Efficient Message Logging to Support Process Replicas in a Volunteer Computing Environment

The context of this research is Volpex, a communication framework based on Put/Get calls to an abstract global space that can seamlessly handle multiple active replicas of communicating processes. Volpex is designed for a heterogeneous and unreliable execution environment where parallel applications need replication as well as check pointing to make continuous progress. Since different instances of the same process can execute in the same logical state at different clock times, communicated data objects must be logged to ensure consistent execution of process replicas. Logging to support redundancy can be the source of a significant overhead in execution time and storage and can limit scalability. In this paper we develop, implement, and evaluate Log on Read and Log on Write logging schemes to support redundant communication. Log on Read schemes log a copy of the data object returned to every Get (or Read) request. On the other hand, Log on Write schemes log the old data object only when a Put request is overwriting a data object. This reduces redundant copying, but identifying the correct data object to return to a Get request is complex. A Virtual Time Stamp (VTS) that captures global execution state is logged along with the data object to make this possible. We develop an optimized Log on Read scheme that minimizes redundancy and an optimized Log on Write scheme that reduces the VTS size and overhead. Experimental results show that the optimizations are effective in terms of storage and time overhead and an optimized Log on Read scheme presents the best tradeoffs for most scenarios.