Warehouse-Scale Computers to Exploit Request-Level and Data-Level Parallelism - Computer Architecture: A Quantitative Approach - page 440

6.3 Computer Architecture of Warehouse-Scale
Computers
Networks are the connective tissue that binds 50,000 servers together. Analogous to the
memory hierarchy of Chapter 2 , WSCs use a hierarchy of networks. Figure 6.5 shows one
example. Ideally, the combined network would provide nearly the performance of a custom
high-end switch for 50,000 servers at nearly the cost per port of a commodity switch designed
for 50 servers. As we shall see in Section 6.6 , the current solutions are far from that ideal, and
networks for WSCs are an area of active exploration.
FIGURE 6.5 Hierarchy of switches in a WSC . (Based on Figure 1.2 of Barroso and Hölzle
[2009] .)
The 19-inch (48.26-cm) rack is still the standard framework to hold servers, despite this
standard going back to railroad hardware from the 1930s. Servers are measured in the number
of rack units (U) that they occupy in a rack. One U is 1.75 inches (4.45 cm) high, and that is the
minimum space a server can occupy.
A 7-foot (213.36-cm) rack offers 48 U, so it's not a coincidence that the most popular switch
for a rack is a 48-port Ethernet switch. This product has become a commodity that costs as litle
as $30 per port for a 1 Gbit/sec Ethernet link in 2011 [ Barroso and Hölzle 2009 ]. Note that the
bandwidth within the rack is the same for each server, so it does not mater where the software