Prefetching from Broadcast Disks

ABSTRACT

Broadcast Disks have been proposed as a means to efficiently deliver data to clients in ``asymmetric'' environments where the available bandwidth from the server to the clients greatly exceeds the bandwidth in the opposite direction. A previous study investigated the use of cost-based caching to improve performance when clients access the broadcast in a demand-driven manner [. achas 95 .]. Such demand-driven access however, does not fully exploit the dissemination-based nature of the broadcast, which is particularly conducive to client {\em prefetching}. With a Broadcast Disk, pages continually flow past the clients so that, in contrast to traditional environments, prefetching can be performed without placing additional load on shared resources. We argue for the use of a simple prefetch heuristic called \PT{} and show that \PT{} balances the cache residency time of a data item with its bandwidth allocation. Because of this tradeoff, \PT{} is very tolerant of variations in the broadcast program. We describe an implementable approximation for \PT{} and examine its sensitivity to access probability estimation errors. The results show that the technique is effective even when the probability estimation is substantially different from the actual values.