Brief announcement

Abstract

Work stealing is a common technique used in the runtime schedulers of parallel languages such as Cilk and parallel libraries such as Intel Threading Building Blocks (TBB). Depth-restricted work stealing is a restriction of Cilk-like work stealing in which a processor blocked on a task at depth d can only steal tasks from other processors at depth greater than d. To support programs coded in a blocking style, i.e., code without explicit continuations, TBB imposes a depth restriction on work stealing to limit the stack space used by a computation.We present a lower bound on the completion time of a computation executed using a depth-restricted work-stealing scheduler. In particular, we construct a computation which on P processors runs a factor of Ω(P) slower with depth-restricted work stealing as compared to unrestricted work stealing. On this pessimal computation, depth-restricted work stealing asymptotically serializes execution while unrestricted work stealing achieves linear speedup.