An Implementation of Exception Handling with Collateral Task Abortion

Abstract

In this paper, an implementation of exception handling for task-parallel languages is proposed such that all running parallel tasks in a try block with an exception are automatically aborted as soon as possible. In parallel tree search, exception handling that allows such a collateral task abortion is useful when the objective is to complete the search as soon as one solution is found or to allow a worker to abort the traversal of a subtree that is found to be redundant by another worker, even when it has been initiated. However, few existing task-parallel languages, such as Cilk Plus and X10, have this capability. In this study, we enhanced a task-parallel language, Tascell, with this capability. Since the Tascell compiler is implemented as a translator to C code, techniques are required for implementing the non-local exit mechanism with cleanup code execution in the “finally” clauses. We achieved this implementation by exploiting nested functions, which are already used in the temporary backtracking mechanism of Tascell. We also modified the task scheduler provided by Tascell such that a worker can abort a task after it is started. When aborting a task, the scheduler also aborts all its descendant tasks. We evaluated our implementation in terms of overheads and time taken to abort tasks.