A Java Task Pool Framework providing Fault-Tolerant Global Load Balancing
Fault tolerance is gaining importance in parallel computing, especially on large clusters. Traditional approaches handle the issue on system-level. Application-level approaches are becoming increasingly popular, since they may be more efficient. This paper presents a fault-tolerant work stealing technique on application level, and describes its implementation in a generic reusable task pool framework for Java. When using this framework, programmers can focus on writing sequential code to solve their actual problem. The framework is written in Java and utilizes the APGAS library for parallel programming. It implements a comparatively simple algorithm that relies on a resilient data structure for storing backups of local pools and other information. Our implementation uses Hazelcast's IMap for this purpose, which is an automatically distributed and fault-tolerant key-value store.?The number of backup copies is configurable and determines how many simultaneous failures can be tolerated. Our algorithm is shown to be correct in the sense that failures are either tolerated and the computed result is the same as in non-failure case, or the program aborts with an error message. Experiments were conducted with the UTS, NQueens and BC benchmarks on up to 144 workers. First, we compared the performance of our framework with that of a non-fault-tolerant variant during failure-free operation. Depending on the parameter settings, the overhead was at most 46.06%. The particular value tended to increase with the number of steals. Second, we compared our framework's performance with that of a related, but less flexible fault-tolerant X10 framework. Here, we did not observe a clear winner. Finally, we measured the overheads for restoring a failed worker and for raising the number of backup copies from one to six, and found both to be negligible.
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