Composability for Application-Specific Transactional Optimizations

Software Transactional Memory (STM) has made great advances towards acceptance into mainstream programming by promising a programming model that greatly reduces the complexity of writing concurrent programs. Unfortunately, the mechanisms in current STM implementations that enforce the fundamental properties of transactions — atomicity, consistency, and isolation — also introduce considerable performance overhead. This performance impact can be so significant that in practice, programmers are tempted to leverage their knowledge of a specific application to carefully bypass STM calls and instead access shared memory directly. While this technique can be very effective in improving performance, it breaks the consistency and isolation properties of transactions, which have to be handled manually by the programmer for the specific application. It also tends to break another desirable property of transactions: composability. In this paper, we identify the composability problem and propose two STM system extensions to provide transaction composability in the presence of direct shared memory reads by transactions. Our proposed extensions give the programmer a similar level of flexibility and performance when optimizing the STM application as the existing practices, while preserving composability. We evaluate our extensions on several benchmarks on a 16-way SMP. The results show that our extensions provide performance competitive with hand-optimized non-composable techniques, while still maintaining transactional composability.