Debugging concurrent programs is known to be difficult due to scheduling non-determinism. The technique of multiprocessor deterministic replay substantially assists debugging by making the program execution reproducible. However, facing the huge replay traces and long replay time, the debugging task remains stunningly challenging for long running executions. We present a new technique, LEAN, on top of replay, that significantly reduces the complexity of the replay trace and the length of the

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... lay time without losing the determinism in reproducing concurrency bugs. The cornerstone of our work is a redundancy criterion that characterizes the redundant computation in a buggy trace. Based on the redundancy criterion, we have developed two novel techniques to automatically identify and remove redundant threads and instructions in the bug reproduction execution. Our evaluation results with several real world concurrency bugs in large complex server programs demonstrate that LEAN is able to reduce the size, the number of threads, and the number of thread context switches of the replay trace by orders of magnitude, and accordingly greatly shorten the replay time. Example A for j =1:M { expected = account.get()+i account.increment(i) assert account.get()==expected expected=account.get()-i account.decrease(i) assert account.get()==expected } account.set(0);