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Transactional Memory (TM) systems must record the memory locations read and written (read and write sets) by concurrent transactions in order to detect conflicts. Some TM implementations use signatures for this purpose, which summarize read and write sets in bounded hardware at the cost of false positives (detection of non-existing conflicts).Read/write signatures are usually implemented as two separate Bloom filters with the same size. In contrast, transactions usually exhibit read/write sets of uneven cardinality, where read sets use to be larger than write sets. Thus, the read filter populates earlier than the write one and, consequently the read signature false positive rate may be high while the write filter has still a low occupation.In this paper, a multiset signature design is proposed which records both the read and write sets in the same Bloom filter without adding significant hardware complexity. Several designs of multiset signatures are analyzed and evaluated. New problems arise related to hardware complexity and the existence of cross false positives, i.e. new false positives coming from the fact that both sets share the same filter. Additionally, multiset signatures are enhanced using locality-sensitive hashing, proposed by the authors in a previous work. Experimental results show that the multiset approach is able to reduce the false positive rate and improve the execution performance in most of the tested codes, without increasing the required hardware area in a noticeable amount.

Reductions are common operations in many realworld applications that may be responsible for a significant part of the computing time. Modern compilers implement parallel reductions by combining privatization, atomic operations and/or locks. In this paper we analyze how to address reductions in the transactional memory (TM) model, which is flourishing together with the modern shared-memory multicore-based parallel architectures. With this purpose, this paper studies which support needs to be added to a TM system to deal with reductions as a special case of conflicting memory accesses.

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