Optimizing the Plush Language's Garbage Collector for Performance

Background on Plush Language

Plush is a toy Lox-like language created to explore interpreter and virtual machine design concepts. It features actor-based parallelism, designed to avoid global VM locks and full VM pauses. The language includes basic interpreter optimizations and a copying Garbage Collector (GC).

Independent Garbage Collection in Plush

A notable feature of Plush is that each actor possesses its own fully independent GC. This design allows each actor to execute a collection cycle without requiring any synchronization. However, the performance of this GC has been disappointing, falling short of the developer's targets.

Performance Goals and Current Limitations

The developer set a personal goal for the Plush GC to collect one million live objects in under 20 milliseconds, aiming for performance suitable for 3D game engine development without noticeable pauses. A microbenchmark showed the current implementation collects one million objects in approximately 117ms on a MacBook Air M5, significantly slower than the target. This performance deficit is attributed to a shortcut taken during the initial implementation of the copying GC.

Copying GC Mechanism

A traditional Cheney copying collector moves objects from a 'from-space' to a 'to-space', utilizing a forwarding pointer in each object's header and using the 'to-space' as a work list to traverse live objects. In Plush, each actor uses a private allocator for objects and a message allocator for inter-actor communication, where objects are copied into the receiver's message allocator when sent.