Revisiting Partial Tracing for Safe, Efficient, and Concurrent Garbage Collection in Unmanaged Languages

Garbage collection (GC) remains a desirable yet elusive goal in unmanaged languages like C/C++ and Rust, where concurrent memory reclamation must be achieved without compiler or runtime support. Existing techniques face fundamental trade-offs among efficiency, safety, and ease of integration: tracing collectors like BDWGC incur costly stop-the-world pauses and unsafe conservative scanning, while reference counting schemes like CIRC are safe but introduce high overhead and require manual handling of cyclic data. We present a safe, efficient, and easy-to-integrate concurrent GC library, revisiting partial tracing (PT), a concept initially conceived by Bacon et al. 22 years ago. PT is a hybrid approach that maintains reference counts for roots, ensuring safety through precise root identification, and traces from objects with non-zero counts, offering ease of integration by handling cyclic garbage. Although PT has historically been considered inefficient due to the high cost of root mutation, we overcome this limitation in two design steps. First, Concurrent Partial Tracing (CPT) introduces phase consensus , enabling concurrent phase coordination without mutator suspension and eliminating most reference-count updates during traversal. Second, Concurrent Deferred Partial Tracing (CDPT) further reduces overhead by replacing atomic root updates with a lightweight, hazard pointer (HP)-based mechanism safeguarded by a phase barrier . We show that CDPT outperforms automatic collectors like BDWGC and CIRC while being comparable to manual schemes like RCU, through both micro-benchmarks on concurrent data structures and a macro-benchmark on Moka, a production cache library.