Approximate Count and Queue Objects in Transactional Memory

Abstract

In Transactional Memory each shared object can be accessed by concurrent transactions which may result to object access conflicts and aborts. Opacity is a precise consistency property which maps a concurrent execution to a legal sequential execution that preserves the real time order of events. However, having precise consistency may result in a large rate of aborts especially in systems that have frequent memory updates. In real applications, there are systems that do not require precise consistency especially when the data is not sensitive. As a means for relaxing consistency, we use the notion of K-opacity where transactions are allowed to read one of the K most recent written values to the objects (and not the latest value only). This increases the throughput and reduces the abort rate by reducing the chance of conflicts. In this paper we apply the K-opacity concept to read/write, count and queue objects, which are common objects used in typical concurrent programs. We use the technique of writer lists to keep track of the transactions and the data being written to the system, in order to control the error rate and to prevent error propagation. We illustrate with an experimental analysis the positive impact of our approach on performance, where higher opacity relaxation (higher values of K) increases the throughput and decreases the aborts rate significantly.