A Hardware Accelerator for Tracing Garbage Collection

A large number of workloads are written in garbage-collected languages. These applications spend up to 10-35% of their CPU cycles on GC, and these numbers increase further for pause-free concurrent collectors. As this amounts to a significant fraction of resources in scenarios ranging from data centers to mobile devices, reducing the cost of GC would improve the efficiency of a wide range of workloads. We propose to decrease these overheads by moving GC into a small hardware accelerator that is located close to the memory controller and performs GC more efficiently than a CPU. We first show a general design of such a GC accelerator and describe how it can be integrated into both stop-the-world and pause-free garbage collectors. We then demonstrate an end-to-end RTL prototype of this design, integrated into a RocketChip RISC-V System-on-Chip (SoC) executing full Java benchmarks within JikesRVM running under Linux on FPGAs. Our prototype performs the mark phase of a tracing GC at 4.2x the performance of an in-order CPU, at just 18.5% the area (an amount equivalent to 64KB of SRAM). By prototyping our design in a real system, we show that our accelerator can be adopted without invasive changes to the SoC, and estimate its performance, area and energy.