Peer-Based Collaborative Caching and Prefetching in Mobile Broadcast - Advanced Operating Systems and Kernel Applications - page 247

is encountered, the overall access pattern
of the neighborhood may change.

effectiVe collaboratiVe
caching and prefetching
SchemeS

For a mobile peer that can request for data
object from its neighbors and can access data
from a broadcast channel, data objects can be
available in three ways: local cache, neighbors'
cache, and broadcast channel. A data object's
availabilities in local cache and neighbors' cache
are not guaranteed, while its availability on the
broadcast channel is assured. The cooperative
cache management scheme in fact determines
which peer to cache what data objects, and as a
result it determines where to access a data object.
Getting data object from different places have
different costs in terms of both access latency and
power consumption. In this situation, when doing
caching management, a mobile peer not only needs
to consider its own access interest, but also needs
to take into account what its neighbors have in
cache and the data objects' broadcast frequencies
on the broadcast channel.
We believe that an effective cooperative cache
management scheme should deal with both the
aforementioned challenges. In the remainder of
this chapter, we discuss two such schemes. One
is a cooperative caching scheme called CPIX
designed for mobile peers whose access to the
broadcast channel is demand-driven. The other
is a cooperative prefetching scheme called ACP
designed for mobile peers whose access to the
broadcast channel is proactive. CPIX could be
used on mobile peers with limited power, while
ACP could be applied on mobile peers for whom
battery is not a problem.
CPIX and ACP share several common desir-
able properties that distinguish them from other
schemes: 1) the mobile peers using the schemes
are autonomous; 2) they consider the data avail-
ability from both the neighbors and the broadcast
channel; 3) they are adaptable to the broadcast
program; 4) they require very few message ex-
changes between mobile peers.

cpix: cooperative caching

PIX (Acharya, 1995) is a well-known individual
caching scheme for mobile peers in broadcast
environment (refer to the Background section for
a brief review of PIX). CPIX (Cooperative PIX)
is an extension of the PIX scheme for the mobile
peers where they can communicate with each
other in a Peer-to-Peer fashion.
The approach CPIX takes is to view the data
availability at other mobile peers (not only the
current neighbors) together with the data avail-
ability on the broadcast channel as the global
data availability , and every mobile peer manages
its own cache space according to its own access
interest and its estimate of global data availabil-
ity. The following observation leads to this idea.
When a local cache miss happens, a mobile peer
can request the data object from its neighbors
and tune into the broadcast channel to wait for
the data object. These two can be done in paral-
lel. The mobile peer is happy as long as it can
get the data object in a short period of time, and
there is no need to distinguish whether the data
object is from a neighboring peer or it is from the
broadcast channel.
It is important to point out that here data
availability does not mean whether the data is
available, since (in the worst case) the data object
is always available from the broadcast channel. It
rather means how long it takes for a mobile peer
to get the data.
Intuitively (as in PIX), data objects that are
frequently accessed and are not readily available
should get the highest priority to be cached locally,
otherwise, many local cache miss es will happen
due to its high access frequency and these local
cache miss es will result in long waiting time since
it is not easy to get them from the neighbors or
broadcast channel.