Will the Guru

Monitoring/Tuning Garbage Collections Flags

• -verbose:gc
  • Prints the Garbage Collector activities such as whether they performed Minor GC, Full GC, etc.
• -XX:-UseAdaptiveSizePolicy
  • Young Generation space like Eden, S0 and S1 area is dynamically being resized by the virtual machine in runtime. This flag is to disable that feature.
• -XX:NewRatio=n
  • The ratio of the size between Young Generation and Old Generation
  • ex) NewRatio=4 means Old Generation is 4 times bigger than the Young Generation
• -XX:SurvivorRatio=n
  • The ratio of each S0 and S1 of total Young Generation size. The rest of them will be Eden space.
  • ex) SurvivorRatio=8 means each S0 and S1 space is 1/8 of the Young Generation. And the rest of them(6/8) will be Eden space.
  • Note that even though we set this value, the virtual machine can update the size in runtime for optimization.
• -XX:MaxTenuringThreshold=n
  • How many generations to an object survive before it becomes an Old Generation object. Which means to be the threshold to become an Old Generation object.
  • 15 is maximum value for this flag, but some Java versions allow above.
    However, it is always a bad idea to set it higher, because this means you’re not going to be using most of the Heap space(Old Generation).

Choosing a Garbage Collector

• Types of Collector
  • Serial: -XX:+UseSerialGC
  • Parallel: -XX:+UseParallelGC
  • Mostly Concurrent: -XX:+UseConcMarkSweepGC, -XX:+UseG1GC
    • G1GC is mostly being set as a default garbage collector after Java 10.