Implementing Signatures for Transactional Memory

Transactional Memory (TM) systems must track the read and write sets--items read and written during a transaction--to detect conflicts among concurrent trans- actions. Several TMs use signatures, which summarize unbounded read/write sets in bounded hardware at a per- formance cost of false positives (conflicts detected when none exists). This paper examines different organizations to achieve hardware-efficient and accurate TM signatures. First, we find that implementing each signature with a single k-hash- function Bloom filter (True Bloom signature) is inefficient, as it requires multi-ported SRAMs. Instead, we advocate using k single-hash-function Bloom filters in parallel (Par- allel Bloom signature), using area-efficient single-ported SRAMs. Our formal analysis shows that both organiza- tions perform equally well in theory and our simulation- based evaluation shows this to hold approximately in prac- tice. We also show that by choosing high-quality hash func- tions we can achieve signature designs noticeably more ac- curate than the previously proposed implementations. Fi- nally, we adapt Pagh and Rodler's cuckoo hashing to im- plement Cuckoo-Bloom signatures. While this representa- tion does not support set intersection, it mitigates false pos- itives for the common case of small read/write sets and per- forms like a Bloom filter for large sets.