(almost) fence-less persist ordering

The semantics and implementation of a memory persistency model can significantly impact the performance achieved on persistent memory systems. The only commercially available and widely used x86 persistency model causes significant performance losses by requiring redundant, expensive fence operations for commonly used undo logging programming patterns. In this work, we propose light-weight extensions to the x86 persistency model to provide some ordering guarantees without an intervening fence operation. Our extension, Themis, eliminates over 91.7% of the fence operations in undo-logging PM programs and improves average performance by 45.8% while incurring only 1.2% increase in data cache size.