RapidMRC is a technique that approximates L2 miss rate curves (MRCs) on commodity systems to enable online optimizations. It tracks memory accesses using hardware performance counters, feeds the data into Mattson's stack algorithm to emulate cache line aging, and generates MRCs from the resulting histogram. This process takes approximately 230ms, providing rapid approximations of L2 MRCs to guide online partitioning of shared caches between applications for performance improvements.