"Once you have twice as much RAM as your working set, then gc is competitive with manual memory allocation. If you have less performance falls off a cliff -- 60x slowdowns -- because tracing interacts pessimally with the LRU algorithm that kernels use to determine which pages to swap to disk. Basically, your gc roots are the hottest data in the program, and since you begin tracing there, LRU means the kernel will put your root set onto disk."