone of large-scale enterprise computing, replacing the traditional mainframe.
For servers, different characteristics are important. First, availability is critical. (We discuss
availability in
Section 1.7
.
) Consider the servers running ATM machines for banks or airline
reservation systems. Failure of such server systems is far more catastrophic than failure of a
single desktop, since these servers must operate seven days a week, 24 hours a day.
Figure 1.3
estimates revenue costs of downtime for server applications.
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