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Golang / Golang Internals and Memory Management Interview Questions

How does Go's garbage collector work? Explain the tri-color mark-and-sweep algorithm.

Go uses a concurrent, tri-color mark-and-sweep garbage collector. The key design goal is to minimize Stop-The-World (STW) pauses to sub-millisecond levels, even on large heaps, allowing Go programs to remain responsive under continuous allocation pressure.

Tri-Color Object States
ColorMeaning
WhiteNot yet visited. At GC end, white objects are unreachable — will be freed.
GreyReachable from a root, but outgoing references not yet scanned.
BlackFully scanned. All references from this object have been processed.

Algorithm phases:

  1. STW start (~100 µs): pauses all goroutines briefly to take a consistent root snapshot and enable the write barrier.
  2. Concurrent mark: GC goroutines run alongside application goroutines, turning grey objects black by scanning their references. Application goroutines continue executing.
  3. Write barrier: while marking is concurrent, the program may create new pointers. The write barrier (Dijkstra/hybrid) intercepts pointer writes and shades the pointed-to object grey so it is not missed.
  4. Mark termination (STW) (~100 µs): a second brief pause to drain the grey queue and disable the write barrier.
  5. Concurrent sweep: white (unreachable) objects are swept back into free lists. This is concurrent — no STW needed.
// Trigger GC manually (rarely needed in production)
import "runtime"
runtime.GC() // triggers a full GC cycle

// Read GC stats
var stats runtime.MemStats
runtime.ReadMemStats(&stats)
fmt.Printf("NumGC: %d\n", stats.NumGC)
fmt.Printf("PauseTotal: %v\n", time.Duration(stats.PauseTotalNs))
fmt.Printf("HeapAlloc: %d MB\n", stats.HeapAlloc/1024/1024)

// GOGC env — controls GC frequency
// GOGC=100 (default): GC triggers when heap grows 100% above last GC live set
// GOGC=200: GC triggers at 200% — less frequent GC, more memory used
// GOGC=off: disables GC (only for benchmarks)

// GOMEMLIMIT (Go 1.19+) — hard memory limit
// GOMEMLIMIT=500MiB triggers GC more aggressively if heap approaches 500 MB

The write barrier overhead (~5–10% CPU in allocation-heavy code) is the price paid for concurrent marking. In Go 1.22+, the runtime adaptively adjusts GC frequency using GOGC together with GOMEMLIMIT to balance latency and throughput automatically.

In Go's tri-color GC, what happens to objects that are still white at the end of the mark phase?
What is the purpose of the write barrier during concurrent GC marking?

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