Efficient CPU power capping is essential for energy saving and fault tolerance in parallel computing clusters, but its effectiveness depends on accurate and timely processor power forecasting with minimal sampling overhead. Existing methods often struggle to balance these factors under scalability constraints, as hardware limitations tightly bound the available sampling resources. This article focuses on the issue of high-precision real-time processor power forecasting while maintaining (or minimally increasing) the total overhead of multiprocessor power forecasting, particularly when the parallelism scale ranges from P to 2P processors or when the problem size scales from M to 2M . We propose CML-PowF , a low-overhead multiprocessor real-time power forecasting approach based on data clustering. CML-PowF integrates two key algorithms: Alg-CEF , which conducts cluster matching on the runtime characteristics of the program at the P / M scale, and models the tradeoff among forecasting error, time span, and sampling overhead. Alg-MSF , which leverages execution patterns from smaller-scale runs to determine the optimal sampling overhead and forecasting time span at the 2P / 2M scale. We evaluate CML-PowF on x86 and ARM platforms with up to 32 computing nodes (2,048 cores). Results show that it achieves 3–6% forecasting error at large scales with only 0.2–0.5% degradation compared to the P / M scale, without increasing total sampling overhead. Integrated with the PowC control system, CML-PowF effectively maintains real-time processor power below target thresholds.