Relinquishment Coherence for Enhancing Directory Efficiency in Chip Multiprocessors

A directory-based chip multiprocessor (CMP) suffers from excessive directory area overhead when its size grows. This work leverages novel relinquishment coherence and superior directory efficiency (RECODE) to lower area overhead. Relinquishment coherence boosts the utilization of a hash-based, set-associative table which holds distinct present-bit vectors (PVs), as it transforms a conflict PV to its variations after sharer relinquishment for hashing alternative sets. Superior directory efficiency is resulted from both boosted table utilization and table width shrunk via dropping "runs of zeros" commonly found in PVs. RECODE Table utilization is elevated by relinquishment coherence, which transforms a conflict PV to its variations after sharer relinquishment for hashing alternative sets. RECODE maintains "exact" directory representations for simple coherent logics and low coherent traffic. RECODE is found to enjoy 3.21x more storage efficiency than conventional bit-tracking directories for a CMP with 64 cores and it is 2.64x more storage efficient than the best directory SCD known so far.