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Java Platform, Standard Edition Troubleshooting Guide
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2 Troubleshooting Memory Leaks

This chapter provides some suggestions for diagnosing problems involving possible memory leaks.

If your application's execution time becomes longer and longer, or if the operating system seems to be performing slower and slower, this could be an indication of a memory leak. In other words, virtual memory is being allocated but is not being returned when it is no longer needed. Eventually the application or the system runs out of memory, and the application terminates abnormally.

This chapter contains the following sections:

Understanding the OutofMemoryError Exception

One common indication of a memory leak is the java.lang.OutOfMemoryError exception. Usually, this error is thrown when there is insufficient space to allocate an object in the Java heap. In this case, The garbage collector cannot make space available to accommodate a new object, and the heap cannot be expanded further. Also, this error may be thrown when there is insufficient native memory to support the loading of a Java class. In a rare instance, a java.lang.OutOfMemoryError may be thrown when an excessive amount of time is being spent doing garbage collection and little memory is being freed.

When a java.lang.OutOfMemoryError exception is thrown, a stack trace is also printed.

The java.lang.OutOfMemoryError exception can also be thrown by native library code when a native allocation cannot be satisfied (for example, if swap space is low).

An early step to diagnose an OutOfMemoryError exception is to determine the cause of the exception. Was it thrown because the Java heap is full, or because the native heap is full? To help you find the cause, the text of the exception includes a detail message at the end, as shown in the following examples.

Exception in thread thread_name: java.lang.OutOfMemoryError: Java heap space
Cause: The detail message "Java heap space" indicates that an object could not be allocated in the Java heap. This error does not necessarily imply a memory leak. The problem can be as simple as a configuration issue, where the specified heap size (or the default size, if it is not specified) is insufficient for the application.

In other cases, and in particular for a long-lived application, the message might be an indication that the application is unintentionally holding references to objects, and this prevents the objects from being garbage collected. This is the Java language equivalent of a memory leak. Note that APIs that are called by an application could also be unintentionally holding object references.

One other potential source of this error arises with applications that make excessive use of finalizers. If a class has a finalize method, then objects of that type do not have their space reclaimed at garbage collection time. Instead, after garbage collection, the objects are queued for finalization, which occurs at a later time. In the Oracle Sun implementation, finalizers are executed by a daemon thread that services the finalization queue. If the finalizer thread cannot keep up, with the finalization queue, then the Java heap could fill up and this type of OutOfMemoryError exception would be thrown. One scenario that can cause this situation is when an application creates high-priority threads that cause the finalization queue to increase at a rate that is faster than the rate at which the finalizer thread is servicing that queue.

Exception in thread thread_name: java.lang.OutOfMemoryError: GC Overhead limit exceeded
Cause: The detail message "GC overhead limit exceeded" indicates that the garbage collector is running all the time and Java program is making very slow progress. After a garbage collection, if the Java process is spending more than approximately 98% of its time doing garbage collection and if it is recovering less than 2% of the heap and has been doing so far the last 5 (compile time constant) consecutive garbage collections, then a java.lang.OutOfMemoryError is thrown. This exception is typically thrown because the amount of live data barely fits into the Java heap having little free space for new allocations.
Action: Increase the heap size. The java.lang.OutOfMemoryError exception for GC Overhead limit exceeded can be turned off with the command line flag -XX:-UseGCOverheadLimit.
Exception in thread thread_name: java.lang.OutOfMemoryError: Requested array size exceeds VM limit
Cause: The detail message "Requested array size exceeds VM limit" indicates that the application (or APIs used by that application) attempted to allocate an array that is larger than the heap size. For example, if an application attempts to allocate an array of 512 MB but the maximum heap size is 256 MB then OutOfMemoryError will be thrown with the reason Requested array size exceeds VM limit.
Action: Usually the problem is either a configuration issue (heap size too small), or a bug that results in an application attempting to create a huge array (for example, when the number of elements in the array is computed using an algorithm that computes an incorrect size).
Exception in thread thread_name: java.lang.OutOfMemoryError: Metaspace
Cause: Java class metadata (the virtual machines internal presentation of Java class) is allocated in native memory (referred to here as metaspace). If metaspace for class metadata is exhausted, a java.lang.OutOfMemoryError exception with a detail MetaSpace is thrown. The amount of metaspace that can be used for class metadata is limited by the parameter MaxMetaSpaceSize, which is specified on the command line. when the amount of native memory needed for a class metadata exceeds MaxMetaSpaceSize, a java.lang.OutOfMemoryError exception with a detail MetaSpace is thrown.
Action: If MaxMetaSpaceSize, has been set on the command-line, increase its value. MetaSpace is allocated from the same address spaces as the Java heap. Reducing the size of the Java heap will make more space available for MetaSpace. This is only a correct trade-off if there is an excess of free space in the Java heap. See the following action for Out of swap space detailed message.
Exception in thread thread_name: java.lang.OutOfMemoryError: request size bytes for reason. Out of swap space?
Cause: The detail message "request size bytes for reason. Out of swap space?" appears to be an OutOfMemoryError exception. However, the Java HotSpot VM code reports this apparent exception when an allocation from the native heap failed and the native heap might be close to exhaustion. The message indicates the size (in bytes) of the request that failed and the reason for the memory request. Usually the reason is the name of the source module reporting the allocation failure, although sometimes it is the actual reason.
Exception in thread thread_name: java.lang.OutOfMemoryError: Compressed class space
Cause: On 64-bit platforms a pointer to class metadata can be represented by a 32-bit offset (with UseCompressedOops). This is controlled by the command line flag UseCompressedClassPointers (on by default). If the UseCompressedClassPointers is used, the amount of space available for class metadata is fixed at the amount CompressedClassSpaceSize. If the space needed for UseCompressedClassPointers exceeds CompressedClassSpaceSize, a java.lang.OutOfMemoryError with detail Compressed class space is thrown.
Action: Increase CompressedClassSpaceSize to turn off UseCompressedClassPointers. Note that there are bounds on the acceptable size of CompressedClassSpaceSize. For example -XX: CompressedClassSpaceSize=4g, exceeds acceptable bounds will result in a message such as

CompressedClassSpaceSize of 4294967296 is invalid; must be between 1048576 and 3221225472.

Note: There is more than one kind of class metadata - klass metadata and other metadata. Only klass metadata is stored in the space bounded by CompressedClassSpaceSize. The other metadata is stored in Metaspace.

Exception in thread thread_name: java.lang.OutOfMemoryError: reason stack_trace_with_native_method
Cause: If the detail part of the error message is "reason stack_trace_with_native_method" and a stack trace is printed in which the top frame is a native method, then this is an indication that a native method has encountered an allocation failure. The difference between this and the previous message is that the allocation failure was detected in a Java Native Interface (JNI) or native method rather than in the JVM code.

Crash Instead of OutOfMemoryError

Sometimes an application crashes soon after an allocation from the native heap fails. This occurs with native code that does not check for errors returned by memory allocation functions.

For example, the malloc system call returns null if there is no memory available. If the return from malloc is not checked, then the application might crash when it attempts to access an invalid memory location. Depending on the circumstances, this type of issue can be difficult to locate.

However, sometimes the information from the fatal error log or the crash dump is sufficient to diagnose this issue. The fatal error log is covered in detail in If the cause of the crash is an allocation failure, then determine the reason for the allocation failure. As with any other native heap issue, the system might be configured with insufficient swap space, another process on the system might be consuming all memory resources, or there might be a leak in the application (or in the APIs that it calls) that causes the system to run out of memory.

Diagnosing Leaks in Java Language Code

Diagnosing leaks in Java language code can be difficult. Usually, it requires very detailed knowledge of the application. In addition, the process is often iterative and lengthy. This section provides information about the tools you can use to diagnose memory leaks in Java language code.


Note:

Besides the tools mentioned in this section, a large number of third-party memory debugger tools are available. The Eclipse Memory Analyzer Tool (MAT), YourKit (www.yourkit.com) are two examples of commercial tools with memory debugging capabilities. There are many others, and no specific product is recommended.

The NetBeans Profiler

The NetBeans Profiler can locate memory leaks very quickly. Commercial memory leak debugging tools may take a long time to locate a leak in a large application. The NetBeans Profiler, however, uses the pattern of memory allocations and reclamations that such objects typically demonstrate. This process includes also the lack of memory reclamations. The profiler can check where these objects were allocated, which often is sufficient to identify the root cause of the leak.

More details can be found at http://profiler.netbeans.org.

The jhat Utility

The jhat utility is useful when debugging unintentional object retention (or memory leaks). It provides a way to browse an object dump, view all reachable objects in the heap, and understand which references are keeping an object alive.

To use jhat you must obtain one or more heap dumps of the running application, and the dumps must be in binary format. After the dump file is created, it can be used as input to jhat. See "The jhat Utility."

Creating a Heap Dump

A heap dump provides detailed information about the allocation of heap memory. There are several ways to create a heap dump:

Obtaining a Heap Histogram

You can try to quickly narrow down a memory leak by examining the heap histogram. It can be obtained in several ways:

Monitoring the Number of Objects Pending Finalization

When the OutOfMemoryError exception is thrown with the "Java heap space" detail message, the cause can be excessive use of finalizers. To diagnose this, you have several options for monitoring the number of objects that are pending finalization:

Diagnosing Leaks in Native Code

Several techniques can be used to find and isolate native code memory leaks. In general there is no single ideal solution for all platforms.

Tracking All Memory Allocation and Free Calls

A very common practice is to track all allocation and free calls of the native allocations. This can be a fairly simple process or a very sophisticated one. Many products over the years have been built up around the tracking of native heap allocations and the use of that memory.

Tools like IBM Rational Purify and the runtime checking functionality of Sun Studio dbx debugger can be used to find these leaks in normal native code situations and also find any access to native heap memory that represents assignments to uninitialized memory or accesses to freed memory. See "Using the dbx Debugger to Find Leaks."

Not all these types of tools will work with Java applications that use native code, and usually these tools are platform-specific. Because the virtual machine dynamically creates code at runtime, these tools can wrongly interpret the code and fail to run at all, or give false information. Check with your tool vendor to ensure that the version of the tool works with the version of the virtual machine you are using.

Many simple and portable native memory leak detecting examples can be found at http://sourceforge.net/. Most of these libraries and tools assume that you can recompile or edit the source of the application and place wrapper functions over the allocation functions. The more powerful of these tools allow you to run your application unchanged by interposing over these allocation functions dynamically. This is the case with the library libumem.so first introduced in Oracle Solaris 9 operating system update 3; see "Using the libumem Tool to Find Leaks."

Tracking Memory Allocation in a JNI Library

If you write a JNI library, then consider creating a localized way to ensure that your library does not leak memory, by using a simple wrapper approach.

The following procedure is an easy localized allocation tracking approach for a JNI library. First, define the following lines in all source files:

#include <stdlib.h>
#define malloc(n) debug_malloc(n, __FILE__, __LINE__)
#define free(p) debug_free(p, __FILE__, __LINE__)

Then you can use the following functions to watch for leaks.

/* Total bytes allocated */
static int total_allocated;
/* Memory alignment is important */
typedef union { double d; struct {size_t n; char *file; int line;} s; } Site;
void *
debug_malloc(size_t n, char *file, int line) 
{ 
    char *rp;
    rp = (char*)malloc(sizeof(Site)+n); 
    total_allocated += n; 
    ((Site*)rp)->s.n = n;
    ((Site*)rp)->s.file = file;
    ((Site*)rp)->s.line = line;
    return (void*)(rp + sizeof(Site));
}
void 
debug_free(void *p, char *file, int line)
{
    char *rp;
    rp = ((char*)p) - sizeof(Site);
    total_allocated -= ((Site*)rp)->s.n;
    free(rp);
}

The JNI library would then need to periodically (or at shutdown) check the value of the total_allocated variable to verify that it made sense. The preceding code could also be expanded to save in a linked list the allocations that remained and report where the leaked memory was allocated. This is a localized and portable way to track memory allocations in a single set of sources. You would need to ensure that debug_free() was called only with the pointer that came from debug_malloc(), and you would also need to create similar functions for realloc(), calloc(), strdup(), and so forth, if they were used.

A more global way to look for native heap memory leaks would involve interposition of the library calls for the entire process.

Tracking Memory Allocation with Operating System Support

Most operating systems include some form of global allocation tracking support.

Using the dbx Debugger to Find Leaks

The dbx debugger includes the Runtime Checking (RTC) functionality, which can find leaks. The dbx debugger is part of Oracle Solaris Studio and also available for Linux.

The following is a sample dbx session:

$ dbx ${java_home}/bin/java
Reading java
Reading ld.so.1
Reading libthread.so.1
Reading libdl.so.1
Reading libc.so.1
(dbx) dbxenv rtc_inherit on
(dbx) check -leaks
leaks checking - ON
(dbx) run HelloWorld
Running: java HelloWorld 
(process id 15426)
Reading rtcapihook.so
Reading rtcaudit.so
Reading libmapmalloc.so.1
Reading libgen.so.1
Reading libm.so.2
Reading rtcboot.so
Reading librtc.so
RTC: Enabling Error Checking...
RTC: Running program...
dbx: process 15426 about to exec("/net/bonsai.sfbay/export/home2/user/ws/j2se/build/solaris-i586/bin/java")
dbx: program "/net/bonsai.sfbay/export/home2/user/ws/j2se/build/solaris-i586/bin/java"
just exec'ed
dbx: to go back to the original program use "debug $oprog"
RTC: Enabling Error Checking...
RTC: Running program...
t@1 (l@1) stopped in main at 0x0805136d
0x0805136d: main       :        pushl    %ebp
(dbx) when dlopen libjvm { suppress all in libjvm.so; }
(2) when dlopen libjvm { suppress all in libjvm.so; }  
(dbx) when dlopen libjava { suppress all in libjava.so; }
(3) when dlopen libjava { suppress all in libjava.so; }  
(dbx) cont                                             
Reading libjvm.so
Reading libsocket.so.1
Reading libsched.so.1
Reading libCrun.so.1
Reading libm.so.1
Reading libnsl.so.1
Reading libmd5.so.1
Reading libmp.so.2
Reading libhpi.so
Reading libverify.so
Reading libjava.so
Reading libzip.so
Reading en_US.ISO8859-1.so.3
hello world
hello world
Checking for memory leaks...

Actual leaks report    (actual leaks:           27  total size:      46851 bytes)

  Total     Num of  Leaked     Allocation call stack
  Size      Blocks  Block
                    Address
==========  ====== =========== =======================================
     44376       4      -      calloc < zcalloc 
      1072       1  0x8151c70  _nss_XbyY_buf_alloc < get_pwbuf < _getpwuid <
                               GetJavaProperties < Java_java_lang_System_initProperties <
                               0xa740a89a< 0xa7402a14< 0xa74001fc
       814       1  0x8072518  MemAlloc < CreateExecutionEnvironment < main 
       280      10      -      operator new < Thread::Thread 
       102       1  0x8072498  _strdup < CreateExecutionEnvironment < main 
        56       1  0x81697f0  calloc < Java_java_util_zip_Inflater_init < 0xa740a89a<
                               0xa7402a6a< 0xa7402aeb< 0xa7402a14< 0xa7402a14< 0xa7402a14
        41       1  0x8072bd8  main 
        30       1  0x8072c58  SetJavaCommandLineProp < main 
        16       1  0x806f180  _setlocale < GetJavaProperties <
                               Java_java_lang_System_initProperties < 0xa740a89a< 0xa7402a14<
                               0xa74001fc< JavaCalls::call_helper < os::os_exception_wrapper 
        12       1  0x806f2e8  operator new < instanceKlass::add_dependent_nmethod <
                               nmethod::new_nmethod < ciEnv::register_method <
                               Compile::Compile #Nvariant 1 < C2Compiler::compile_method <
                               CompileBroker::invoke_compiler_on_method <
                               CompileBroker::compiler_thread_loop 
        12       1  0x806ee60  CheckJvmType < CreateExecutionEnvironment < main 
        12       1  0x806ede8  MemAlloc < CreateExecutionEnvironment < main 
        12       1  0x806edc0  main 
         8       1  0x8071cb8  _strdup < ReadKnownVMs < CreateExecutionEnvironment < main 
         8       1  0x8071cf8  _strdup < ReadKnownVMs < CreateExecutionEnvironment < main

The output shows that the dbx debugger reports memory leaks if memory is not freed at the time the process is about to exit. However, memory that is allocated at initialization time and needed for the life of the process is often never freed in native code. Therefore, in such cases the dbx debugger can report memory leaks that are not leaks in reality.

Note that the example used two suppress commands to suppress the leaks reported in the virtual machine, libjvm.so and the Java support library, libjava.so.

Using the libumem Tool to Find Leaks

First introduced in Oracle Solaris 9 operating system update 3, the libumem.so library and the modular debugger mdb can be used to debug memory leaks. Before using libumem, you must preload the libumem library and set an environment variable as follows:

$ LD_PRELOAD=libumem.so
$ export LD_PRELOAD
$ UMEM_DEBUG=default
$ export UMEM_DEBUG

Now, run the Java application, but stop it before it exits. The following example uses truss to stop the process when it calls the _exit system call:

$ truss -f -T _exit java MainClass arguments

At this point you can attach the mdb debugger, as follows:

$ mdb -p pid
>::findleaks

The ::findleaks command is the mdb command to find memory leaks. If a leak is found, this command prints the address of the allocation call, buffer address, and nearest symbol.

It is also possible to get the stack trace for the allocation that resulted in the memory leak by dumping the bufctl structure. The address of this structure can be obtained from the output of the ::findleaks command. The description of the commands to perform these functions, as well as more information about using libumem to identify memory managements bugs, is located at

http://docs.oracle.com/docs/cd/E19424-01/820-4814/geogv/index.html

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