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

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?

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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, eg what the VM thread is responsible for.

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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.

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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
  };

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