Design and Analysis of Field-Logging Write Barriers

Write barriers are a fundamental mechanism that most production garbage collection algorithms depend on. They inform the collector of mutations to the object graph, enabling partial heap collections, concurrent collection, and reference counting. While in principle, write barriers remember only the pointers within the object graph that were changed and do so just once, widely-used write barriers are less precise, sacrificing temporal and/or spatial precision to performance. The idea of precisely remembering just the pointers that were changed is not new, however implementing performant field-precise barriers has proved elusive. We describe a technique for efficiently implementing field-logging barriers. We implement a number of barriers and evaluate them on a range of x86 hardware. A generational field-logging barrier performs with 0.1% to 1% mutator overhead compared to a highly tuned object-logging barrier, while a preliminary implementation of a reference counting field-logging barrier performs with around 1% to 2% overhead compared to a highly tuned object-logging reference counting barrier. These results suggest that garbage collection algorithms that require the precision of exactly remembering field mutations without sacrificing performance may now be possible, adding a new mechanism to the design toolkit available to garbage collection researchers.