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Common GC problems

Problems that show in garbage collection logs, with what they mean, how they appear in a log and what to look at next.

Java

• Pause / Stop-the-world

  A period during which every application thread is halted so that the collector can work on a heap that does not change under its feet.

• Full GC

  A stop-the-world collection of the entire heap: mark, then compact. The slowest collection; in a healthy application it is rare or absent.

• Humongous object (G1)

  In G1, an object whose size is at least half a region. It is allocated directly in contiguous old regions and bypasses the young generation.

• Evacuation failure / to-space exhausted (G1)

  G1 could not copy a live object because no free region was left to receive it. The log shows to-space exhausted.

• Concurrent mode failure (CMS)

  CMS only. The old generation filled up before the concurrent cycle finished, so the JVM stopped the application and ran a full compacting collection.

• GC overhead limit exceeded

  An OutOfMemoryError raised when the JVM spends nearly all its time in GC and recovers almost nothing.

• Explicit GC (System.gc())

  A collection requested by code or by a tool instead of by the collector itself, with the cause System.gc() or Diagnostic Command.

• Metaspace

  Native memory, outside the Java heap, holding the metadata of loaded classes: structures, methods and constant pools. It replaced the permanent generation in JDK 8.

Go

• Mark assist

  Marking work done by an allocating goroutine itself, when the program allocates faster than the background workers can mark.

• GC CPU limiter

  A safeguard added in Go 1.19 that caps the CPU time the collector can take, around 50 percent, when the memory limit is reached.

.NET

• Large object heap (LOH)

  A separate area for objects of 85,000 bytes or more. It is collected only together with generation 2.

• GC suspension

  Before most collections the runtime must suspend all managed threads. The time this takes is the suspension time.