Algorithms and data structures for compressed-memory machines

An overview of a set of algorithms and data structures developed for compressed-memory machines is given. These include 1) very fast compression and decompression algorithms, for relatively small fixed-size lines, that are suitable for hardware implementation; 2) methods for storing variable-size compressed lines in main memory that minimize overheads due to directory size and storage fragmentation, but that are simple enough for implementation as part of a system memory controller; 3) a number of operating system modifications required to ensure that a compressed-memory machine never runs out of memory as the compression ratio changes dynamically. This research was done to explore the feasibility of computer architectures in which data are decompressed/compressed on cache misses/writebacks. The results led to and were implemented in IBM Memory Expansion Technology (MXT), which for typical systems yields a factor of 2 expansion in effective memory size with generally minimal effect on performance.