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 garbage collection (see Java Concurrent and Parallel GC) but, is there a difference between the two parallel algorithms?

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Using -XX  UseParNewGC along with -XX  UseConcMarkSweepGC  will cause higher pause time for Minor GCs  when compared to -XX  UseParallelGC   This is because  promotion of objects from Young to Old Generation will require running a Best-Fit algorithm  due to old generation fragmentation  to find an address for this object  Running such an algorithm is not required when using -XX  UseParallelGC  as  UseParallelGC can be configured only with MarkandCompact Collector  in which case there is no fragmentation

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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   x000D   x000D   lt table border  1  style  width 100   gt  x000D     lt tr gt  x000D       lt td align  center  gt Young collector lt  td gt  x000D       lt td align  center  gt Old collector lt  td gt  x000D       lt td align  center  gt JVM option lt  td gt  x000D     lt  tr gt  x000D     lt tr gt  x000D       lt td gt Serial  DefNew  lt  td gt  x000D       lt td gt Serial Mark-Sweep-Compact lt  td gt  x000D       lt td gt -XX  UseSerialGC lt  td gt  x000D     lt  tr gt  x000D     lt tr gt  x000D       lt td gt Parallel scavenge  PSYoungGen  lt  td gt  x000D       lt td gt Serial Mark-Sweep-Compact  PSOldGen  lt  td gt  x000D       lt td gt -XX  UseParallelGC lt  td gt  x000D     lt  tr gt  x000D     lt tr gt  x000D       lt td gt Parallel scavenge  PSYoungGen  lt  td gt  x000D       lt td gt Parallel Mark-Sweep-Compact  ParOldGen  lt  td gt  x000D       lt td gt -XX  UseParallelOldGC lt  td gt  x000D     lt  tr gt  x000D     lt tr gt  x000D       lt td gt Serial  DefNew  lt  td gt  x000D       lt td gt Concurrent Mark Sweep lt  td gt  x000D       lt td gt  x000D         lt p gt -XX  UseConcMarkSweepGC lt  p gt  x000D         lt p gt -XX -UseParNewGC lt  p gt  x000D       lt  td gt  x000D     lt  tr gt  x000D     lt tr gt  x000D       lt td gt Parallel  ParNew  lt  td gt  x000D       lt td gt Concurrent Mark Sweep lt  td gt  x000D       lt td gt  x000D         lt p gt -XX  UseConcMarkSweepGC lt  p gt  x000D         lt p gt -XX  UseParNewGC lt  p gt  x000D       lt  td gt  x000D     lt  tr gt  x000D     lt tr gt  x000D       lt td colspan  2  gt G1 lt  td gt  x000D       lt td gt -XX  UseG1GC lt  td gt  x000D     lt  tr gt  x000D   lt  table gt  x000D   x000D   x000D    Conclusion    Apply -XX  UseParallelGC when you require parallel collection method over YOUNG generation ONLY   but still  use serial-mark-sweep method as OLD generation collection Apply -XX  UseParallelOldGC when you require parallel collection method over YOUNG generation  automatically sets -XX  UseParallelGC  AND OLD generation collection Apply -XX  UseParNewGC  amp  -XX  UseConcMarkSweepGC when you require parallel collection method over YOUNG generation AND require CMS method as your collection over OLD generation memory 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

User · Answer

UseParNewGC usually knowns as  quot parallel young generation collector quot  is same in all ways as the parallel garbage collector  -XX  UseParallelGC   except that its more sophiscated and effiecient  Also it can be used with a  quot concurrent low pause collector quot   See Java GC FAQ  question 22 for more information  Note that there are some known bugs with UseParNewGC

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After a lot of searching  the best explanation I ve found is from Java Performance Tuning website in Question of the month  1 4 1 Garbage collection algorithms  January 29th  2003  Young generation garbage collection algorithms The  original  copying collector  Enabled by default   When this collector kicks in  all application threads are stopped  and the copying collection proceeds using one thread  which means only one CPU even if on a multi-CPU machine   This is known as a stop-the-world collection  because basically the JVM pauses everything else until the collection is completed  The parallel copying collector  Enabled using -XX  UseParNewGC   Like the original copying collector  this is a stop-the-world collector  However this collector parallelizes the copying collection over multiple threads  which is more efficient than the original single-thread copying collector for multi-CPU machines  though not for single-CPU machines   This algorithm potentially speeds up young generation collection by a factor equal to the number of CPUs available  when compared to the original singly-threaded copying collector  The parallel scavenge collector  Enabled using -XX UseParallelGC   This is like the previous parallel copying collector  but the algorithm is tuned for gigabyte heaps  over 10GB  on multi-CPU machines  This collection algorithm is designed to maximize throughput while minimizing pauses  It has an optional adaptive tuning policy which will automatically resize heap spaces  If you use this collector  you can only use the the original mark-sweep collector in the old generation  i e  the newer old generation concurrent collector cannot work with this young generation collector    From this information  it seems the main difference  apart from CMS cooperation  is that UseParallelGC supports ergonomics  while UseParNewGC doesn t

[java] Difference between -XX:+UseParallelGC and -XX:+UseParNewGC

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