IBM J. Res. Dev

Transactional memory support in the IBM POWER8 processor

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With multi-core processors, parallel programming has taken on greater importance. Traditional parallel programming techniques based on critical sections controlled by locking have several well-known drawbacks. To allow for more efficient parallel programming with higher performance, the IBM POWER8™ processor implements a hardware transactional memory facility. Transactional memory allows groups of load and store operations to execute and commit as a single atomic unit without the use of traditional locks, thereby improving performance and simplifying the parallel programming model. The POWER8 transactional memory facility provides a robust capability to execute transactions that can survive interrupts. It also allows non-speculative accesses within transactions, which facilitates debugging and thread-level speculation. Unique challenges caused by implementing transactional memory on top of the Power ISA (Instruction Set Architecture) weakly consistent memory model are addressed. We detail the Power ISA transactional memory architecture, the POWER8 implementation of this architecture, and two practical uses of this architecture - Transactional Lock Elision (TLE) and Thread-Level Speculation (TLS) - and provide performance results for these uses.