Inside the jvm
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This document discusses the structure and components of the Java Virtual Machine (JVM). It describes the key elements of the JVM including class files, bytecode, interpreters, memory areas like the Java stack, heap, and method area. It also discusses threads in the JVM, their states, and how thread dumps can be obtained. Garbage collection techniques like serial GC, parallel GC, CMS, and G1GC are covered. Out of memory errors and how to generate heap dumps are also summarized.
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Inside the jvm
- 1. Inside the JVM STRUCTURE OF JAVA VIRTUAL MACHINE Development Center @ Pikicast.com 김병부 ( Benjamin )
- 2. Benjamin : about me Pikicast Netmarble U-Angel Joins.com and etc
- 3. 因果關係 : 인과관계 System Crash, Service Crash. Application Crash 서비스가 죽는 것은 항상 원인이 있습니다 시스템이 원인인지, 어플리케이션이 원인인지, 데이터 베이스가 원인인지 알아야 같은 현상을 막을 수 있습니다. 서비스는 한번에 죽지 않습니다. 서서히 죽어갈 뿐입니다.
- 4. List of JVM List of JVM?? The Java Virtual Machine Specification Hotspot JVM Open JVM IBM JVM TinyVM Dalvik JM ?? Stack Machine vs Register Machine
- 5. JVM : Java Virtual Machine #1 Class File Bytecode 의 집합 Interpreter / JIT Compiler Java Stack Heap Thread 들이 공유하는 메모리 영역 Method Area Thread 들이 공유하는 메모리 영역 Class 변수, 상수, Class 정보, Method 정보
- 6. Thread 1st Keyword inside of the JVM
- 7. Thread State : java.lang.Thread.State NEW RUNNABLE TIMED_WAITING WAITING BLOCKED TERMINATED Since Java 1.5
- 9. Thread :list of thread in the Hotspot JVM Thread 종류 Non – Daemon Thread : E.g. Thread, VM Thread Daemon Thread : Garbage Collector Thread, JMX Monitoring Thread System Threads VM thread GC threads Periodic task thread – timer events i.e. interrupts for schedule Compiler thread – compiles byte code to native code in runtime
- 10. Thread : Multi-Thread Each thread has .. Program Counter (PC) Stack Native stack Stack Frame Multi Thread – Context Switching & Scheduling Thread-Safe Keyword – synchronized
- 11. JVM Stack JVM Stack per Thread!!! Stack Frame per Method Local Variable Array Operand Stack Reference to Constant Pool
- 12. Thread dump : How to dump JVM Thread jstack [pid] or kill –3 [pid]Thread Dump Date JVM Type, Mode Thread name Thread Priority Thread ID Native Thread ID Thread STATUS Thread STACK info
- 13. Thread Dump Tool : IBM Thread and Monitor Dump Analyzer for Java
- 14. Heap Area 2nd Keyword inside of the JVM
- 15. JVM Memory Structure : with JVM options Basic Structure Depends on GC options Promotion Minor GC vs Major GC Young Generation Old and Perm Generation STW – Stop the world From Java8, No more PERM gen. Metaspace (Native Heap)
- 16. What is difference between Method Area and PERM GEN? Actually the Method Area is a part of the PERM GEN A third generation closely related to the tenured generation is the permanent generation. The permanent generation is special because it holds data needed by the virtual machine to describe objects that do not have an equivalence at the Java language level. For example objects describing classes and methods are stored in the permanent generation.
- 17. Thread and Heap : Reference from Stack to Heap
- 18. Garbage Collection GC options Serial GC, Parallel GC – Mark, Sweep, Compact Incremental GC CMS G1GC
- 19. CMS vs G1GC CMS GC Concurrent – Mark – Sweep GC more CPU, G1GC Garbage-First GC Region Recommend over 4G memory
- 20. Heap Dump : How to dump JVM Heap jmap –dump:format=b,file=[file name].hprof [PID] JVM 메모리 사이즈 만큼의 binary 파일이 생성된다. Binary 파일이므로 분석툴이 필요함.
- 21. Heap Dump Tool : IBM HeapAnalyzer Or Eclipse Memory Analyzer
- 23. OOME Case : Out Of Memory Exception java.lang.OutOfMemoryError : Java heap space java.lang.OutOfMemoryError : PermGen space java.lang.OutOfMemoryError : Request array size exceeds VM limit
- 24. Conclusion Conclusion inside of the JVM
- 25. Note Dump 는 10초 간격으로 3번씩 뜬다. Perm 영역에서도 OOM 이 발생할 수도 있다. Hotspot JVM > Open JVM Async Process 가 만능은 아님 MultiThread 도 Pool로 관리하자 Thread-safe and Pool Management 우리에겐 G1GC 가 가장 맞는 케이스 Throughput based tuning Response time based tuning
- 26. Reference http://blog.jamesdbloom.com/JVMInternals.html http://d2.naver.com/helloworld/1230 http://blog.takipi.com/garbage-collectors-serial-vs-parallel-vs-cms-vs-the-g1-and- whats-new-in-java-8/ http://www.slideshare.net/MonicaBeckwith/garbage-first-garbage-collector-g1-gc- migration-to-expectations-and-advanced-tuning http://www.poalsoft.com/blog/?p=34
- 27. Thank you
Editor's Notes
- Java byte code is interpreted however this is not as fast as directly executing native code on the JVM’s host CPU. To improve performance the Oracle Hotspot VM looks for “hot” areas of byte code that are executed regularly and compiles these to native code. The native code is then stored in the code cache in non-heap memory. In this way the Hotspot VM tries to choose the most appropriate way to trade-off the extra time it takes to compile code verses the extra time it take to execute interpreted code.
- 데몬 스레드는 다른 비데몬 스레드가 없다면 동작을 중지한다. 사용자가 직접 스레드를 생성하지 않더라도, Java 애플리케이션이 기본적으로 여러 개의 스레드를 생성한다. 대부분이 데몬 스레드인데 가비지 컬렉션이나, JMX 등의 작업을 처리하기 위한 것이다. 'static void main(String[] args)' 메서드가 실행되는 스레드는 비데몬 스레드로 생성되고, 이 스레드가 동작을 중지하면 다른 데몬 스레드도 같이 동작을 중지하게 되는 것이다. 이 main 메서드가 실행되는 스레드를 HotSpot VM에서는 VM Thread라고 부른다.
- Each frame contains: - Local variable array - Return value - Operand stack Reference to runtime constant pool for class of the current method Program counter – CPU 명령어 포인터
- Reference to Constant Pool : 각 클래스와 인터페이스의 상수뿐만 아니라, 메서드와 필드에 대한 모든 레퍼런스까지 담고 있는 테이블이다. 즉, 어떤 메서드나 필드를 참조할 때 JVM은 런타임 상수 풀을 통해 해당 메서드나 필드의 실제 메모리상 주소를 찾아서 참조한다.
- OLD generation == Tenured generation