Thread dump

Thread dump

If an application seems stuck, or is running out of resources, a thread dump will reveal the state of the server.

Java's thread dumps are a vital tool for server debugging. Because servlets are intrinsically multithreaded, it is very possible to create deadlocks without realizing it, or to have runaway threads that consume resources and cause OutOfMemory exceptions. That's especially true when you start adding third-party software like databases, EJB, and Corba ORBs.

Thread dump using /resin-admin

The Resin administration pages provide a simple way of providing a thread dump by clicking on the "thread" tab. The thread page will show the current state of all the threads in the server. The /resin-admin thread dump is usually the easiest method for getting a thread dump.

Thread dump using JDK 5 tools

jps and jstack are useful tools included in JDK 5, providing a quick command line method for obtaining stack traces of all current threads.

(prompt) jps
12903 Jps
20087 Resin
 
(prompt) jstack 20087
Attaching to process ID 20087, please wait...
Debugger attached successfully.
Client compiler detected.
JVM version is 1.5.0-beta2-b51
Thread 12691: (state = BLOCKED)
 - java.lang.Object.wait(long) (Compiled frame; information may be imprecise)
 - com.caucho.util.ThreadPool.runTasks() @bci=111, line=474 (Compiled frame)
 - com.caucho.util.ThreadPool.run() @bci=85, line=423 (Interpreted frame)
 - java.lang.Thread.run() @bci=11, line=595 (Interpreted frame)

Thread 12689: (state = BLOCKED)
 - java.lang.Object.wait(long) (Compiled frame; information may be imprecise)
 - com.caucho.util.ThreadPool.runTasks() @bci=111, line=474 (Compiled frame)
 - com.caucho.util.ThreadPool.run() @bci=85, line=423 (Interpreted frame)
 - java.lang.Thread.run() @bci=11, line=595 (Interpreted frame)

...

Thread dump by sending a signal

On Windows, ctrl-break may produce a thread dump.

On Unix, kill -QUIT will produce a thread dump. If Resin is running in a console window, Ctrl-\ sends a QUIT signal and produces a thread dump.

Thread dump if signalling doesn't work

You get a thread dump without signalling the process by starting the JVM with some extra arguments to allow a debugger to attach. You can then attach with the debugger at any time to get a thread dump. This technique works on all operating systems.

Here are some step by step instructions:

• Start Resin with some extra arguments that allow a debugger to attach:

(prompt) -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=5432

• Wait until you believe the application is in a state of deadlock or there are runaway threads.

• In another terminal (window), use jdb to connect to the running instance of Resin:

$JAVA_HOME/bin/jdb -connect com.sun.jdi.SocketAttach:hostname=localhost,port=5432

jdb will show something like:

Set uncaught java.lang.Throwable
Set deferred uncaught java.lang.Throwable
Initializing jdb ...
>

• Use the "suspend" command and then the "where all" command to get a thread dump:

> suspend
 
All threads suspended.

> where all
 
tcpConnection-6862-3:
 
 [1] java.lang.Object.wait (native method)
 [2] com.caucho.server.TcpServer.accept (TcpServer.java:650)
 [3] com.caucho.server.TcpConnection.accept (TcpConnection.java:208)
 [4] com.caucho.server.TcpConnection.run (TcpConnection.java:131)
 [5] java.lang.Thread.run (Thread.java:536) tcpConnection-543-2:
 [1] java.lang.Object.wait (native method)
 [2] com.caucho.server.TcpServer.accept (TcpServer.java:650)
 [3] com.caucho.server.TcpConnection.accept (TcpConnection.java:208)
 [4] com.caucho.server.TcpConnection.run (TcpConnection.java:131)
 [5] java.lang.Thread.run (Thread.java:536)
 
...

• Use the "resume" command to resume the process

> resume

Unix users (and Cygwin users on Windows) will recognize the opportunity to make a script:

#!/bin/sh
echo -e "suspend\nwhere all\nresume\nquit" | $JAVA_HOME/bin/jdb -connect \
  com.sun.jdi.SocketAttach:hostname=localhost,port=5432

There appears to be no overhead or performance penalties involved in having the JVM start with the options that allow a debugger to attach.

Thread dumps on Windows and pre JDK 5

Before JDK 5, there was limited ability to produce thread dumps. jvmstat (http://java.sun.com/performance/jvmstat/) from Sun would allow some limited capabilities with JDK 1.4.1 and JDK 1.4.2. This tool (version 3.0) was bundled in JDK 5.

Windows was that much worse off because if you installed Resin as a service, you would not get a console that could receive a CTRL-BREAK signal.

There's a relatively cheap commercial tool available at http://www.adaptj.com/.

Understanding the thread dump

In any case, you'll eventually get a trace that looks something like the following (each JDK is slightly different):

Full thread dump:
 
"tcpConnection-8080-2" daemon waiting on monitor [0xbddff000..0xbddff8c4]
        at java.lang.Object.wait(Native Method)
        at com.caucho.server.TcpServer.accept(TcpServer.java:525)
        at com.caucho.server.TcpConnection.accept(TcpConnection.java:190)
        at com.caucho.server.TcpConnection.run(TcpConnection.java:136)
        at java.lang.Thread.run(Thread.java:484)

"tcpConnection-8080-1" daemon waiting on monitor [0xbdfff000..0xbdfff8c4]
        at java.lang.Object.wait(Native Method)
        at com.caucho.server.TcpServer.accept(TcpServer.java:525)
        at com.caucho.server.TcpConnection.accept(TcpConnection.java:190)
        at com.caucho.server.TcpConnection.run(TcpConnection.java:136)
        at java.lang.Thread.run(Thread.java:484)
 
"tcpConnection-8080-0" daemon waiting on monitor [0xbe1ff000..0xbe1ff8c4]
        at java.lang.Object.wait(Native Method)
        at com.caucho.server.TcpServer.accept(TcpServer.java:525)
        at com.caucho.server.TcpConnection.accept(TcpConnection.java:190)
        at com.caucho.server.TcpConnection.run(TcpConnection.java:136)
        at java.lang.Thread.run(Thread.java:484)
 
"tcp-accept-8080" runnable [0xbe7ff000..0xbe7ff8c4]
        at java.net.PlainSocketImpl.socketAccept(Native Method)
        at java.net.PlainSocketImpl.accept(PlainSocketImpl.java:413)
        at java.net.ServerSocket.implAccept(ServerSocket.java:243)
        at java.net.ServerSocket.accept(ServerSocket.java:222)
        at com.caucho.server.TcpServer.run(TcpServer.java:415)
        at java.lang.Thread.run(Thread.java:484)
 
"resin-cron" daemon waiting on monitor [0xbe9ff000..0xbe9ff8c4]
        at java.lang.Thread.sleep(Native Method)
        at com.caucho.util.Cron$CronThread.run(Cron.java:195)
 
"resin-alarm" daemon waiting on monitor [0xbebff000..0xbebff8c4]
        at java.lang.Thread.sleep(Native Method)
        at com.caucho.util.Alarm$AlarmThread.run(Alarm.java:268)
 
"Signal Dispatcher" runnable [0..0]
 
"Finalizer" daemon waiting on monitor [0xbf3ff000..0xbf3ff8c4]
        at java.lang.Object.wait(Native Method)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:108)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:123)
        at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:162)

"Reference Handler" daemon waiting on monitor [0xbf5ff000..0xbf5ff8c4]
        at java.lang.Object.wait(Native Method)
        at java.lang.Object.wait(Object.java:420)
        at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:110)
 
"main" waiting on monitor [0xbfffd000..0xbfffd210]
        at java.lang.Thread.sleep(Native Method)
        at com.caucho.server.http.ResinServer.waitForExit(ResinServer.java:674)
        at com.caucho.server.http.ResinServer.main(ResinServer.java:821)
        at com.caucho.server.http.HttpServer.main(HttpServer.java:95)

Each thread is named. Here are some of the common names:

Thread Name	Description
tcp-accept-8080	Resin thread listening for new connections on port 8080.
tcpConnection-8080-3	Resin servlet thread handling a connection from port 8080.
tcp-cron	Resin's run-at thread
tcp-alarm	Resin's alarm thread

There should be one tcp-accept-xxx thread for each http and srun that Resin's listening for. The tcp-accept-xxx thread should almost always be in socketAccept.

There should be several tcpConnection-xxx-n threads. Each of these is the servlet thread. On a busy server, these can appear anywhere in your code. If several appear in one location, you've likely got some sort of deadlock or at least a slow lock. Idle threads are either in tcpAccept or httpRequest or runnerRequest (for keepalive threads.)

For deadlocks, look at the "waiting on monitor" threads and any case where lots of threads are stuck at the same location.