What do the different (HotSpot) JVM thread types do?

Question

I see there are six thread types implemented into the HotSpot JVM: VMThread, CGCThread, PGCThread, JavaThread, CompilerThread and WatcherThread. However I don't know which thread type is doing what exactly. Here is what I understood so far:

VMThread: run VM tasks like the garbage collector.
CGCThread: Concurrent garbage collector.
PGCThread: Parallel garbage collector (differences with CGC?).
JavaThread: Program's threads, I guess.
CompilerThread: A thread for the compiler?
WatcherThread: ?

Additional question: what about other JVMs?

Parallel vs concurrent: http://stackoverflow.com/questions/1897993/difference-between-concurrent-programming-and-parallel-programming — m0skit0, Mar 06 '13 at 15:37
PCG vs CGC in Hotspot: http://www.oracle.com/technetwork/java/javase/gc-tuning-6-140523.html#available_collectors — assylias, Mar 06 '13 at 15:54
Some info about the WatcherThread here: https://blogs.oracle.com/ksrini/entry/we_take_java_performance_very — assylias, Mar 07 '13 at 13:49

score 5 · Accepted Answer · answered Aug 14 '14 at 16:27

You can find a brief explanation on the OpenJDK website:

The main kinds of VM threads are as follows:

VM thread: This singleton instance of VMThread is responsible for executing VM operations, which are discussed below

Periodic task thread: This singleton instance of WatcherThread simulates timer interrupts for executing periodic operations within the VM

GC threads: These threads, of different types, support parallel and concurrent garbage collection

Compiler threads: These threads perform runtime compilation of bytecode to native code

Signal dispatcher thread: This thread waits for process directed signals and dispatches them to a Java level signal handling method

You might want to read the whole Thread Management paragraph since it continues further explanations, e.g. what the VM thread is responsible for.

score 3 · Answer 2 · answered Mar 07 '13 at 15:47

Ok, thanks to comments, we have the beginning of an answer:

1) Since the garbage collector has a stop-the-world mecanism, there exist besides tunings two ways to reduce these pauses:

With parallel GCs running via PGCThreads such that, if n cores are available, then n threads can be run during pauses to shorten them.
With a concurrent GC, running via a CGCThread and finishing the job of the regular GC off pauses, concurrently with the main program thread.

2) The CompilerThread runs the Just-In-Time compiler.

3) The WatcherThread simulates timer interrupts every 50ms to run periodic operations in the VM.

score 1 · Answer 3 · answered Oct 24 '17 at 17:04

And I would add that there are 7 threads type in JVM! Don't miss os_thread

path: Defined in: /hotspot/src/share/vm/runtime/os.hpp

  enum ThreadType {
    vm_thread,
    cgc_thread,        // Concurrent GC thread
    pgc_thread,        // Parallel GC thread
    java_thread,       // Java, CodeCacheSweeper, JVMTIAgent and Service threads.
    compiler_thread,
    watcher_thread,
    os_thread
  };

What do the different (HotSpot) JVM thread types do?

3 Answers3

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