There are a variety of knobs throughout the JSC heap subsystem that are hardcoded but could easily be runtime options in Options.h. E.g.: (1) minEdenToOldGenerationRatio: the ration of Eden to Old gen size over which we will do a FullCollection. (2) proportionalHeapSize: the amount we will grow the Heap at the end of a FullCollection (3) other stuff I can't remember right now.