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Chapter 16Memory Management MechanicsPaid chapter

The Garbage Collector — Pacing, Assists, GOMEMLIMIT, and Green Tea

Understand how Go's GC paces itself, what GC assists are, why allocation rate is the primary lever, how GOGC/GOMEMLIMIT interact, and what the Green Tea experiment changes.

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What this chapter teaches

Common belief

GC pauses are the cost; tune GOGC to fix latency.

What actually happens

STW pauses are sub-millisecond; the real latency cost is GC assists — your allocating goroutines forced to do mark work. The primary lever is allocation rate, not GOGC.

Mechanisms covered
GOGC (default 100) sets the ratio of new allocation to live heap before the next cycle. GOMEMLIMIT (Go 1.19+) adds a soft memory ceiling.
Allocation rate is the primary lever: reducing allocs/op reduces GC frequency and assist tax. Tuning GOGC/GOMEMLIMIT without reducing allocation rate trades memory for latency.
Green Tea GC (Go 1.25, experimental, GOEXPERIMENT=greenteagc): marks at memory-page granularity for spatial locality rather than per-object. Many workloads spend ~10% (up to ~40%) less time in GC. Planned to become the default in Go 1.26. This connects directly to scan/noscan locality from Chapter 15.
tracebenchexperiment
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