Java Heap Analysis in JRockit

This article will provide you an overview and tutorial on how you can perform an initial analysis and problem isolation of a JRockit Java Heap problem using the jrcmd tool. A deeper analysis and tutorial using JRockit Mission Control and Heap Dump analysis (JRockit version R28+ only) will be covered in future articles.

For a quick overview of the JRockit Java Heap Space, please consult the article below:
JRockit Java Heap Space

JRCMD tool overview

jrcmd is a free tool that is available out-of-the-box in the JRockit binaries. It allows you generate and collect crucial data from your runtime JRockit VM such as:

-        Java process memory space breakdown (Java Heap vs Native memory spaces)
-        Java Heap diagnostic (histogram)
-        Java loaded classes
-        On-demand JRockit Heap Dump generation (version R28+ only)
-        Thread Dump generation
-        More…

For this article, we created a simple Java program leaking internally. We will use this program to demonstrate how you can leverage jrcmd to perform your initial analysis.

Sample Java memory leak program

This simple Java program is simply adding String data to a static HashMap and slowly leaking to the point of the JVM running out of Java Heap memory. This program will allow you to visualize a slowly growing Java Heap leak via JRockit jrcmd. Please note that a Java Heap size of 128 MB (-Xms128m –Xmx128m) was used for this example.

/**
 * JavaHeapLeakSimulator
 * @author Pierre-Hugues Charbonneau
 * http://javaeesupportpatterns.blogspot.com
 */
public class JavaHeapLeakSimulator {

        private final static int NB_ITERATIONS = 500000000;

        // ~1 KB data footprint
        private final static String LEAKING_DATA_PREFIX = "datadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadata";

        // Map used to stored our leaking String instances
        private static Map<String, String> leakingMap;

        static {
               leakingMap = new HashMap<String, String>();
        }

        /**
         * @param args
         */
        public static void main(String[] args) {

               System.out.println("Java Heap Leak Simulator 1.0");
               System.out.println("Author: Pierre-Hugues Charbonneau");
               System.out.println("http://javaeesupportpatterns.blogspot.com/");

               try {

                       for (int i = 0; i < NB_ITERATIONS; i++) {

                              String data = LEAKING_DATA_PREFIX + i;
                             
                              // Add data to our leaking Map data structure...
                              leakingMap.put(data, data);
                             
                              // Slowdown the Java program so we can monitor the leak before the OutOfMemoryError condition
                              Thread.sleep(1);
                       }

               } catch (Throwable any) {
                       if (any instanceof java.lang.OutOfMemoryError) {
                              System.out.println("OutOfMemoryError triggered! "
                                             + any.getMessage() + " [" + any + "]");

                       } else {
                              System.out.println("Unexpected Exception! " + any.getMessage()
                                             + " [" + any + "]");
                       }
               }

               System.out.println("JavaHeapLeakSimulator done!");
        }
}

JRCMD - initial execution

JRCMD can be executed from the local server hosting the JVM that you want to monitor or remotely via JRockit Mission Control. The executable is located within the JRockit JDK that you are using:

<JRockit_JDK_HOME>/bin/jrcmd

The default jrcmd execution will return the list of active JRockit Java process Id that you can monitor:

C:\Apps\Weblogic1035\jrockit_160_24_D1.1.2-4\bin>jrcmd
5360 org.ph.javaee.tool.oom.JavaHeapLeakSimulator
5952
6852 jrockit.tools.jrcmd.JrCmd

JRCMD - Java Heap monitoring

The next step is to start monitoring the Java Heap memory usage and histogram. A Java Heap histogram is a snapshot of the biggest pools of Java Class instances. This allow you to pinpoint the leaking data type. Ple

You can either chose between print_object_summary (quick summary) or heap_diagnostics (complete breakdown).

C:\Apps\Weblogic1035\jrockit_160_24_D1.1.2-4\bin>jrcmd 5360 heap_diagnostics

Invoked from diagnosticcommand
======== BEGIN OF HEAPDIAGNOSTIC =========================

Total memory in system: 8465022976 bytes
Available physical memory in system: 5279170560 bytes
-Xmx (maximal heap size) is 134217728 bytes
Heapsize: 134217728 bytes
Free heap-memory: 123592704 bytes


--------- Detailed Heap Statistics: ---------
90.9% 3948k     5468  +3948k [C
 3.0% 128k     5490   +128k java/lang/String
 2.1% 92k     3941    +92k java/util/HashMap$Entry
 1.2% 50k      461    +50k java/lang/Class
 0.8% 35k       21    +35k [Ljava/util/HashMap$Entry;
 0.6% 24k        7    +24k [B
 0.3% 15k      305    +15k [Ljava/lang/Object;
 0.3% 14k      260    +14k java/net/URL
 0.2% 6k      213     +6k java/util/LinkedHashMap$Entry
 0.1% 4k      211     +4k java/io/ExpiringCache$Entry
 0.1% 2k        4     +2k [Ljrockit/vm/FCECache$FCE;
 0.0% 1k       50     +1k [Ljava/lang/String;
 0.0% 1k       10     +1k java/lang/Thread
 0.0% 1k       61     +1k java/util/Hashtable$Entry
 0.0% 1k        7     +1k [I
 0.0% 0k       19     +0k java/util/HashMap
 0.0% 0k       19     +0k java/lang/ref/WeakReference
 0.0% 0k        7     +0k [Ljava/util/Hashtable$Entry;
 0.0% 0k       19     +0k java/util/Locale
 0.0% 0k       11     +0k java/lang/ref/SoftReference
 0.0% 0k        1     +0k [S
…………………………………………………

- The first column correponds to the Class object type contribution to the Java Heap footprint in %
- The second column correponds to the Class object type memory footprint in K
- The third column correponds to the # of Class instances of a particular type
- The fourth column correponds to the delta - / + memory footprint of a particular type

As you can see from the above snapshot, the biggest data type is [C (char in our case) & java.lang.String. In order to see which data types are leaking, you will need to generate several snapshots. The frequency will depend of the leaking rate. In our example, find below another snapshot taken after 5 minutes:

# After 5 minutes
--------- Detailed Heap Statistics: ---------
93.9% 26169k    28746 +12032k [C
 2.4% 674k    28768   +295k java/lang/String
 2.3% 637k    27219   +295k java/util/HashMap$Entry
 0.9% 259k       21   +128k [Ljava/util/HashMap$Entry;
 0.2% 50k      462     +0k java/lang/Class
 0.1% 24k        7     +0k [B

# After 5 more minutes
--------- Detailed Heap Statistics: ---------
94.5% 46978k    50534 +20809k [C
 2.4% 1184k    50556   +510k java/lang/String
 2.3% 1148k    49007   +510k java/util/HashMap$Entry
 0.5% 259k       21     +0k [Ljava/util/HashMap$Entry;
 0.1% 50k      462     +0k java/lang/Class

The third & fourth column are showing a constant increase. As you can see, the leaking data in our case are [C, java.lang.String and java.util.HashMap$Entry which all increased from ~4 MB to 28 MB, 50 MB and growing…

It is easy to pinpoint the leaking data type(s) with this approach but what about the source (root cause) of the leaking data type(s)? This is where jrcmd is no longer useful. Deeper memory leak analysis will require you to use either JRockit Mission Control or Heap Dump analysis (JRockit R28+ only).

A final point, before you can conclude on a true Java Heap leak, please ensure that jrcmd snapshots are taken after at least one Full GC in between the captures (what you are interested in is OldGen leak e.g. Java objects surviving major GC collections).

JRCMD Thread Dump generation

Thread Dump analysis is crucial for stuck Thread related problems but can also be useful to troubleshoot certain types of Java Heap problem. For example, it can pinpoint the source of a sudden Java Heap increase by exposing the culprit Thread(s) allocating a large amount of memory on the Java Heap in a short amount of time. Thread Dump can be generated using jrcmd print_threads option.

** Thread Dump captured from our sample Java program after removing the Thread.sleep() and increasing the Java Heap capacity **

C:\Apps\Weblogic1035\jrockit_160_24_D1.1.2-4\bin>jrcmd 5808 print_threads
5808:

===== FULL THREAD DUMP ===============
Mon Apr 09 09:08:08 2012
Oracle JRockit(R) R28.1.3-11-141760-1.6.0_24-20110301-1429-windows-ia32

"Main Thread" id=1 idx=0x4 tid=6076 prio=5 alive, native_blocked
    at jrockit/vm/Allocator.getNewTla(II)V(Native Method)
    at jrockit/vm/Allocator.allocObjectOrArray(Allocator.java:354)[optimized]
    at java/util/Arrays.copyOfRange(Arrays.java:3209)[inlined]
    at java/lang/String.<init>(String.java:215)[inlined]
    at java/lang/StringBuilder.toString(StringBuilder.java:430)[optimized]
    at org/ph/javaee/tool/oom/JavaHeapLeakSimulator.main(JavaHeapLeakSimulator.java:38)
    at jrockit/vm/RNI.c2java(IIIII)V(Native Method)
    -- end of trace
……………………………………….

We can see that our sample Java program is creating a lot of java.lang.String objects from the “Main Thread” executing our JavaHeapLeakSimulator program.

Conclusion

I hope this article has helped you understand you can leverage the JRockit jrcmd tool for quick Java Heap analysis. I’m looking forward for your comments and questions.

Future articles will include a deeper JRockit Java Heap and Heap Dump analysis tutorial.