[Discussion/20/Discussion/Discussion] Consider a social networking website
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 websites use memcached as a
caching layer before the backend storage/database tiers. Assume that at any given
time the average user is browsing megabytes of content, and on any given day the
average user uploads megabytes of content.
a. [20] For the social networking website 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?
b. [Discussion] 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?
c. [Discussion] 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.
d. [Discussion] 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
servers on separate nodes in the same rack, or (3) memcached servers on the
same racks and storage servers collocated on separate racks.