Difference between -XX:+UseParallelGC and -XX:+UseParNewGC

Question

They are algorithms for the young generation garbage collection.

The second one (UseParNewGC) gets activated automatically with the concurrent tenured generation ga

Answer 1

Parallel GC

• XX:+UseParallelGC Use parallel garbage collection for scavenges. (Introduced in 1.4.1)
• XX:+UseParallelOldGC Use parallel garbage collection for the full collections. Enabling this option automatically sets -XX:+UseParallelGC. (Introduced in 5.0 update 6.)

UseParNewGC

UseParNewGC A parallel version of the young generation copying collector is used with the concurrent collector (i.e. if -XX:+ UseConcMarkSweepGC is used on the command line then the flag UseParNewGC is also set to true if it is not otherwise explicitly set on the command line).

Perhaps the easiest way to understand was combinations of garbage collection algorithms made by Alexey Ragozin

  

    
Young collector
    
Old collector
    
JVM option
  
  

    
Serial (DefNew)
    
Serial Mark-Sweep-Compact
    
-XX:+UseSerialGC
  
  

    
Parallel scavenge (PSYoungGen)
    
Serial Mark-Sweep-Compact (PSOldGen)
    
-XX:+UseParallelGC
  
  

    
Parallel scavenge (PSYoungGen)
    
Parallel Mark-Sweep-Compact (ParOldGen)
    
-XX:+UseParallelOldGC
  
  

    
Serial (DefNew)
    
Concurrent Mark Sweep
    

      
-XX:+UseConcMarkSweepGC
      
-XX:-UseParNewGC
    
  
  

    
Parallel (ParNew)
    
Concurrent Mark Sweep
    

      
-XX:+UseConcMarkSweepGC
      
-XX:+UseParNewGC
    
  
  

    
G1
    
-XX:+UseG1GC
  

Conclusion:

1. Apply -XX:+UseParallelGC when you require parallel collection method over YOUNG generation ONLY, (but still) use serial-mark-sweep method as OLD generation collection
2. Apply -XX:+UseParallelOldGC when you require parallel collection method over YOUNG generation (automatically sets -XX:+UseParallelGC) AND OLD generation collection
3. Apply -XX:+UseParNewGC & -XX:+UseConcMarkSweepGC when you require parallel collection method over YOUNG generation AND require CMS method as your collection over OLD generation memory
4. You can't apply -XX:+UseParallelGC or -XX:+UseParallelOldGC with -XX:+UseConcMarkSweepGC simultaneously, that's why your require -XX:+UseParNewGC to be paired with CMS otherwise use -XX:+UseSerialGC explicitly OR -XX:-UseParNewGC if you wish to use serial method against young generation