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Cilkprof is a scalability profiler for multithreaded Cilk computations. Unlike its predecessor Cilkview, which analyzes only the whole-program scalability of a Cilk computation, Cilkprof collects work (serial running time) and span (critical-path length) data for each call site in the computation to assess how much each call site contributes to the overall work and span. Profiling work and span in this way enables a programmer to quickly diagnose scalability bottlenecks in a Cilk program. Despite the detail and quantity of information required to collect these measurements, Cilkprof runs with only constant asymptotic slowdown over the serial running time of the parallel computation. As an example of Cilkprof's usefulness, we used Cilkprof to diagnose a scalability bottleneck in an 1800-line parallel breadth-first search (PBFS) code. By examining Cilkprof's output in tandem with the source code, we were able to zero in on a …