Non-Volatile Hardware Transactional Memory: Challenges, Advancements, and Future Directions

Transactional memory (TM) has emerged as a powerful paradigm to simplify concurrent programming. Nowadays, hardware-based TM (HTM) implementations are available in several mainstream CPUs (e.g., by ARM, Intel and IBM). Due to their hardware nature, HTM implementations spare the cost of software instrumentation and can efficiently detect conflicts by extending existing cache- coherency protocols. However, their cache-centric approach also imposes a number of limitations that impact how effectively such systems can be used in practice.

This talk investigates the challenges that arise when leveraging existing HTM systems in conjunction with another recent disrup- tive hardware technology, namely Non-Volatile Memory (NVM). NVM, such as Intel Optane DC, provide much higher density than existing DRAM, while attaining competitive performance and pre- serving DRAM's byte addressability. However, the cache-centric approach adopted by existing HTM implementations raises a crucial problem when these are used in conjunction with NVM: since CPU caches are volatile, existing HTM fail to guarantee that data updated by committed transactions are atomically persisted to NVM.

I will overview how this problem has been so far tackled in the literature, with a focus on solutions that do not assume ad-hoc hard- ware mechanisms not provided by current HTM implementations, but that rather rely on hardware-software co-design techniques to ensure consistency on unmodified existing HTM systems. I will conclude by presenting ongoing research directions that depart from state of the art approaches in a twofold way: i) they assume the availability of durable caches, i.e., systems equipped with addi- tional power sources that ensure that cache contents can be safely persisted to NVM upon crashes; ii) they assume a weaker isolation levels at the TM level, namely Snapshot Isolation, which despite being more relaxed than the reference consistency model for TM systems (e.g., opacity), can still ensure correct execution of a wide range of applications while enabling new optimizations to boost the efficiency HTM applications operating on NVM.