Thread dump in Java

Differences between stack and heap in terms of multithreading

Stack is a piece of memory that is closely related to threads. Each thread has its own stack, which stores local variables, method parameters and a call stack. A variable stored on the stack of one thread is not visible to another.

Heap is a shared piece of memory that is shared among all threads. Objects, whether local or of any other level, are created on the heap. To improve performance, a thread usually caches values ​​from the heap onto its stack, in which case the volatile keyword is used to indicate to the thread that a variable should be read from the heap.

How do I share data between two threads?

Data between threads can be shared using a shared object or parallel data structures such as BlockingQueue.

To control the stack size of a thread -Xss JVM startup parameter is used.

Thread dump

HotSpot-based Java runtimes only generate an HPROF dump. The developer has several interactive dump generation methods and one event-based dump method at his disposal.

Interactive methods:

  • Using Ctrl+Break: if the command line option -XX:+HeapDumpOnCtrlBreak is set for the running application, then a dump of the HPROF format is generated along with a stream dump when a Ctrl+Break or SIGQUIT event occurs (usually generated using kill -3), which is initiated through the console. This option may not be available in some versions. In this case, you can try using the following option: -Xrunhprof:format=b,file=heapdump.hprof
  • Using the jmap tool: the jmap utility, supplied with the /bin/ directory of the JDK, allows you to request an HPROF dump from a running process.
  • Using the operating system: you can use the non-destructive gcore command or the destructive kill -6 or kill -11 commands to create a kernel file. Then extract the heap dump from the kernel file using the jmap utility.
  • Using the JConsole tool. The dumpHeap operation is exposed in JConsole as a HotSpotDiagnostic MBean. This operation requests the generation of a dump in HPROF format.

Event based method:

  • OutOfMemoryError Event: If the -XX:+HeapDumpOnOutOfMemoryError command line option is set for a running application, an HPROF dump is generated when an OutOfMemoryError occurs. This is ideal for "production" systems, as it is almost mandatory for diagnosing memory problems and does not come with constant performance overhead. In older releases of HotSpot Java runtime environments, this event does not have a limit on the number of heap dumps per JVM; newer releases allow a maximum of one heap dump for this event per JVM startup.

Read also:


Comments

Post a Comment

Popular posts from this blog

Methods for reading XML in Java

Image
DOM (Document Object Model) - object - reads XML, recreating it in memory in the form of an object structure, while the XML document is represented as a set of tags - nodes. Each node can have an unlimited number of child nodes. Each child can also contain several levels of descendants or none at all. Thus, the result is a kind of tree. Pluses: Easy to program. If there are many objects in XML with cross-references to each other, it is enough to go through the document twice: first time create objects without references and fill in the name-object dictionary, the second time - restore links. If an error occurs in XML, a half-created XML structure remains in memory, which will be automatically destroyed. Suitable for both reading and writing. Minuses: Low speed. Wastes a lot of memory. SAX (Simple API for XML) - event-driven - reads an XML document, responding to emerging events (opening or closing tag, string, attribute) by calling the event handlers provided by the appl...
Read more

XML, well-formed XML and valid XML

Image
XML, the eXtensible Markup Language, is a language with a simple formal syntax, well suited for creating and manipulating documents with programs, and at the same time easy to read and create documents by humans. XML is extensible, it does not fix the markup used in documents, and the developer is free to create markup in accordance with the needs of a particular area, being limited only by the syntax rules of the language. DTD A DTD, Document Type Definition, is a predefined set of rules that defines the relationships between elements and attributes. For example, the DTD for HTML says that the DIV tag must be inside the BODY tag and can appear multiple times, TITLE - in HEAD and only once, and SCRIPT - both there and there as many times as you want. A DTD is usually described directly in the document as a formulation line starting with <!DOCTYPE ... > or in a separate file. Difference between well-formed XML and valid XML Depending on the level of compliance, the document...
Read more

ArrayList and LinkedList in Java, memory usage and speed

Image
How much extra memory is needed when calling ArrayList.add()? If there is enough space in the array to accommodate a new element, then no additional memory is required. Otherwise, a new array is created, 1.5 times larger than the existing one (this is true for JDK above 1.7, in earlier versions the size of the increase is different). How much extra memory is allocated when you call LinkedList.add()? One new instance of the nested Node class is created. How much memory to store one byte primitive in LinkedList? Each LinkedList element stores a link to the previous element, the next element, and a data link. private static class Node<E> { E item; Node<E> next; Node<E> prev; //... } For 32-bit systems, each link is 32 bits (4 bytes). The object (header) of the nested Node class itself takes 8 bytes. 4 + 4 + 4 + 8 = 20 bytes, and since the size of each object in Java is a multiple of 8, so we get 24 bytes. A byte primitive occupies 1 by...
Read more