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 transactions. Several TMs use signatures, which summarize unbounded read/write sets in bounded hardware at a performance 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 (Parallel Bloom signature), using area-efficient single-ported SRAMs. Our formal analysis shows that both organizations perform equally well in theory and our simulation- based evaluation shows this to hold approximately in practice. We also show that by choosing high-quality hash functions we can achieve signature designs noticeably more accurate than the previously proposed implementations. Finally, we adapt Pagh and Rodler's cuckoo hashing to implement Cuckoo-Bloom signatures. While this representation does not support set intersection, it mitigates false positives for the common case of small read/write sets and performs like a Bloom filter for large sets.