10_GC.md

GC

What is GC?

• Objects stored in the Heap area during Java runtime will continue to accumulate if not cleaned up, potentially causing an OutOfMemory Exception
• To prevent this, the JVM periodically collects and cleans up unused objects through Garbage Collection

Heap Area

• A virtual memory space for storing objects
• Stores objects and arrays created with the new operator

Heap Structure

• young : Where relatively young references live
  • eden
    • Where freshly created references exist among the young generation
  • survivor
    • Two areas exist
    • Where references that survived eden temporarily reside
• old : Where references that have survived a certain number of times reside
• permanent: Where metadata information from the method area is recorded

Minor GC and Major (Full) GC

GC is classified as Minor GC or Major GC depending on the area where it is performed

• Minor GC
  • Deletes objects from the Young Generation (Eden and Survivor 1, 2) areas from memory
• Major GC
  • Among objects that were not deleted during the Minor GC process and were moved to the Old Generation area, those determined to be unused are deleted from memory
  • Causes
    • When the Old Generation area's memory is full and objects can no longer be allocated
    • When there are many objects in the Old Generation area that are no longer in use
    • When the application runs for a long time and objects in the Old Generation area are continuously created and destroyed
• Cautions
  • When a Major GC occurs, the JVM stops application execution to perform GC, which is called the Stop-the-World phenomenon
  • Since all threads are stopped, no further work is executed, and performance degrades
    • It is important to ensure GC runs at an appropriate frequency to reduce Stop-the-World time and minimize the time threads are stopped