Consider a social networking Web site with 100 million active users posting updates about themselves (in text and pictures) as well as browsing and interacting with updates in their social networks. To provide low latency, Facebook and many other Web sites use memcached as a caching layer before the backend storage/database tiers.

a. Estimate the data generation and request rates per user and across the entire site.

b. For the social networking Web site discussed here, how much DRAM is needed to host its working set? Using servers each having 96 GB DRAM, estimate how many local versus remote memory accesses are needed to generate a user’s home page?

c. Now consider two candidate memcached server designs, one using conventional Xeon processors and the other using smaller cores, such as Atom processors. Given that memcached requires large physical memory but has low CPU utilization, what are the pros and cons of these two designs?

d. Today’s tight coupling between memory modules and processors often requires an increase in CPU socket count in order to provide large memory support. List other designs to provide large physical memory without proportionally increasing the number of sockets in a server. Compare them based on performance, power, costs, and reliability.

e. The same user’s information can be stored in both the memcached and storage servers, and such servers can be physically hosted in different ways. Discuss the pros and cons of the following server layout in the WSC:

(1) Memcached collocated on the same storage server,

(2) Memcached and storage server on separate nodes in the same rack, or

(3) Memcached servers on the same racks and storage servers collocated on separate racks.