Toward a Theory of Input Acceptance for Transactional Memories

Abstract

As opposed to database transactional systems, transactional memory (TM) systems are constrained by real-time while treating their input workload. Nevertheless, there is no clear formalization of how a TM should react regarding to a specific input. While TM performance is often measured in terms of throughput, i.e., commit-rate (by time unit), we consider the commit-abort ratio of a TM for a given input, as the number of transactions this TM commits over the total number of input transactions. Building onto this, this paper defines the input acceptance of TMs depending on their commit-abort ratio on input classes. To this end, we exhibit several classes of workloads and identify bounds on the input acceptance of existing TM designs. Additionally, we propose a serializable STM that presents high input acceptance at the cost of a more complex algorithm than existing STMs. Finally, experimental validation compares the presented TM designs in terms of input acceptance with realistic workloads.

Inherent Limitations on Disjoint-Access Parallel Implementations of Transactional Memory

“…Indeed, none of the serializable STM implementations presented in the literature, e.g. [5,7,20,24], provides disjoint-access parallelism and wait-free, invisible read-only transactions. In fact, the impossibility results hold also for STMs that satisfy the even weaker condition of snapshot isolation known from the database literature [19,25] and suggested as an efficient alternative to serializability for STMs [23].…”

On Avoiding Spare Aborts in Transactional Memory

“…Gramoli et al [6] referred to the problem of spare aborts and introduced the notion of commit-abort ratio, which is the ratio between the number of committed transactions and the overall number of transactions in the run. Clearly, the commit-abort ratio depends on the choice of the transaction that should be aborted in case of a conflict.…”

“…Input acceptance is also a notion presented by Gramoli et al [6] -a TM accepts a certain input pattern (sequence of invocations) if it commits all of its transactions. The authors compared different TMs according to their input acceptance patterns.…”

“…The commit-abort ratio (τ ) [6] is the ratio between the number of committed transactions and the overall number of transactions in the history. Unfortunately, no online TM may guarantee optimal commit-abort ratio.…”

“…Previous works introduced two related notions: commit-abort ratio [6] and permissiveness [7]. The latter stipulates that in runs that do not violate correctness, no aborts should happen.…”