Flexible Decoupled Transactional Memory Support

A high-concurrency Transactional memory (TM) implementation needs to track concurrent accesses, buffer speculative updates, and manage conflicts. We propose that the requisite hardware mechanisms be decoupled from one another. Decoupling (a) simplifies hardware development, by allowing mechanisms to be developed independently; (b) enables software to manage these mechanisms and control policy (e.g., conflict management strategy and laziness of conflict detection); and (c) makes it easier to use the hardware for purposes other than TM. We present a system, FlexTM (FLEXible Transactional Memory), that employs three decoupled hardware mechanisms: read and write signatures, which summarize per-thread access sets; per-thread conflict summary tables, which identify the threads with which conflicts have occurred; and a lazy versioning mechanism, which maintains the speculative updates in the local cache and employs a thread-private buffer (in virtual memory) only in the rare event of an overflow. The conflict summary tables allow lazy conflict management to occur locally, with no global arbitration (they also support eager management). All three mechanisms are kept software-accessible, to enable virtualization and to support transactions of arbitrary length. In experiments with a prototype on the Simics/GEMS testbed, FlexTM provides a 5 times speedup over high-quality software TM, with no loss in policy flexibility. Our analysis highlights the importance of lazy conflict detection, which maximizes concurrency and helps to ensure forward progress. Eager detection provides better overall system utilization in a mixed-programming environment. We also present a preliminary case study in which FlexTM components aid in the development of a tool to detect memory-related bugs