JVM Garbage collection

Question

In general ( as I am aware that there is a standard JVM implementation from Oracle/sun and other third parties as well like MS) , Does JVM create only one Garbage collection thread running as daemon to collect the garbage objects OR does JVM spawn more than one thread to accomplish the Garbage collection?

Answer 1

The "throughput collector" which is enabled with -XX:+UseParallelGC and is the default collector uses multiple threads. The "concurrent low pause collector" enabled with -XX:+UseConcMarkSweepGC uses one thread for concurrent collector but its stop-the-world collections are parallel.

Only the rarely used single threaded gc -XX:+UseSerialGC is single threaded.

http://www.oracle.com/technetwork/java/gc-tuning-5-138395.html

http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html

Answer 2

-XX:ConcGCThreads=n -- Number of threads concurrent garbage collectors will use. The default value varies with the platform on which the JVM is running.

http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html#G1Options

There could be more garbage collector threads but you should not rely on their number, running sequence or anything. There are essential things however, you can rely on. for example: Object.finalize() will be called once and only once.

Also check out Tuning garbage collector, about the question:

http://www.oracle.com/technetwork/java/gc-tuning-5-138395.html#1.1.%20Types%20of%20Collectors%7Coutline

Answer 3

Oracle's Garbage-First GC algorithm (available in Java 8 and the default option in Java 9) is a parallel/concurrent GC algorithm, so there is more than one thread involved. Specifically, there are a number of threads used for garbage collection:

• ParallelGC Threads are the threads that are used during a "stop the world" collection phase
• Parallel Marking Threads (or Concurrent GC Threads) are threads used to mark regions as candidates for cleanup and run without stopping application threads
• G1 concurrent refinement Threads are responsible for marking changes to individual region's remembered set of references

G1GC can be enabled by setting -XX:+UseG1GC (note that while available in Java 7 G1GC at that point in time was unreliable so don't use it in production (and since Java 7 has been end-of-lifed you shouldn't use that in production either)).

sources https://blogs.oracle.com/g1gc/entry/g1gc_faq and http://www.oracle.com/technetwork/articles/java/g1gc-1984535.html