Tradeoffs in transactional memory virtualization

For transactional memory (TM) to achieve widespread acceptance, transactions should not be limited to the physical resources of any specific hardware implementation. TM systems should guar-antee correct execution even when transactions exceed schedul-ing quanta, overflow the capacity of hardware caches and physi-cal memory, or include more independent nesting levels than what is supported in hardware. Existing proposals for TM virtualization are either incomplete or rely on complex hardware implementa-tions, which are an overkill if virtualization is invoked infrequently in the common case. We present eXtended Transactional Memory (XTM), the first TM virtualization system that virtualizes all aspects of transactional execution (time, space, and nesting depth). XTM is implemented in software using virtual memory support. It operates at page gran-ularity, using private copies of overflowed pages to buffer mem-ory updates until the transaction commits and snapshots of pages to detect interference between transactions. We also describe two enhancements to XTM that use limited hardware support to ad-dress key performance bottlenecks.We compare XTM to hardware-based virtualization using both real applications and synthetic mi-crobenchmarks. We show that despite being software-based, XTM and its enhancements are competitive with hardware-based alter-natives. Overall, we demonstrate that XTM provides a complete, flexible, and low-cost mechanism for practical TM virtualization