Age-based garbage collection

Abstract

Modern generational garbage collectors look for garbage among the young objects, because they have high mortality; however, these objects include the very youngest objects, which clearly are still live. We introduce new garbage collection algorithms, called age-based, some of which postpone consideration of the youngest objects. Collecting less than the whole heap requires write barrier mechanisms to track pointers into the collected region. We describe here a new, efficient write barrier implementation that works for age-based and traditional generational collectors. To compare several collectors, their configurations, and program behavior, we use an accurate simulator that models all heap objects and the pointers among them, but does not model cache or other memory effects. For object-oriented languages, our results demonstrate that an older-first collector, which collects older objects before the youngest ones, copies on average much less data than generational collectors. Our results also show that an older-first collector does track more pointers, but the combined cost of copying and pointer tracking still favors an older-first over a generational collector in many cases. More importantly, we reopen for consideration the question where in the heap and with which policies copying collectors will achieve their best performance. © 1999 ACM.