We have investigated the performance characteristics of hardware transactional memory (HTM) on the BlueGene/Q computer in comparison with conventional concurrency control mechanisms, using a molecular dynamics application as an example. Benchmark tests, along with overhead-cost and scalability analysis, quantify relative performance advantages of HTM over other mechanisms. We found that the bookkeeping cost of HTM is high but that the rollback cost is low. We propose transaction fusion and

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... ally-compact scheduling techniques to reduce the overhead of HTM with minimal programming. A strong scalability benchmark shows that the fused HTM has the shortest runtime among various concurrency control mechanisms without extra memory. Based on the performance characterization, we derive a decision tree in the concurrencycontrol design space for multithreading application.