USENIX Technical Program - Abstract - USENIX Annual
Conference, General Session - June 2000
Performing Replacement in Modem Pools
Yannis Smaragdakis, Georgia Institute of Technology; Paul Wilson,
University of Texas, Austin
We examine a policy for managing modem pools that disconnects users only
if not enough modems are available for other users to connect. Managing
the modem pool then becomes a replacement problem, similar to buffer
cache management (e.g., in virtual memory systems). When a new
connection request is received, the system needs to find a user to
``replace''. In this paper we examine such demand-disconnect schemes
using extensive activity data from actual ISPs. We discuss various
replacement policies and propose CIRG: a novel replacement algorithm
that is well suited for modem pools. In general, the choice of algorithm
is significant. A naive algorithm (e.g., one that randomly replaces any
user who has been inactive for a while) incurs many tens of percent more
``faults'' (i.e., disconnections of users who are likely to want to be
active again soon) than the LRU algorithm, which, in turn, incurs 10%
more faults than CIRG. For good replacement algorithms, the impact can
be significant in terms of resource requirements. We show that the same
standards of service as a system that does not disconnect idle users can
be achieved with up to 13% fewer modems.
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