Warehouse-Scale Computers to Exploit Request-Level and Data-Level Parallelism - Computer Architecture: A Quantitative Approach - page 444

FIGURE 6.8 The Layer 3 network used to link arrays together and to the Internet
[ Greenberg et al. 2009 ] . Some WSCs use a separate border router to connect the Internet to
the datacenter Layer 3 switches.
Most applications fit on a single array within a WSC. Those that need more than one array
use sharding or partitioning , meaning that the dataset is split into independent pieces and then
distributed to different arrays. Operations on the whole dataset are sent to the servers hosting
the pieces, and the results are coalesced by the client computer.
Example
What is the average memory latency assuming that 90% of accesses are local to
the server, 9% are outside the server but within the rack, and 1% are outside the
rack but within the array?
Answer
The average memory access time is
or a factor of more than 120 slowdown versus 100% local accesses. Clearly, loc-
ality of access within a server is vital for WSC performance.