what may cause waiting state thread increase all the time in Java 8

Question

Today I found my Java 8 apps have many thread is in WAITING state:

[arthas@1]$ thread --state RUNNABLE
Threads Total: 3427, NEW: 0, RUNNABLE: 17, BLOCKED: 0, WAITING: 3114, TIMED_WAITING: 296, TERMINATED: 0                                                                              
ID             NAME                                         GROUP                          PRIORITY       STATE          %CPU           TIME           INTERRUPTED    DAEMON         
124            pool-11-thread-25                            main                           5              RUNNABLE       75             0:0            false          false          
53             as-command-execute-daemon                    system                         10             RUNNABLE       23             0:0            false          true           
133            Thread-20                                    main                           5              RUNNABLE       1              0:2            false          true           
28             Apollo-RemoteConfigLongPollService-1         Apollo                         5              RUNNABLE       0              0:0            false          true           
32             Attach Listener                              system                         9              RUNNABLE       0              0:0            false          true           
99             DestroyJavaVM                                main                           5              RUNNABLE       0              0:39           false          false          
4              Signal Dispatcher                            system                         9              RUNNABLE       0              0:0            false          true           
19             grpc-default-worker-ELG-1-1                  main                           5              RUNNABLE       0              0:0            false          true           
21             grpc-default-worker-ELG-1-2                  main                           5              RUNNABLE       0              0:0            false          true           
97             http-nio-11003-Acceptor                      main                           5              RUNNABLE       0              0:0            false          true           
85             http-nio-11003-BlockPoller                   main                           5              RUNNABLE       0              0:0            false          true           
96             http-nio-11003-ClientPoller                  main                           5              RUNNABLE       0              0:0            false          true           
54             lettuce-nioEventLoop-4-1                     main                           5              RUNNABLE       0              0:0            false          true           
70             lettuce-nioEventLoop-4-2                     main                           5              RUNNABLE       0              0:0            false          true           
36             nioEventLoopGroup-3-1                        system                         10             RUNNABLE       0              0:0            false          false          
42             nioEventLoopGroup-3-2                        system                         10             RUNNABLE       0              0:0            false          false          
37             nioEventLoopGroup-4-1                        system                         10             RUNNABLE       0              0:0            false          false          
Affect(row-cnt:0) cost in 120 ms.

that have 3000+ thread is in WAITING state, now I pick a random WAITING thread pool thread, shows like this::

[arthas@1]$ thread 4410
"pool-96-thread-10" Id=4410 WAITING on java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject@3e27c029
    at sun.misc.Unsafe.park(Native Method)
    -  waiting on java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject@3e27c029
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039)
    at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:1088)
    at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:809)
    at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1074)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1134)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at java.lang.Thread.run(Thread.java:748)

Affect(row-cnt:0) cost in 16 ms.

but the problem is I don't know where started the thread and what make the waiting thread increase. Is there any way to find out where to start the thread or why the WAITING thread increase? I am now using Java ThreadExecutor. Right now the waiting thread is up to 6000+. I add a custom config:

@Configuration
public class ScheduleConfig implements SchedulingConfigurer {

    @Override
    public void configureTasks(ScheduledTaskRegistrar taskRegistrar) {
        taskRegistrar.setScheduler(Executors.newScheduledThreadPool(30));
    }

}

user_768644 · Answer

The stack trace you have shown is 'situation normal': That is a threadpool executor thread that is ready to do work, but the queue of work is empty. In this case, 'waiting' means: I'm waiting for a job to do, not: "I have stuff to do, but cannot do because I am waiting for stuff to be finished first".

Now, 3000 threads is itself somewhat of a concern; each thread has its own stack space. How large that is depends on your -Xss parameter, but they tend to go from 64k to 1MB. If it's 1MB, that's... 3GB of stack space, that's... suboptimal. This number (how many threads you have waiting around for a job to accept) should also not be growing much after a VM has heated up.

If all/most of those WAITING threads have a similar trace, then there are really only two options:

You made an executor and keep asking it, over time, to add more and more threads. I doubt this, but it's possible.
You keep making executors. Don't do this.

The idea behind an executor is that you make only one, or at least very very few of these.

If you MUST create them as part of your running app (vs. the normal thing, of creating jobs and feeding them to the singleton executor), then be aware they are effectively resources: if you don't 'close' them, your process will require more and more resources, until eventually the VM will crash when it runs out.

To close them, you invoke shutdown() which is asking nicely, and shutdownNow() which is more aggressive and will any not-yet-picked-up jobs permanently undone.

So, to recap:

You are making new executors during normal processing in your app. Search for new ScheduledThreadPoolExecutor in your codebase and inspect the situation. Add some logging if you must to see this in action.
Then, most likely, you want to fix this and stop making new executors in the first place - just make one, once, and feed jobs into this one executor.
If truly it makes sense to make them, use some guardian construct to ensure that you also clean them up when you're done using them. You can search for how to do so safely; it's a bit complicated, as you need to decide what to do with any jobs in the queue that are not yet done. If that's not an issue, it's easy: .shutdown() will get the job done.

what may cause waiting state thread increase all the time in Java 8

1 Answers