Two hardware-based approaches for deterministic multiprocessor replay

Modern computer systems are inherently nondeterministic due to a variety of events that occur during an execution, including I/O, interrupts, and DMA fills. The lack of repeatability that arises from this nondeterminism can make it difficult to develop and maintain correct software. Furthermore, it is likely that the impact of nondeterminism will only increase in the coming years, as commodity systems are now shared-memory multiprocessors. Such systems are not only impacted by the sources of nondeterminism in uniprocessors, but also by the outcome of memory races among concurrent threads. In an effort to help ease the pain of developing software in a nondeterministic environment, researchers have proposed adding deterministic replay capabilities to computer systems. A system with a deterministic replay capability can record sufficient information during an execution to enable a replayer to (later) create an equivalent execution despite the inherent sources of nondeterminism that exist. With the ability to replay an execution verbatim, many new applications may be possible: Debugging: Deterministic replay could be used to provide the illusion of a time-travel debugger that has the ability to selectively execute both forward and backward in time. Security: Deterministic replay could also be used to enhance the security of software by providing the means for an in-depth analysis of an attack, hopefully leading to rapid patch deployment and a reduction in the economic impact of new threats. Fault Tolerance: With the ability to replay an execution, it may also be possible to develop hot-standby systems for critical service providers using commodity hardware. A virtual machine (VM) could, for example, be fed, in real time, the replay log of a primary server running on a physically separate machine. The standby VM could use the replay log to mimic the primary's execution, so that in the event that the primary fails, the backup can take over operation with almost zero downtime.