|By Jinwoo Hwang||
|May 8, 2010 06:45 AM EDT||
When we encounter a java.lang.OutOfMemoryError, we often find that Java heap dumps, along with other artifacts, are generated by the Java Virtual Machine. If you feel like jumping right into a Java heap dump when you get a java.lang.OutOfMemoryError, don't worry, it's a normal thought. You may be able to discover something serendipitously, but it's not always the best idea to analyze Java heap dumps, depending on the situation you are facing. We first need to investigate the root cause of the java.lang.OutOfMemoryError.
Only after the root cause is identified can we decide whether or not to analyze Java heap dumps. What is a java.lang.OutOfMemoryError? Why in the world does it occur? Let's find out.
What Is a java.lang.OutOfMemoryError?
A java.lang.OutOfMemoryError is a subclass of java.lang.VirtualMachineError that is thrown when the Java Virtual Machine is broken or has run out of resources that are necessary to continue the operation of the Java Virtual Machine. Obviously, memory is the exhausted resource for a java.lang.OutOfMemoryError, which is thrown when the Java Virtual Machine cannot allocate an object due to memory constraints. Unfortunately, the Java specification of java.lang.OutOfMemoryError does not elaborate further on what kind of memory it's talking about.
There are six different types of runtime data areas, or memory areas, in the Java Virtual Machine (see Figure 1).
- Program Counter Register
- Java Virtual Machine Stack
- Method Area
- Runtime Constant Pool
- Native Method Stack
The Program Counter Register, also known as the pc register, stores the address of the Java byte code instruction that is currently being executed (just like the processor register in your central processing unit of the device from which you are reading or printing this article). You will not see a java.lang.OutOfMemoryError from the pc register since a program counter is not conventionally considered as a memory.
Java Virtual Machine Stacks contain frames where data, return values, and partial execution results are stored. Java Virtual Machine Stacks can be expanded during runtime. If there's not enough memory for the expansion of an existing Java Virtual Machine stack, or for the creation of a new Java Virtual Machine stack for a new thread, the Java Virtual Machine will throw a java.lang.OutOfMemoryError.
The Heap is where instances of Java classes and arrays are allocated. A java.lang.OutOfMemoryError will be thrown when there is not enough memory available for instances of Java classes or arrays.
The Method Area stores class-related information, the runtime constant pool, for instances, the code for methods and constructors, and field/method data. If there's not enough memory in the method area, you will encounter java.lang.OutOfMemoryError.
The Runtime Constant Pool contains constants such as field references and literals. A java.lang.OutOfMemoryError will be thrown when not enough memory is available for the construction of the runtime constant pool area.
Native Memory Stacks store conventional stacks, also known as C stacks, to support native methods that are written in a non-Java language such as C/C++. Native memory stacks can be expanded during runtime. If there's not enough memory for the expansion of an existing native memory stack or for the creation of a new native memory stack for a new thread, you would see a java.lang.OutOfMemoryError.
You may have seen a java.lang.StackOverflowError, which is completely different from a java.lang.OutOfMemoryError. A java.lang.StackOverflowError is thrown when native memory stacks or Java Virtual Machine stacks need more memory than is configured. In most IBM Java Virtual Machine implementations, the -Xmso command-line option controls the stack size for operation system threads or native thread, and the -Xss command-line option controls the stack size for Java threads. In some implementations, such as Sun Microsystems HotSpot Java Virtual Machine, the Java methods share stack frames with C/C++ native code. The maximum stack size for a thread can be configured with the -Xss Java command-line option. The default sizes of these options vary by platform and implementation, but are usually between 256 Kbytes-1024 Kbytes. Please refer to the documentation of your Java virtual machine for more specific information. We will cover more about java.lang.StackOverflowError in a separate article.
Now that we understand which memory areas could cause a java.lang.OutOfMemoryError, let's take a look at actual error messages. What does a java.lang.OutOfMemoryError look like and how can I address each symptom? Have you ever seen a java.lang.OutOfMemoryError similar to the following?
java.lang.OutOfMemoryError: Requested array size exceeds VM limit
This error message indicates that there is a memory request for an array but that's too large for a predefined limit of a virtual machine. What do we do if we encounter this kind of java.lang.OutOfMemoryError? We need to check the source code to make sure that there's no huge array created dynamically or statically. Fortunately, latest virtual machines usually do not have this limit.
java.lang.OutOfMemoryError: PermGen space
You will see an OutOfMemoryError when the Permanent Generation area of the Java heap is full, like the above message.
On some Java Virtual Machines, such as Sun Microsystems' HotSpot Java Virtual Machine, a dedicated memory area called permanent generation (or permanent region) stores objects that describe classes and methods. We can visualize the usage of a permanent generation with the IBM Pattern Modeling and Analysis Tool for the Java Garbage Collector.
In Figure 2 we enabled the "Max Perm" button and the "Used Tenured" button to visualize permanent generation usage and its maximum size. We can see that the used amount of permanent generation reaches its maximum limit. That's why we're getting the java.lang.OutOfMemoryError: PermGen space message. If there's no memory leak, we can just use the -XX:MaxPermSize command-line option to increase the maximum limit of the permanent generation. For example,
will set the maximum size of the permanent generation to 128 Mbytes.
So far we've seen a Java.lang.OutOfMemoryError due to exhaustion in the Java heap or an area in the Java heap such as permanent generation. Surprisingly, a Java.lang.OutOfMemoryError can be thrown when the Java Virtual Machine cannot find any more memory in the native memory as well as in the Java heap. How can we tell whether it's caused by the Java heap or native memory?
In the following message, there's no information in the message whether java.lang.OutOfMemoryError is caused by the Java heap or native memory:
JVMDUMP013I Processed dump event "systhrow", detail "java/lang/OutOfMemoryError".
In the following case, the Java virtual machine is kind enough to tell us that there's native memory exhaustion. In the message, the Java virtual machine says "allocateMemory failed" which means a native memory allocation failed:
java.lang.OutOfMemoryError: JVMCI046: allocateMemory failed
In the following message, there's no clue as to whether it's native memory or a Java heap. Fortunately we have a line number, 20, and the source code file name, HeapExhaustionSimulator.java. This might be Java heap related.
JVMDG274: Dump Handler has Processed OutOfMemory.
Exception in thread "main" java.lang.OutOfMemoryError
In the following message, there's no clue whether it's native memory or a Java heap. But "sun.misc.Unsafe.allocateMemory(Native Method)" indicates that it might be native memory related.
Exception in thread "main" java.lang.OutOfMemoryError
at sun.misc.Unsafe.allocateMemory(Native Method)
In the following message, the Java Virtual Machine indicates that the Java heap space is related to the java.lang.OutOfMemoryError.
java.lang.OutOfMemoryError: Java heap space
Dumping heap to java_pid6280.hprof ...
Heap dump file created [50549348 bytes in 1.444 secs]
You may have seen a java.lang.OutOfMemoryError similar to the following:
java.lang.OutOfMemoryError: requested NNN bytes for MMMM. Out of swap space?
Literally you could check the operating system configuration for swap space. It seems that the Java Virtual Machine is not sure if the swap space is the root cause of the problem (?).We can check whether this Java Virtual Machine is consuming too much native memory .We also need to make sure there's enough memory for this JVM and no other processes are consuming most of memory resource. The last thing we can try is to find any known defects related to the module, MMMM.
java.lang.OutOfMemoryError: unable to create new native thread
This kind of message is seen when you have an excessive number of threads or if the native memory is exhausted and a thread is attempting to be created.
What Is a Java Heap Dump?
We've learned that a Java heap is a runtime data area where all class instances and arrays are allocated and shared among all Java Virtual Machine threads during execution of the JVM. A Java heap dump is a snapshot of a Java heap at a specific time. It's like taking a picture of a busy warehouse at a given time. If we look at the picture, we can identify what items were available at that time. Some items may be shipped to Canada a few minutes later, but you can see them in the picture because they were there at the time of the snapshot.
Because the Java specification does not mention the Java heap dump, there are different forms of Java heap dump implementations from different Java Virtual Machines. The IBM Java heap dump provides information mostly about the Java heap.
The Sun Microsystems hprof Java heap dump provides information about the Java Virtual Machine stacks, the runtime constant pool as well as the Java heap.
How Can I Generate Java Heap Dumps?
A Java heap dump is usually automatically generated by the Java Virtual Machine, but you can also force Java heap dump generation. On most IBM Java Virtual Machines, Java heap dumps are generated automatically when the Java heap becomes exhausted. On most Sun Microsystems JVMs, you need to configure the virtual machine to generate Java heap dumps. If you want to generate a Java heap dump when a java.lang.OutOfMemoryError occurs, you need to set the -XX:+HeapDumpOnOutOfMemoryError command-line option on certain releases of Sun's JVM. You could also use a HPROF profiler by using the -agentlib:hprof=heap=dump command-line option. You could also use jmap if your Sun JVM provides the utility. For example, jmap -dump 1234 will generate the Java heap dump from the process whose identifier is 1234. You could utilize JConsole by calling the HotSpotDiagnostic MBean and the dumpHeap operation if it's available from your Sun JVM.
If you want to generate Java heap dumps for Java virtual machine crashes (an unexpected termination of process) or user signals on IBM JVMs, you can set the environment variable IBM_HEAPDUMP or IBM_HEAP_DUMP to TRUE. For example, you can send the IBM Java virtual machine the signal SIGQUIT for the Linux operating systems and AIX operating systems or SIGINT(Control-Break key combination) for Windows to generate Java heap dumps. The IBM JVM provides an API, com.ibm.jvm.Dump.HeapDump(), that you can invoke from application code to generate Java heap dumps programmatically.
Please refer to the documentation of your JVM for detailed information since these options vary by platform and implementation.
Where Can I Find Java Heap Dumps?
You can find Java heap dumps in the current working directory of the Java Virtual Machine process, unless you specify a different location. You can specify the location with the environment variable IBM_HEAPDUMPDIR or _CEE_DMPTARG on IBM JVMs. If there's not enough space available for Java heap dumps or the JVM cannot acquire write-permission in the location, Java heap dumps are generated to the operating system's temporary directory on the IBM JVM. Please refer to your operating system manual for the location of the system's temporary directory and its configuration.
What Do Java Heap Dumps Look Like and How Can I read Them?
Nowadays, the majority of Java heap dump formats are generated in binary. Thus you might want to use a tool unless your brain can interpret hexadecimal codes without any headaches.
The IBM HeapAnalyzer is one of the most popular Java heap analysis tools. It can analyze all Java heap dump formats provided by Sun, HP and most of the Java heap dump formats provided by IBM. It's powered by object query engines and patent-pending heuristic analysis engines. I've been developing the IBM HeapAnalyzer from scratch since 2003, spending my vacation, weekends and weeknights on it. The IBM HeapAnalyzer was so successful that IBM decided to make the IBM HeapAnalyzer an official IBM software product and bundle it with existing products. So I donated all the source code of IBM HeapAnalyzer to IBM to run it on an Eclipse-based interface. Now the IBM HeapAnalyzer has a daughter, MDD4J, which always reminds me of my late daughter lost while I was working on the MDD4J project. The IBM HeapAnalyzer has been the top technology at IBM alphaWorks for six consecutive years since its inception as of March 2009.
Whether your Java heap dump is in binary or text/ascii format, the heap dump contains information about all the live objects that are on the Java heap such as address, object size, and referenced addresses. Let's take a look at the text/ascii format Java heap dumps since binary heap dumps have similar information but are in hexadecimal format to save disk space.
Ask someone to architect an Internet of Things (IoT) solution and you are guaranteed to see a reference to the cloud. This would lead you to believe that IoT requires the cloud to exist. However, there are many IoT use cases where the cloud is not feasible or desirable. In his session at @ThingsExpo, Dave McCarthy, Director of Products at Bsquare Corporation, will discuss the strategies that exist to extend intelligence directly to IoT devices and sensors, freeing them from the constraints of ...
Jul. 23, 2016 02:30 PM EDT Reads: 1,693
WebRTC is bringing significant change to the communications landscape that will bridge the worlds of web and telephony, making the Internet the new standard for communications. Cloud9 took the road less traveled and used WebRTC to create a downloadable enterprise-grade communications platform that is changing the communication dynamic in the financial sector. In his session at @ThingsExpo, Leo Papadopoulos, CTO of Cloud9, discussed the importance of WebRTC and how it enables companies to focus...
Jul. 23, 2016 02:15 PM EDT Reads: 607
The best-practices for building IoT applications with Go Code that attendees can use to build their own IoT applications. In his session at @ThingsExpo, Indraneel Mitra, Senior Solutions Architect & Technology Evangelist at Cognizant, provided valuable information and resources for both novice and experienced developers on how to get started with IoT and Golang in a day. He also provided information on how to use Intel Arduino Kit, Go Robotics API and AWS IoT stack to build an application tha...
Jul. 23, 2016 01:00 PM EDT Reads: 805
Is your aging software platform suffering from technical debt while the market changes and demands new solutions at a faster clip? It’s a bold move, but you might consider walking away from your core platform and starting fresh. ReadyTalk did exactly that. In his General Session at 19th Cloud Expo, Michael Chambliss, Head of Engineering at ReadyTalk, will discuss why and how ReadyTalk diverted from healthy revenue and over a decade of audio conferencing product development to start an innovati...
Jul. 23, 2016 12:00 PM EDT Reads: 808
With an estimated 50 billion devices connected to the Internet by 2020, several industries will begin to expand their capabilities for retaining end point data at the edge to better utilize the range of data types and sheer volume of M2M data generated by the Internet of Things. In his session at @ThingsExpo, Don DeLoach, CEO and President of Infobright, discussed the infrastructures businesses will need to implement to handle this explosion of data by providing specific use cases for filterin...
Jul. 23, 2016 12:00 PM EDT Reads: 1,104
Early adopters of IoT viewed it mainly as a different term for machine-to-machine connectivity or M2M. This is understandable since a prerequisite for any IoT solution is the ability to collect and aggregate device data, which is most often presented in a dashboard. The problem is that viewing data in a dashboard requires a human to interpret the results and take manual action, which doesn’t scale to the needs of IoT.
Jul. 23, 2016 11:00 AM EDT Reads: 1,811
So, you bought into the current machine learning craze and went on to collect millions/billions of records from this promising new data source. Now, what do you do with them? Too often, the abundance of data quickly turns into an abundance of problems. How do you extract that "magic essence" from your data without falling into the common pitfalls? In her session at @ThingsExpo, Natalia Ponomareva, Software Engineer at Google, provided tips on how to be successful in large scale machine learning...
Jul. 23, 2016 11:00 AM EDT Reads: 1,136
What does it look like when you have access to cloud infrastructure and platform under the same roof? Let’s talk about the different layers of Technology as a Service: who cares, what runs where, and how does it all fit together. In his session at 18th Cloud Expo, Phil Jackson, Lead Technology Evangelist at SoftLayer, an IBM company, spoke about the picture being painted by IBM Cloud and how the tools being crafted can help fill the gaps in your IT infrastructure.
Jul. 23, 2016 10:45 AM EDT Reads: 1,961
"C2M is our digital transformation and IoT platform. We've had C2M on the market for almost three years now and it has a comprehensive set of functionalities that it brings to the market," explained Mahesh Ramu, Vice President, IoT Strategy and Operations at Plasma, in this SYS-CON.tv interview at @ThingsExpo, held June 7-9, 2016, at the Javits Center in New York City, NY.
Jul. 23, 2016 10:00 AM EDT Reads: 1,037
"delaPlex is a software development company. We do team-based outsourcing development," explained Mark Rivers, COO and Co-founder of delaPlex Software, in this SYS-CON.tv interview at 18th Cloud Expo, held June 7-9, 2016, at the Javits Center in New York City, NY.
Jul. 23, 2016 10:00 AM EDT Reads: 1,909
Whether your IoT service is connecting cars, homes, appliances, wearable, cameras or other devices, one question hangs in the balance – how do you actually make money from this service? The ability to turn your IoT service into profit requires the ability to create a monetization strategy that is flexible, scalable and working for you in real-time. It must be a transparent, smoothly implemented strategy that all stakeholders – from customers to the board – will be able to understand and comprehe...
Jul. 23, 2016 09:45 AM EDT Reads: 1,730
Traditional IT, great for stable systems of record, is struggling to cope with newer, agile systems of engagement requirements coming straight from the business. In his session at 18th Cloud Expo, William Morrish, General Manager of Product Sales at Interoute, outlined ways of exploiting new architectures to enable both systems and building them to support your existing platforms, with an eye for the future. Technologies such as Docker and the hyper-convergence of computing, networking and sto...
Jul. 23, 2016 09:45 AM EDT Reads: 961
SYS-CON Events announced today that LeaseWeb USA, a cloud Infrastructure-as-a-Service (IaaS) provider, will exhibit at the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. LeaseWeb is one of the world's largest hosting brands. The company helps customers define, develop and deploy IT infrastructure tailored to their exact business needs, by combining various kinds cloud solutions.
Jul. 23, 2016 09:15 AM EDT Reads: 1,016
The cloud market growth today is largely in public clouds. While there is a lot of spend in IT departments in virtualization, these aren’t yet translating into a true “cloud” experience within the enterprise. What is stopping the growth of the “private cloud” market? In his general session at 18th Cloud Expo, Nara Rajagopalan, CEO of Accelerite, explored the challenges in deploying, managing, and getting adoption for a private cloud within an enterprise. What are the key differences between wh...
Jul. 23, 2016 09:00 AM EDT Reads: 1,925
It’s 2016: buildings are smart, connected and the IoT is fundamentally altering how control and operating systems work and speak to each other. Platforms across the enterprise are networked via inexpensive sensors to collect massive amounts of data for analytics, information management, and insights that can be used to continuously improve operations. In his session at @ThingsExpo, Brian Chemel, Co-Founder and CTO of Digital Lumens, will explore: The benefits sensor-networked systems bring to ...
Jul. 23, 2016 08:15 AM EDT Reads: 1,443
Much of IT terminology is often misused and misapplied. Modernization and transformation are two such terms. They are often used interchangeably even though they mean different things and have very different connotations. Indeed, it is somewhat safe to assume that in IT any transformative effort is likely to also have a modernizing effect, and thus, we can see these as levels of improvement efforts. However, many businesses are being led to believe if they don’t transform now they risk becoming ...
Jul. 23, 2016 08:00 AM EDT Reads: 1,058
SYS-CON Events announced today the Enterprise IoT Bootcamp, being held November 1-2, 2016, in conjunction with 19th Cloud Expo | @ThingsExpo at the Santa Clara Convention Center in Santa Clara, CA. Combined with real-world scenarios and use cases, the Enterprise IoT Bootcamp is not just based on presentations but with hands-on demos and detailed walkthroughs. We will introduce you to a variety of real world use cases prototyped using Arduino, Raspberry Pi, BeagleBone, Spark, and Intel Edison. Y...
Jul. 23, 2016 08:00 AM EDT Reads: 1,248
Large scale deployments present unique planning challenges, system commissioning hurdles between IT and OT and demand careful system hand-off orchestration. In his session at @ThingsExpo, Jeff Smith, Senior Director and a founding member of Incenergy, will discuss some of the key tactics to ensure delivery success based on his experience of the last two years deploying Industrial IoT systems across four continents.
Jul. 23, 2016 08:00 AM EDT Reads: 1,400
SYS-CON Events announced today that Venafi, the Immune System for the Internet™ and the leading provider of Next Generation Trust Protection, will exhibit at @DevOpsSummit at 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. Venafi is the Immune System for the Internet™ that protects the foundation of all cybersecurity – cryptographic keys and digital certificates – so they can’t be misused by bad guys in attacks...
Jul. 23, 2016 07:45 AM EDT Reads: 1,052
Identity is in everything and customers are looking to their providers to ensure the security of their identities, transactions and data. With the increased reliance on cloud-based services, service providers must build security and trust into their offerings, adding value to customers and improving the user experience. Making identity, security and privacy easy for customers provides a unique advantage over the competition.
Jul. 23, 2016 07:45 AM EDT Reads: 976