The Impossibility of Ensuring Snapshot Isolation in Genuine Replicated STMs

In order to scale to large multiprocessors or clusters, transactional memories must reduce reliance on synchronisation. Therefore, we consider two favorable formal properties, namely Snapshot Isolation (SI, by which read-only transactions commit without synchronisation), and Genuine Partial Replication (GPR, by which a processor synchronises for a transaction only if it maintains a copy of some data item accessed by that transaction). We show that, unless the read-set of every transaction is known in advance, the combination of SI+GPR is impossible. To circumvent this impossibility result, we propose to weaken SI such that snapshots are allowed to be non-monotonic.