java - 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?

Answer 1

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.