Deconstructing the garbage-first collector

Garbage-First is among today's most widely used garbage collectors. It is used in the HotSpot and OpenJDK virtual machines, and shares algorithmic foundations with three other important contemporary collectors: Shenandoah, C4, and ZGC. However, the design of the core algorithms and the performance tradeoffs they manifest have not been carefully analyzed in the literature. In this work, we deconstruct the G1 algorithm and re-implement it from first principles. We retrospectively develop a concurrent, region-based evacuating collector, CRE, which captures the principal design elements shared by G1, Shenandoah, C4, and ZGC. We then evaluate the impact of each of the major elements of G1 on performance, including pause time, remembered set footprint and barrier overheads. We find that G1's concurrent marking and generational collection reduces the 95-percentile GC pauses by 64% and 93% respectively. We find that the space overhead of G1's remembered sets is very low, typically under 1%. We also independently measure the barriers used by G1 and find that they have an overhead of around 12% with respect to total performance. This analysis gives users and collector designers insights into the garbage-first collector and the other fixed-size region-based concurrent evacuating collectors, which we hope will lead to better use of the collectors and provoke future improvements.