Peer-Based Collaborative Caching and Prefetching in Mobile Broadcast - Advanced Operating Systems and Kernel Applications - page 240

a neighbor may be cheaper than getting from the
broadcast channel if the mobile peer has to moni-
tor the broadcast channel for a long time before
the data object appears.
A cooperative caching (or prefetching) strategy
that takes the sharing between peers into account
is more effective than an individual caching
(or prefetching) scheme. For example, a simple
cooperative strategy could be: a peer caches lo-
cally only the data objects that its neighbors do
not have. In this way, the overall data availability
among the neighborhood is improved, and then
the response time is improved.
We believe that a good cooperative cache
(or prefetching) management scheme for mobile
peers in a broadcast environment should meet
two requirements.

when making local caching decisions.

In this chapter, we discuss both cooperative
caching and cooperative prefetching in a broad-
cast environment. For cooperative caching, we
discuss in detail a scheme called CPIX (Coopera-
tive PIX). CPIX is a modified version of GCM
(Wu, 2006). CPIX extends the well-known PIX
caching scheme (designed for broadcast environ-
ment) (Acharya, 1995) to the cooperative scenario.
CPIX considers two important factors: local
access frequency and the global availability of
data objects. Access frequency helps to identify
critical objects that should be locally cached to
improve local cache hits (and hence reduce wait-
ing time and energy consumption). Global data
availability is used to identify the data objects
that are neither widely cached by other mobile
peers nor are frequently broadcast by the server.
Note that here data availability not only means
whether the data is available globally, but also
means how long it takes for a mobile peer to get
the data. For cooperative prefetching, we discuss in
detail a scheme called ACP (Announcement-based
Cooperative Prefetching) (Wu, 2005). The basic
idea of ACP is to let a mobile peer make prefetch-
ing decisions based on its neighbors' prefetching
decisions while keeping them autonomous. The
objective is to help peers avoid prefetching the
same data objects, and to improve data availability.
Both CPIX and ACP are designed for push-based
broadcast environments where the mobile peers
can move freely. The schemes consider not only
a mobile peer's local access pattern, but also the
data availability from other mobile peers and from
the broadcast channel.
In the remainder of the chapter, we give the
background knowledge on mobile broadcast and
individual caching and prefetching schemes,
review briefly some cooperative caching and
prefetching schemes, and present in detail CPIX
and ACP and a performance study of their ef-
fectiveness.

• First, mobile peers should retain their au-

tonomy. By autonomy we mean that a mobile
peer can make a caching (or prefetching)
decision based on its own knowledge (about
itself, its neighborhood, and the broadcast
channel) and does not require a leader (or
super-peer) to make central decisions. Be-
cause the peers are moving and their cache
contents change with time, to have accurate
and real-time knowledge of neighbors' cache
content is very expensive. In such a highly
dynamic environment leader selection and
information synchronization will incur
a very high communication (and battery
energy) cost.

• Second, it should consider not only the data

availability from the neighbors, but also
the data availability on the broadcast chan-
nel. A data object can be available in three
places: local cache, neighbor's cache, and the
broadcast channel. In a non-uniform mobile
broadcast, popular data objects are broadcast
more frequently and they have a high avail-
ability on the broadcast channel. A mobile
peer should consider both the availability
from neighbors and from broadcast channels