Approximately Opaque Multi-version Permissive Transactional Memory

In multi-version transactional memory read-only transactions do not have to abort, while update transactions may abort. There are situations where system delays do not allow to have precise consistency, such as in large scale network and database applications, due to network delays or other factors. In order to cope with such systems, we introduce here the notion of approximate consistency in transactional memory. We define K-opacity as a relaxed consistency property where read instructions in a read-only transaction may read one of K most recent written values, while read instructions in an update transaction read always the latest value. The relaxed consistency for read-only transactions has two benefits: (i) it reduces space requirements, since a new object version is saved once every K object updates, which reduces the total number of saved object versions by a factor of K, and (ii) it reduces the number of aborts, since there is smaller chance for read-only transactions to abort update transactions. This framework allows to have worst-case consistency guarantees and simultaneously good performance characteristics. In addition to correctness proofs, we demonstrate the performance benefits of our approach with experimental analysis. We tested our algorithm for different values of K using different benchmarks and we observed that when we increase K the number of aborts decreases and at the same time the throughput increases.