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04_GC_Algorithm.md

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Garbage Collection Algorithm


Parallel GC

Applied as the default GC in Java 8

  • Performs minor collection in parallel
    • Can significantly reduce GC overhead
  • A GC designed for applications that handle medium to large-scale datasets running on multi-processor or multi-thread hardware
  • Parallel Compaction
    • A feature that allows the Parallel Collector to perform major collection in parallel
    • When the -XX:+UseParallelGC option is specified, Parallel Compaction is used by default
  • However, the use of multi-threaded collection is limited to the Young generation only
    • Young Generation Collection algorithm: Parallel Scavenge
    • Old Generation Collection algorithm: Serial Mark-Sweep-Compact

G1 (Garbage First Collector)

Applied as the default GC in Java 11

  • A Collector designed for multi-processor machines with large memory
  • The G1 Collector can be enabled with the -XX:+UseG1GC option
  • Note that G1 GC is the only one that is not a Generational GC
    • The G1 collector is a unique collector that does not divide the heap into Young and Old regions
    • G1 eliminates the physical generation distinction and treats the entire heap as 1MB-sized regions
      • It is organized in a grid pattern and uses approximately 2000 regions
    • The name G1 means that collection starts from the region most filled with garbage
      • When a region full of garbage is found, the collector runs immediately
  • Live objects in Old regions are moved to other Old regions, and compaction occurs
  • In G1, the concepts of Young and Old generations are not fixed
    • The set of regions where objects are newly allocated is the Young generation
    • The set of regions where promotion occurs is the Old generation

ZGC

Applied as the default GC in Java 17

ZGC is a scalable low-latency GC that can handle heaps ranging from 8MB to 16TB in size, and can be enabled using the -XX:+UseZGC option

  • Low-latency
    • ZGC increases responsiveness by minimizing pauses for memory collection
    • It is a concurrent GC where application threads continue to run during collection operations, and pause times are very short
      • This allows maintaining high responsiveness even in large-scale memory environments
      • Maximum latency is less than 1 millisecond
  • Very Large Heap Support
    • ZGC can handle heaps as large as 16TB
      • This makes it suitable for applications that perform large-scale data processing or memory-intensive tasks
  • Non-contiguous Collection
    • Instead of collecting the entire heap at once, ZGC divides it into small region units and performs garbage collection
    • By processing only portions of the collection work, it can distribute pause times
  • Parallel Processing
    • ZGC utilizes multi-threads to process GC operations in parallel
    • This efficiently distributes GC operations and reduces overall collection time
  • Efficiency
    • It is efficient because there is no need to clean up the heap during collection
      • Typical GCs trigger stop-the-world events to perform memory collection operations
        • During this time, all application threads are paused for GC operations
        • These pause times can affect application responsiveness and performance
      • In contrast, ZGC maintains very short pause times during the heap cleanup process
    • ZGC continues to run application threads during collection operations and only triggers very short pauses when necessary
      • ZGC focuses on increasing application responsiveness by minimizing pause times