Minor GC vs Full GC — Scope, Pause Duration, Causes, and Log Signatures

Q: Minor GC vs Full GC — Scope, Pause Duration, Causes, and Log Signatures

**Minor GC** collects only the young generation (Eden + active Survivor). It runs frequently — every few seconds in a busy app — completes in 10-50ms, and is healthy. It is triggered when Eden fills. **Full GC** stops all application threads and collects the entire heap (young + old + Metaspace). It can take 1-30 seconds depending on heap size and is always pathological in modern collectors. Common triggers: promotion failure, to-space exhausted (G1), Metaspace OOM, explicit `System.gc()`, allocation failure when heap is undersized. In Java 9+ unified logs, look for `Pause Young (Normal)` (minor) vs `Pause Full (G1 Evacuation Pause)` with a reason in parentheses (full). Treat any non-rare full GC as a problem to investigate.

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Core answer

Minor GC collects only the young generation (Eden + active Survivor). It runs frequently — every few seconds in a busy app — completes in 10-50ms, and is healthy. It is triggered when Eden fills. Full GC stops all application threads and collects the entire heap (young + old + Metaspace). It can take 1-30 seconds depending on heap size and is always pathological in modern collectors. Common triggers: promotion failure, to-space exhausted (G1), Metaspace OOM, explicit System.gc(), allocation failure when heap is undersized. In Java 9+ unified logs, look for Pause Young (Normal) (minor) vs Pause Full (G1 Evacuation Pause) with a reason in parentheses (full). Treat any non-rare full GC as a problem to investigate.

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Interview answer

Minor GC and full GC describe entirely different events with completely different operational implications.

Minor GC:

Collects only the young generation — Eden plus the active Survivor space
Typically 10-50ms — runs frequently in healthy apps; background noise
Triggered when Eden fills
Log marker: Pause Young (Normal) in Java 9+

Full GC:

Collects the entire heap — young, old, and Metaspace
Stops all application threads — pauses can take seconds on large heaps
Always worth investigating in a modern collector
Log marker: Pause Full with a reason in parentheses

G1 has a healthy middle ground — mixed GC:

Pause Mixed collects young gen plus a few old-gen regions per cycle
Spreads old-gen reclamation across many small pauses
Replaces what older collectors called major GC

Common full GC triggers, with mitigations:

(System.gc()) from libraries → -XX:+DisableExplicitGC or -XX:+ExplicitGCInvokesConcurrent
(To-space exhausted) in G1 → increase -Xmx or -XX:G1ReservePercent
(Metadata GC Threshold) → cap Metaspace and fix classloader leak
(Allocation Failure) → heap is undersized
Memory leak → capture heap dump, analyze in Eclipse MAT, fix in code

The diagnostic discipline:

Always enable GC logging in production: -Xlog:gc* on Java 9+
Treat minor GC as background; investigate every full GC
Use GCEasy or JMC for visual analysis
Pair with -XX:+HeapDumpOnOutOfMemoryError so you have evidence when OOM strikes

In an interview, the differentiator is distinguishing G1's Pause Mixed from Pause Full — most candidates conflate them, missing the fact that mixed GC is the healthy operation while full GC is the fallback.

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