SnCTM: reducing false transaction aborts by adaptively changing the source of conflict detection

Abstract

Optimistic concurrency provided by Transactional Memory (TM) makes it a good candidate for maintaining synchronization in future multi-core processors. Speculative execution and bulk level conflict detection enable TM to provide synchronization at fine grain without the complexity of managing fine grain locks. Early hardware TM systems proposed to store the information needed for checking conflicts in the Level 1 (L1) cache, thereby limiting the size of a transaction to the size of the L1 cache. The introduction of signatures to TM systems removed this limitation and allowed transactions to be of any size. However signatures produce false positives which leads to performance degradation in TM systems. The objective of introducing signatures to TM is that the size of a transaction can be bigger than the L1 cache. Once signatures are integrated to a TM system, they are used to detect conflicts regardless of the size of a transaction. This means signatures are being used even for transactions that can store their read and write sets in the L1 cache. Based on this observation we propose SnCTM, a TM system that adaptively changes the source used to detect conflicts. In our approach, when a transaction fits in the L1 cache, cache line information is used to detect conflicts and signatures are used otherwise. By adaptively changing the source, SnCTM achieved up to 4.62 and 2.93 times speed-up over a baseline TM using lazy versioning and lazy conflict detection with two commonly used signature configurations. We also show that our system, even with a smaller signature (64 bit), can achieve performance comparable to a system with a perfect signature (8k bit).