Supporting nested transactional memory in logTM

Abstract

Nested transactional memory (TM) facilitates software composition by letting one module invoke another without either knowing whether the other uses transactions. Closed nested transactions extend isolation of an inner transaction until the toplevel transaction commits. Implementations may flatten nested transactions into the top-level one, resulting in a complete abort on conflict, or allow partial abort of inner transactions. Open nested transactions allow a committing inner transaction to immediately release isolation, which increases parallelism and expressiveness at the cost of both software and hardware complexity.This paper extends the recently-proposed flat Log-based Transactional Memory (LogTM) with nested transactions. Flat LogTM saves pre-transaction values in a log, detects conflicts with read (R) and write (W) bits per cache block, and, on abort, invokes a software handler to unroll the log. Nested LogTM supports nesting by segmenting the log into a stack of activation records and modestly replicating R/W bits. To facilitate composition with nontransactional code, such as language runtime and operating system services, we propose escape actions that allow trusted code to run outside the confines of the transactional memory system.

Improving Parallelism in Hardware Transactional Memory

“…Since the introduction of the HTM design by Herlihy and Moss [20], numerous attempts have been made to improve and extend it (e.g., References [3,27,30,31,40,46] to give just a very few examples). The scope of this article precludes elaborating on all these efforts.…”

Fractal

“…Nesting can be trivially supported by ignoring the boundaries of all nested transactions, treating them as part of the top-level one. Some HTMs exploit nesting to implement partial aborts [44]: they track the speculative state of a nested transaction separately while it executes, so conflicts that occur while the nested transaction runs do not abort the top-level one. Even with partial aborts, HTMs ultimately merge nested speculative state into the top-level transaction, resulting in large atomic regions that are hard to support in hardware [2,12,19,20] and make conflicts more likely.…”

“…Prior work has explored relaxed nesting semantics, like open nesting [41,44,46] and early release [55], which relax isolation to improve performance. FRACTAL is orthogonal to these techniques and could be extended to support them, but we do not see the need on the applications we study.…”

On Open Nesting in Distributed Transactional Memory

“…Moravan et al [10] implement closed and open nesting in their previously proposed LogTM HTM. They implement the nesting models by maintaining a stack of log frames, similar to the run-time activation stack, with one frame for each nesting level.…”