|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.
A critical component of any IoT project is what to do with all the data being generated. This data needs to be captured, processed, structured, and stored in a way to facilitate different kinds of queries. Traditional data warehouse and analytical systems are mature technologies that can be used to handle certain kinds of queries, but they are not always well suited to many problems, particularly when there is a need for real-time insights.
Jul. 27, 2016 04:30 PM EDT Reads: 1,846
Big Data, cloud, analytics, contextual information, wearable tech, sensors, mobility, and WebRTC: together, these advances have created a perfect storm of technologies that are disrupting and transforming classic communications models and ecosystems. In his session at @ThingsExpo, Erik Perotti, Senior Manager of New Ventures on Plantronics’ Innovation team, provided an overview of this technological shift, including associated business and consumer communications impacts, and opportunities it ...
Jul. 27, 2016 04:30 PM EDT Reads: 157
You think you know what’s in your data. But do you? Most organizations are now aware of the business intelligence represented by their data. Data science stands to take this to a level you never thought of – literally. The techniques of data science, when used with the capabilities of Big Data technologies, can make connections you had not yet imagined, helping you discover new insights and ask new questions of your data. In his session at @ThingsExpo, Sarbjit Sarkaria, data science team lead ...
Jul. 27, 2016 04:15 PM EDT Reads: 1,100
Extracting business value from Internet of Things (IoT) data doesn’t happen overnight. There are several requirements that must be satisfied, including IoT device enablement, data analysis, real-time detection of complex events and automated orchestration of actions. Unfortunately, too many companies fall short in achieving their business goals by implementing incomplete solutions or not focusing on tangible use cases. In his general session at @ThingsExpo, Dave McCarthy, Director of Products...
Jul. 27, 2016 04:00 PM EDT Reads: 1,733
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. 27, 2016 04:00 PM EDT Reads: 1,048
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. 27, 2016 03:30 PM EDT Reads: 961
SYS-CON Events announced today that 910Telecom 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. Housed in the classic Denver Gas & Electric Building, 910 15th St., 910Telecom is a carrier-neutral telecom hotel located in the heart of Denver. Adjacent to CenturyLink, AT&T, and Denver Main, 910Telecom offers connectivity to all major carriers, Internet service providers, Internet backbones and ...
Jul. 27, 2016 11:00 AM EDT Reads: 706
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. 27, 2016 10:30 AM EDT Reads: 1,224
Manufacturers are embracing the Industrial Internet the same way consumers are leveraging Fitbits – to improve overall health and wellness. Both can provide consistent measurement, visibility, and suggest performance improvements customized to help reach goals. Fitbit users can view real-time data and make adjustments to increase their activity. In his session at @ThingsExpo, Mark Bernardo Professional Services Leader, Americas, at GE Digital, discussed how leveraging the Industrial Internet a...
Jul. 27, 2016 09:45 AM EDT Reads: 459
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. 27, 2016 09:30 AM EDT Reads: 2,075
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. 27, 2016 09:15 AM EDT Reads: 1,367
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. 27, 2016 09:15 AM EDT Reads: 1,165
Amazon has gradually rolled out parts of its IoT offerings in the last year, but these are just the tip of the iceberg. In addition to optimizing their back-end AWS offerings, Amazon is laying the ground work to be a major force in IoT – especially in the connected home and office. Amazon is extending its reach by building on its dominant Cloud IoT platform, its Dash Button strategy, recently announced Replenishment Services, the Echo/Alexa voice recognition control platform, the 6-7 strategic...
Jul. 27, 2016 09:00 AM EDT Reads: 468
For basic one-to-one voice or video calling solutions, WebRTC has proven to be a very powerful technology. Although WebRTC’s core functionality is to provide secure, real-time p2p media streaming, leveraging native platform features and server-side components brings up new communication capabilities for web and native mobile applications, allowing for advanced multi-user use cases such as video broadcasting, conferencing, and media recording.
Jul. 27, 2016 08:45 AM EDT Reads: 944
IoT generates lots of temporal data. But how do you unlock its value? You need to discover patterns that are repeatable in vast quantities of data, understand their meaning, and implement scalable monitoring across multiple data streams in order to monetize the discoveries and insights. Motif discovery and deep learning platforms are emerging to visualize sensor data, to search for patterns and to build application that can monitor real time streams efficiently. In his session at @ThingsExpo, ...
Jul. 27, 2016 08:45 AM EDT Reads: 1,076
Verizon Communications Inc. (NYSE, Nasdaq: VZ) and Yahoo! Inc. (Nasdaq: YHOO) have entered into a definitive agreement under which Verizon will acquire Yahoo's operating business for approximately $4.83 billion in cash, subject to customary closing adjustments. Yahoo informs, connects and entertains a global audience of more than 1 billion monthly active users** -- including 600 million monthly active mobile users*** through its search, communications and digital content products. Yahoo also co...
Jul. 27, 2016 08:00 AM EDT Reads: 607
There will be new vendors providing applications, middleware, and connected devices to support the thriving IoT ecosystem. This essentially means that electronic device manufacturers will also be in the software business. Many will be new to building embedded software or robust software. This creates an increased importance on software quality, particularly within the Industrial Internet of Things where business-critical applications are becoming dependent on products controlled by software. Qua...
Jul. 27, 2016 06:30 AM EDT Reads: 1,494
In addition to all the benefits, IoT is also bringing new kind of customer experience challenges - cars that unlock themselves, thermostats turning houses into saunas and baby video monitors broadcasting over the internet. This list can only increase because while IoT services should be intuitive and simple to use, the delivery ecosystem is a myriad of potential problems as IoT explodes complexity. So finding a performance issue is like finding the proverbial needle in the haystack.
Jul. 27, 2016 04:45 AM EDT Reads: 2,294
Machine Learning helps make complex systems more efficient. By applying advanced Machine Learning techniques such as Cognitive Fingerprinting, wind project operators can utilize these tools to learn from collected data, detect regular patterns, and optimize their own operations. In his session at 18th Cloud Expo, Stuart Gillen, Director of Business Development at SparkCognition, discussed how research has demonstrated the value of Machine Learning in delivering next generation analytics to imp...
Jul. 27, 2016 04:30 AM EDT Reads: 2,517
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. 27, 2016 04:00 AM EDT Reads: 1,576