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README.md

HeapAudit

HeapAudit is a java agent which audits heap allocations for JVM processes.

HeapAudit runs in three modes:

  • STATIC: This requires a simple integration hook to be implemented by the java process of interest. The callback hook defines how the allocations are recorded and the callback code is only executed when the java agent is loaded.
  • DYNAMIC: This injects HeapQuantile recorders to all matching methods and dumps heap allocations to stdout when removed. Be aware, a lot of recorders, including nested ones, may be injected if the supplied matching pattern is not restrictive enough.
  • HYBRID: This launches like the static use case but dynamically determines where to inject recorders.

Best way to understand what HeapAudit can do for you is to check out some of the sample tutorials and play around with the examples!

Building and testing the HeapAudit java agent

Build project with Maven:

$ mvn clean package

The built jar will be in 'target/'.

NOTE: The built jar package references the file name with version suffix in its manifest boot class path. If the built jar package were to be renamed, the path may be required to be specified on the java command line.

Because the included tests must be executed with the java agent attached, they must run in the verify phase instead of in the test phase as unit tests:

$ mvn verify

Implementing the HeapAudit hook

Currently, two recorders are provided with HeapAudit:

  • HeapActivity prints each heap allocation to stdout as they occur
  • HeapQuantile accumulates allocations and dumps out summary at the end

Both of the above inherit from the base class HeapRecorder. Additional recording behavior can be extended by implementing the record method in HeapRecorder.

class MyRecorder extends HeapRecorder {
    @Override public void record(String name,
                                 int count,
                                 long size) {
        System.out.println("Allocated " + name + "[" + count + "] " + size + " bytes");
    }
}

Registering the HeapAudit recorder

Recording starts when it is registered and stops when it is unregistered. Each recorder can be registered globally across all threads or local to the current. The following example shows how to register the HeapActivity recorder across all threads. The output will display as allocations occur.

HeapActivity r = new HeapActivity();
HeapRecorder.register(r, HeapRecorder.Threading.Global);
MyObject o = new MyObject();
HeapRecorder.unregister(r, HeapRecorder.Threading.Global);

The HeapQuantile recorder requires an extra step at the end to tally up the results. The following example shows how to register the HeapQuantile recorder only on the current thread and displays the summary at the end.

HeapQuantile r = new HeapQuantile();
HeapRecorder.register(r, HeapRecorder.Threading.Local);
MyObject o = new MyObject();
HeapRecorder.unregister(r, HeapRecorder.Threading.Local);
for (HeapQuantile.Stats s: r.tally(HeapRecorder.Threading.Local, true)) System.out.println(s);

Launching the HeapAudit java agent

Launch HeapAudit statically along with the process of interest (requires MyTest to implement the integration hook to register heap recorders).

$ java -javaagent:heapaudit.jar MyTest

Launch HeapAudit dynamically by injecting to the process of interest (does not require MyTest to have any prior intrumentations). The recorder data is dumped to the console upon exiting.

$ java -jar heapaudit.jar 999 -Icom/foursquare/test/MyTest@test.+
Press <enter> to exit HeapAudit...

The JDK's tools.jar library is required to launch HeapAudit dynamically. If launching within JRE, specify the -Xbootclasspath command line arg to point to the tools.jar file.

$ java -Xbootclasspath/a:/usr/local/lib/tools.jar -jar heapaudit.jar 999 -Icom/foursquare/test/MyTest@test.+
Press <enter> to exit HeapAudit...

Alternatively, an interactive menu will show if options are left off of the command line.

$ java -jar heapaudit.jar
999     MyTest
PID: 999
OPTIONS: -Icom/foursquare/test/MyTest@test.+
Press <enter> to exit HeapAudit...

Another hybrid way is to statically instrument the process of interest at launch time with dynamically injected recorders. This will provide an inclusive collection from the moment the targeted process starts to the moment it exits while not requiring prior code changes.

$ java -javaagent:heapaudit.jar=-Icom/foursquare/test/MyTest@test.+ MyTest

Additional options can be passed to HeapAudit to customize which classes and/or methods are not to be instrumented for recording allocations. For additional information on how to specify the options, see HeapAudit.java.

$ java -javaagent:heapaudit.jar="-Acom/foursquare/test/.+" MyTest

Understanding HeapQuantile output

The HeapQuantile recorder is intended to collect a concise summary of all the allocations while still providing a meaningful breakdown of types, sizes and occurrences.

For instance, given the following code:

public void Test() {
    int array1D = new int[9];
    int array3D = new int[8][29][5];
    HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
    for (int i = 0; i < 25; ++i) map.put(i, i);
}

the HeapQuantile recorder collects the following output:

HEAP: MyTest@test()V
      - int[9] (56 bytes) x1
      - int[1160] (9464 bytes) x1
      - java.util.HashMap (48 bytes) x1
      - java.util.HashMap$Entry (32 bytes) x25
      - java.util.HashMap$Entry[16] (80 bytes) x1
      - java.util.HashMap$Entry[32] (144 bytes) x1

The output starts with the HEAP prefix followed by the method signature in which the recorder was injected. The subsequent lines each show a bucket of captured allocations with the format: <TYPE>[<AVG_ELEMENTS>] (<AVG_BYTES> bytes) x<OCCURRENCES>

You will notice that multiple lines of the same type may appear. This is because each array type is further broken into separate buckets by element count over power of 2s.

Troubleshooting

Some libraries may not be instrumentable. This often manifests in some useless error while instrumenting the target code, i.e. some generic error in the form of a java.lang.NoClassDefFoundError exception. While some of the errors may be attributed to bugs in HeapAudit, this should not block you from auditing the rest of your code.

To identify the offending library, run with "-D.+" and search for the last line of the debug output that starts with the word CLASS. You can subsequently avoid instrumenting the identified class via the -A flag.

For instance, we recently switched to use jrockit JVM and it would not run with HeapAudit attached. Upon troubleshooting with "-D.+", we noticed that many of the classes under the jrockit/ namespace causes exception to be thrown during startup. We subsequently ran HeapAudit with "-Ajrockit/.+" and everything returned back to normal. See HeapSettings.java for list of namespaces avoided by default.

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