Thread-level speculation on off-the-shelf hardware transactional memory

Abstract

Thread-level speculation can speed up a single-thread application by splitting its execution into multiple tasks and speculatively executing those tasks in multiple threads. Efficient thread-level speculation requires hardware support for memory conflict detection, store buffering, and execution rollback, and in addition, previous research has also proposed advanced optimization facilities, such as ordered transactions and data forwarding. Recently, implementations of hardware transactional memory (HTM) are coming into the market with minimal hardware support for thread-level speculation. However, few implementations offer advanced optimization facilities. Thus, it is important to determine how well thread-level speculation can be realized on the current HTM implementations, and what optimization facilities should be implemented in the future. In our research, we studied thread-level speculation on the off-the-shelf HTM implementation in Intel TSX. We manually modified potentially parallel benchmarks in SPEC CPU2006 for thread-level speculation. Our experimental results showed that thread-level speculation resulted in up to an 11% speed-up even without the advanced optimization facilities, but actually degraded the performance in most cases. In contrast to our expectations, the main reason for the performance loss was not the lack of hardware support for ordered transactions but the transaction aborts due to memory conflicts. Our investigation suggests that future hardware should support not only ordered transactions but also memory data forwarding, data synchronization, multi-version cache, and word-level conflict detection for thread-level speculation.