JVM Troubleshooting

HotSpot Garbage Collectors

• Young Generation

  • Serial
    • is a stop-the-world
    • copying collector that uses a single GC thread
  • ParNew
    • is a stop-the-world
    • copying collector that uses multiple GC threads
  • ParallelScavenge
    • is a stop-the-world
    • copying collector that uses multiple GC threads

• Old Generation

  • Serial Old (Mark Sweep Compact)
    • is a stop-the-world
    • mark-sweep-compact collector that uses a single GC thread
  • CMS
    • is a mostly concurrent
    • low-pause collector
  • Parellel Old
    • is a compacting collector that uses multiple GC threads

• Both

  • G1
    • is the Garbage First collector for large heaps
    • provides reliable short GC pauses
    • has generations but uses different memory layout

How to enable the Collectors

• UseSerialGC : Serial + Serial Old
• UseParNewGC : ParNew + Serial Old
• UseConcMarkSweepGC : ParNew + CMS + Serial Old.
  • CMS is used most of the time to collect the old generation. Serial Old is used when a concurrent mode failure occurs.
• UseParallelGC : Parallel Scavenge + Parallel Old
• UseG1GC : G1 GC for both generations

Collectors

Mark-Sweep-Compact Collector

• -XX:+UseSerialGC or –XX:+UseParNewGC selects Serial Old collector for old generation
• Stop-the-world old generation collector.
• Old is collected using mark-sweep-compact.
• Mark phase: marks all live objects.
• Sweep phase: sweeps over the heap identifying garbage.
• Slide phase: the GC performs a sliding compaction, sliding the live objects towards the start of the Heap.

Parallel Collector

• -XX:+UseParallelGC
  • Young Generation collected with Parallel Scavenge
  • Old Generation collected with Parallel collector
• It is also called the throughput collector
• Stop-the-world collections
• Default on Server type machines up until JDK 9
• Collection is performed in parallel on multiple cores

Concurrent Mark Sweep Collector (CMS)

• -XX:+UseConcMarkSweepGC
  • Young gen – ParNew collector
  • Old gen – CMS collector
• Low-latency collector, Mostly concurrent
• No heap compaction – fragmentation
• Free lists link unallocated regions
• Allocations expensive as compared to bump-the-pointer allocations
• Additional overhead on young collections
• Larger heap size requirement and floating garbage
• Deprecated in Java 9

G1 Collector

• Server-style garbage collector, targeted for multi-processor machines with large memories
• Meets garbage collection (GC) pause time goals with a high probability, while achieving high throughput.
• Better GC ergonomics
• Low pauses without fragmentation
• Parallelism and concurrency in collections
• G1 is a compacting collector.
• Fully supported in Oracle JDK 7 update 4 and later releases
• Default collector in JDK 9

References

• GC Tuning Guide