Transactions as the Foundation of a Memory Consistency Model

We argue for transactions as the synchronization primitive of an ordering-based memory consistency model. Rather than define transactions in terms of locks, our model defines locks, conditions, and atomic/volatile variables in terms of transactions. A traditional critical section, in particular, is a region of code, bracketed by transactions, in which certain data have been privatized. Our memory model, originally published at OPODIS’08, is based on the database notion of strict serializability (SS). In an explicit analogy to the DRF0 of Adve and Hill, we demonstrate that SS provides the appearance of transactional sequential consistency (TSC) for programs that are transactional data-race free (TDRF). We argue against relaxation of the total order on transactions, but show that selective relaxation of the relationship between program order and transaction order (selective strict serializability—SSS) can allow the implementation of transaction-based locks to be as efficient as conventional locks. We also show that condition synchronization (in the form of the transactional retry primitive) can be accommodated in our model without explicit mention of speculation, opacity, or aborted transactions. Finally, we compare SS and SSS to the notion of strong isolation (SI), arguing that SI is neither sufficient for TSC nor necessary in programs that are TDRF.