Fundamentals of Quantitative Design and Analysis - Computer Architecture: A Quantitative Approach - page 7

FIGURE 1.3 Costs rounded to nearest $100,000 of an unavailable system are shown by
analyzing the cost of downtime (in terms of immediately lost revenue), assuming three
different levels of availability and that downtime is distributed uniformly . These data are
from Kembel [2000] and were collected and analyzed by Contingency Planning Research.
A second key feature of server systems is scalability. Server systems often grow in response
to an increasing demand for the services they support or an increase in functional require-
ments. Thus, the ability to scale up the computing capacity, the memory, the storage, and the
I/O bandwidth of a server is crucial.
Finally, servers are designed for efficient throughput. That is, the overall performance of
the server—in terms of transactions per minute or Web pages served per second—is what is
crucial. Responsiveness to an individual request remains important, but overall efficiency and
cost-efectiveness, as determined by how many requests can be handled in a unit time, are
the key metrics for most servers. We return to the issue of assessing performance for diferent
types of computing environments in Section 1.8 .
Clusters/Warehouse-Scale Computers
The growth of Software as a Service (SaaS) for applications like search, social networking,
video sharing, multiplayer games, online shopping, and so on has led to the growth of a class
of computers called clusters . Clusters are collections of desktop computers or servers connec-
ted by local area networks to act as a single larger computer. Each node runs its own operating
system, and nodes communicate using a networking protocol. The largest of the clusters are
called warehouse-scale computers (WSCs), in that they are designed so that tens of thousands of
servers can act as one. Chapter 6 describes this class of the extremely large computers.
Price-performance and power are critical to WSCs since they are so large. As Chapter 6 ex-
plains, 80% of the cost of a $90M warehouse is associated with power and cooling of the com-
puters inside. The computers themselves and networking gear cost another $70M and they
must be replaced every few years. When you are buying that much computing, you need
to buy wisely, as a 10% improvement in price-performance means a savings of $7M (10% of
$70M).
WSCs are related to servers, in that availability is critical. For example, Amazon.com had
$13 billion in sales in the fourth quarter of 2010. As there are about 2200 hours in a quarter, the