What does java “VM thread” do?

Question

I use jstack to output the thread info. And there is a thread: \"VM Thread\" prio=10 tid=0x0878b400 nid=0x760a runnable

What is this thread used to do? It takes 50%

Answer 1

The VM thread is defined here as:

This thread waits for operations to appear that require the JVM to reach a safe-point. The reason these operations have to happen on a separate thread is because they all require the JVM to be at a safe point where modifications to the heap can not occur. The type of operations performed by this thread are "stop-the-world" garbage collections, thread stack dumps, thread suspension and biased locking revocation.

There's also some information provided in a de-duplicated SO answer here.

Answer 2

How do you know that this thread is taking 50% usage? Being runnable doesn't mean its consuming cpu.

AFAIK this is used for internal java operation, possibly involved in GC.

Which version of java are you using? I would check you have an up to date version of Java and see if this still happens.

I would suggest you memory profile your application to see how much objects you are creating and see if you can reduce your object count to reduce any GC work load.

Answer 3

I think it is the Garbage Collection thread. It does Garbage Collection.