Optimal folding of a paging drum in a three level memory system

• Authors: Lee J. Scheffier
• Venue: SOSP '73 Proceedings of the fourth ACM symposium on Operating system principles
• Year: 1973

This paper describes a drum space allocation and accessing strategy called “folding”, whereby effective drum storage capacity can be traded off for reduced drum page fetch time. A model for the “folded drum” is developed and an expression is derived for the mean page fetch time of the drum as a function of the degree of folding. In a hypothetical three-level memory system of primary (directly addressable), drum, and tertiary (usually disk) memories, the tradeoffs among drum storage capacity, drum page fetch time, and page fetch traffic to tertiary memory are explored. An expression is derived for the mean page fetch time of the combined drum-tertiary memory system as a function of the degree of folding. Measurements of the MULTICS three-level memory system are presented as examples of improving multi-level memory performance through drum folding. A methodology is suggested for choosing the degree of folding most appropriate to a particular memory configuration.