Analyze and respond to a Tomcat heap dump

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Get the PID

If a heap dump is unexpectedly or automatically created, ensure that the JVM associated with the heap dump is still running properly. The heap dump files can be used to determine the PID associated with the heap dump, and then the PID can be used to determine the JVM associated with the heap dump. The heap dump files are in this format:

java_pid<pid>.hprof

 

Is the PID in use

Let's say the PID is 12345. The ps command can be used to determine if the PID is still associated wth the JVM. In this example, the output of the ps command only displays the grep command, which means that the PID is no longer in use, which means that the JVM that was associated with the PID is no longer running, or was automatically restarted as part of the heap dump. If there is a significant amount of output, the JVM is still running. However, the JVM is probably in a bad way. For example, the JVM may be out of memory.

~]# ps -ef | grep 12345
root  12345  1  0  21:54  pts/0  00:00:00  grep 12345

 

Kill the PID if in use

When the JVM is in a bad way, you first will want to kill the PID, and the start the JVM.

~]# kill -9 12345

 

Determine the JVM associated with the PID

You can search the logs with the PID to determine what JVM was associated with the PID. This command will usually produce quite a bit of output, as this command searches every file at and below the specified directory for the string (12345 in this example). This may help you find the JVM that had the PID associated with the heap dump. What you are looking for is line "Dumping heap to /path/to/java_pidxxxxx.hprof" in the JVMs catalina.out log.

~]# grep -R /path/to/logs/directory -ie '12345'
Dumping heap to /path/to/java_pid12345.hprof

 

Ensure the JVM is running

Once you know the JVM that had the PID with the heap dump, determine if the JVM was restarted. You can check the catalina.log file for the event "Starting service Catalina" to determine when the JVM was last restarted.

~]# cat catalina.log
Mar 16, 2018 1:25:56 AM org.apache.catalina.core.StandardService startInternal
INFO: Starting service Catalina

 

Check for out of memory

If event OutOfMemory is in the catalina.out log before the heap dump event, this means that the JVM heap dumped because the JVM ran out of memory. If this was due to perm gen memory (Java 7) or metaspace (Java 8), there is no need to analyze the heap dump. Instead, you would look into the perm gen or metaspace event, due to the perm gen event being the cause of the heap dump. If this was due to a java heap dump, you would analyze the heap dump. If this was due to a GC overhead limit exceeded, this means the JVMs garbage collector is not working well.

java.lang.OutOfMemoryError: PermGen space
java.lang.OutOfMemoryError: Metaspace
java.lang.OutOfMemoryError: Java heap space
java.lang.OutOfMemoryError: GC overhead limit exceeded

 

Check for SEVERE events in catalina.log

When a WAR is deployed to a Tomcat JVM, some threads associated with the prior instance of the application may remain active, thus retaining objects in the heap. Check the catalina.log for SEVERE errors.

SEVERE: The web application [/yourapp] created a ThreadLocal with key of type [java.lang.ThreadLocal] (value [java.lang.ThreadLocal@544d8937]) and a value of type [org.apache.cxf.BusFactory.BusHolder] (value [org.apache.cxf.BusFactory$BusHolder@2e6ba51e]) but failed to remove it when the web application was stopped. Threads are going to be renewed over time to try and avoid a probable memory leak.

When the new WAR is deployed and becomes active, the new WAR creates new threads and places objects in the heap. For example, perhaps the old and new war both have an object in the heap for a SQL connection, when only a single object is needed for the SQL connection. Thus, both the old and new WAR are using the heap, which causes a large footprint in the heap, which can lead to an out of memory situation. For some reason, the garbage collector doesn’t know to remove the objects with the old WAR from the heap. A quick fix is to restart the JVM to remove the threads associated with the prior instance of the application.

 

Transfer the heap dump

If the heap dump was not caused by an out of memory perm gen space event, then you will want to transfer the heap dump from the Tomcat server to another PC, such as your personal laptop. You never want to analyze a heap dump on the Tomcat server, as this will use significant resources, such as memory and CPU, which will impact the performance of the Tomcat server.

 

Check for a memory leak

If you have a heap visualizer tool, use the tool to determine if there is a memory leak

 

Check the Garbage Collection interval

Check the interval.

 

 

Analyze the heap dump

IBMs Heap Analyzer can be used to analyze a heap dump. IBMs Heap Analyzer can be installed as a tool in IBMs Support Assistant Team Server

 

In IBMs Heap Analzyer, select File > Open, select the java_pid<pid>.hprof file, and select Open. The analysis will identify the Java classes that contain objects that are taking up heap space. In this example, the java/lang/Object class is using 27.59% of the heap.

 

A class is a container that contains one or more objects. In this example, there is a class called Dog that contains an object called Puppy with a value of Old Yeller. The programmer that created the application that has classes taking up heap space will need to determine if a code change can be made to the application to reduce the heap space being used by the class.

Public class Dog {
  Public static void main(String []args){
    Puppy myPuppy = new Puppy( "Old Yeller" );
  }
}

 

 



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