Understand Concurrent Garbage Collection

Collect While Mutator Runs

Stop-the-world pauses hurt latency. Concurrent mark-sweep runs marker alongside mutator with write barriers to catch new references. G1, ZGC, and Go's concurrent sweeper reduce pause times to milliseconds or sub-ms.

Tri-Color Invariant

White: unvisited. Gray: visited, children pending. Black: fully scanned. Barrier ensures black objects do not point to white without gray intervention.

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• Floating garbage: whites reachable later but collected
• Remark phase fixes drift from concurrent stores
• Sweep can be lazy per-page
• Correctness harder than stop-the-world

Connecting to real compilers

Production compilers embed this step inside a longer pipeline. GCC flows through cpp, cc1, assembly, and ld; Clang uses the driver, Sema, LLVM IR passes, and a target backend. LLVM bitcode, JVM bytecode, and WASM are other familiar IRs at the same layer. The exercise isolates one pass so you can test it alone before chaining it to the next stage.

Why for this exercise

You will understand concurrent garbage collection algorithms. This exercise asks you to model tri-color marking, write barriers, and how the collector maintains correctness while the mutator allocates.