Concurrent Remembered Set Refinement in Generational Garbage Collection

Generational garbage collection divides a heap up into two or more generations, and usually collects a youngest generation most frequently. Collection of the youngest generation requires identification of pointers into that generation from older generations; a data structure that supports such identification is called a remembered set. Various remembered set mechanisms have been proposed; these generally require mutator code to execute a write barrier when modifying pointer fields. Remembered set data structures can vary in their precision: an imprecise structure requires the garbage collector to do more work to find old-to-young pointers. Generally there is a tradeoff between remembered set precision and barrier cost: a more precise remembered set requires a more elaborate barrier. Many current systems tend to favor more efficient barriers in this tradeoff, as shown by the widespread popularity of relatively imprecise card marking techniques. This imprecision becomes increasingly costly as the ratio between old- and young-generation sizes grows. We propose a technique that maintains more precise remembered sets that scale with old-generation size, using a barrier whose cost is not significantly greater than card marking.