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  • 1. 去哪儿-技术部培训Something About JVM
  • 2. Java Process Life Cycle Parse the command line options Establish the heap sizes and the compiler type (client or server) Establishes the environment variables Creates the VM using JNI_CreateJavaVM Find the main class The java main method is invoked using CallStaticVoidMethod DestroyJVM www.qunar.com
  • 3. VM Class Loading When does it happen • bytecode resolution • Class.forName(), classLoader.loadClass(), reflection APIs, and JNI_FindClass • VM startup • java.lang.String • java.lang.System • java.lang.Thread, java.lang.ThreadGroup • java.lang.reflect.Method • java.lang.ref.Finalizer • java.lang.Class www.qunar.com
  • 4. VM Class Loading Class Loader Delegation www.qunar.com
  • 5. VM Class Loading Class loaders are organized in a tree Loading Order • looks up in its own cache, if found, return • If fails, delegates the request to the parent, if found, return • If fails, loads the class itself and searches its own path, if found, return • If fails, a ClassNotFoundException is thrown www.qunar.com
  • 6. VM Class Loading Phases of Class Loading www.qunar.com
  • 7. JVM Memory Layoutwww.qunar.com
  • 8. JVM Memory Layout For One Thread • PC register • The address of a JVM instruction being executed now • JVM stack • jinfo -flag ThreadStackSize {PID} • Native method stack Shared By All Threads • Method area (PermGen) • Runtime constant pool • Included in Method Area • Heap • A target of garbage collection www.qunar.com
  • 9. JVM Memory Layout Computing The Size of An Object (X86_64) • Arrays of boolean, byte, char, short, int: 2 * 8 (Object header) + 4 (length-field) + sizeof(primitiveType) * length -> align result up to a multiple of 8 • Arrays of objects: 2 * 8 (Object header) + 4 (length-field) + 4 (dead space due to alignment restrictions) + 8 * length • Arrays of longs and doubles: 2 * 8 (Object header) + 4 (length-field) + 4 (dead space due to alignment restrictions) + 8 * length www.qunar.com
  • 10. JVM Memory Layout Computing The Size of An Object • java.lang.Object: 2 * 8 (Object header) • other objects: sizeofSuperClass + 8 * nrOfLongDoubleAndObjectFields + 4 * nrOfntAndFloatFields + 2 * nrOfShortAndCharFields + 1 * nrOfByteAndBooleanFields -> align result up to a multiple of 8 www.qunar.com
  • 11. JVM Garbage Collection Structure of heap HotSpot GC Options Generational GC Monitoring GC www.qunar.com
  • 12. Structure of Heapwww.qunar.com
  • 13. Structure of Heap Eden are space there objects are allocated Survivor spaces are used to receive object during young (or minor GC) Tenured space is for long lived objects Permanent space is the Method Area and Runtime Constant Pool mentioned earlier www.qunar.com
  • 14. Hotspot GC Options Serial generational collector (-XX:+UseSerialGC). Parallel for young space, serial for old space generational collector (-XX:+UseParallelGC). Parallel young and old space generational collector (- XX:+UseParallelOldGC). Concurrent mark sweep with serial young space collector (-XX:+UseConcMarkSweepGC –XX:-UseParNewGC). Concurrent mark sweep with parallel young space collector (-XX:+UseConcMarkSweepGC – XX:+UseParNewGC). G1 garbage collector (-XX:+UseG1GC, not covered in this talk) www.qunar.com
  • 15. Generational GC Young GC • Write Barrier • Always Stop-The-World • Parallel • ParNew Full GC • Parallel Mark Sweep • Concurrent Mark Sweep www.qunar.com
  • 16. Young GC What Triggers Young GC • New objects are allocated in Eden • Once Eden becomes full Young GC is triggered Goal of Young GC • To clear fresh garbage in Eden space What does Young GC • live objects are copied to another space, and then whole space is marked as free memory How to find live objects • find all root references (Root references for young GC are references from stack and all references from old space) www.qunar.com
  • 17. Young GC Dirty Cards Write Barrier • prevent scaning all objects in old gen to find root references • card page size is 512 bytes • the card table is implemented as a simple array of bytes • each 512 byte page of memory has associated byte in card table • each time when program modifies reference in memory, it should mark modified memory page as dirty www.qunar.com
  • 18. Young GCwww.qunar.com
  • 19. Young GCWhile JVM is updating references to relocated object, memorypages get marked again, so we can be sure that on next youngGC only dirty pages has references to young space www.qunar.com
  • 20. Young GC Object Promotion • -XX:+AlwaysTenure makes JVM to promote objects directly to old space instead of survivor space • once survivor space is full, all remaining live object are relocated directly to old space • if object has survived certain number of young space collections, it will be promoted to old space • –XX:MaxTenuringThreshold – e.g., -XX:MaxTenuringThreshold=16 # Copy object between survivor space at most 16 times • –XX:TargetSurvivorRatio – e.g., -XX:TargetSurvivorRatio=90 # Keep each survivor space about 90% full www.qunar.com
  • 21. Young GC Time of young GC • Tstack_scan – can be considered application specific constant • Tcard_scan – is proportional to size of old space. Literally, JVM have to check single byte of table for each 512 bytes of heap (e.g. 8G of heap -> 16m to scan) • Told_scan – is proportional to number of dirty cards in old space at the moment of young GC. www.qunar.com
  • 22. Full GC What triggers full gc • Old Generation being full or promotion failure • Calls to • System.gc() • Runtime.gc() • through distributed „RMI‟ • If Xms and Xmx have different values and the old gen grows • if PermSize and MaxPermSize have different values and the Permanent Space grows www.qunar.com
  • 23. Full GC Mark-Sweep Model • Starting with the root objects • Objects represented by all active stack frames and all the static variables • Marks each node it visits • Objects not marked are reclaimed and returned to the free list • an array of lists that chained together free heap chunks of predetermined size www.qunar.com
  • 24. Full GC Serial Mark-Sweep-Compact GC Vs. CMS www.qunar.com
  • 25. Full GC CMS • -XX: +UseConcMarkSweepGC • triggered when amount of free memory in old space falls below certain threshold • -XX:+UseCMSInitiatingOccupancyOnly • -XX:CMSInitiatingOccupancyFraction=70 (this will force CMS cycle to start when more than 70% of old space is used) • Yields lower full gc pause time and better responsiveness of your application • Falls back to STW full gc when: • CMS is not fast enough for dealing with garbage • Collection cycle has been started too late • fragmentation of old space www.qunar.com
  • 26. Full GC CMS • Initial mark – STW, collecting root references • Concurrent mark – traverses through object graph in tenured space marking living objects • Concurrent pre clean - try to account references changed during previous mark phase, to reduce time of STW remark phase • Remark – STW, to account references which have been changed during concurrent mark phase • Concurrent sweep • Concurrent reset www.qunar.com
  • 27. Monitoring Java GCwww.qunar.com
  • 28. CompressedOops What is an object instance • a block of memory set aside on the heap • contain instance data and other meta information • represented in hotpot by the oopDesc data- structure • an object is referenced via an oop: • a native memory address when running on 32-bit architecture • An 32-bit index into the heap with - XX:+UseCompressedOops on 64-bit architecture www.qunar.com
  • 29. CompressedOops oopsDesc Layout www.qunar.com
  • 30. CompressedOops QUESTION? • So how do we calculate object size with CompressedOops enabled www.qunar.com
  • 31. jinfo Prints Java configuration information for a given Java process or core file or a remote debug server. Common Usage • sudo -u tomcat /home/q/java/default/bin/jinfo - flag ThreadStackSize ${pid} • sudo -u tomcat /home/q/java/default/bin/jinfo - flag UseCompressedOops ${pid} • sudo -u tomcat /home/q/java/default/bin/jinfo – sysprops ${pid} www.qunar.com
  • 32. Jstack usage Prints Java stack traces of Java threads for a given Java process or core file or a remote debug server Common Usage • sudo -u tomcat /home/q/java/default/bin/jstack ${pid} | less www.qunar.com
  • 33. Jstack explanationwww.qunar.com
  • 34. Jstack explanation Thread Information • Thread name • When using Java.lang.Thread class to generate a thread, the thread will be named Thread-(Number), whereas when using java.util.concurrent.ThreadFactory class, it will be named pool-(number)-thread-(number) • Priority • the priority of the threads. • Thread ID • the unique ID for the threads • Thread status • the status of the threads • Thread callstack • The call stack information of the threads www.qunar.com
  • 35. Jstack explanation Thread ID • Each java thread is mapped one-on-one to os thread • Nid is the hexadecimal representation of native thread id • [root@l-bnb3.dev.cn6 ~]# ps -eLf | grep $(printf "%dn" 0x4454) www.qunar.com
  • 36. Jstack explanation Thread Status www.qunar.com
  • 37. Jstack explanation NEW: created but has not been processed yet. RUNNABLE: The thread is occupying the CPU and processing a task. BLOCKED: waiting for a different thread to release its lock in order to get the monitor lock. WAITING: The thread is waiting indefinitely by using a wait, join or park method. TIMED_WAITING: The thread is waiting by using a sleep, wait, join or park method for up to a specified waiting time. www.qunar.com
  • 38. jstat Displays performance statistics for an instrumented HotSpot Java virtual machine (JVM) Common Usage • sudo -u tomcat /home/q/java/default/bin/jstat - gcutil ${pid}1000 1000 www.qunar.com
  • 39. Jmap & mat Prints shared object memory maps or heap memory details of a given process or core file or a remote debug server. Common Usage • sudo -u tomcat /home/q/java/default/bin/jmap - histo:live ${pid} | less • sudo -u tomcat /home/q/java/default/bin/jmap -heap ${pid} | less • sudo -u tomcat /home/q/java/default/bin/jmap - dump:live,format=b,file=/home/q/memdump/memMap .20121030.hprof ${pid} www.qunar.com
  • 40. Jmap & mat Analyze hprof dump file with MAT • Eclipse mat 1.2.0 is required, otherwise you will get incorrect statistics about memory occupation due to the mishandling of CompressedOops in 1.1.0 • Key Concepts • Incoming/outgoing references • Shallow/retained heap www.qunar.com
  • 41. Jvm core dump analysis the first thing is: Dont Panic try following the steps below: 1. log in as root 2. executing ulimit -aH, check whether the “core file sizes” is specified as unlimited, if not, executing ulimit -c unlimited www.qunar.com
  • 42. Jvm core dump analysis check whether theres enough space left in your current work dir 4. running the java process again using the command obtained from the previous "ps aux|grep java" 5. waiting for the java process crashing again 6. collecting the core dump file from the current work dir 7. doing like that: • gdb /home/q/java/jdk1.6.0_20/bin/java core.9537 8. there are ways to obtain java-related information from core dump file: • obtaining java memory dump: /home/q/java/jdk1.6.0_20/bin/jmap -dump:format=b,file=dump.1013.hprof /home/q/java/jdk1.6.0_20/jre/bin/java core.26408 • obtaining jstack info: /home/q/java/jdk1.6.0_20/bin/jstack /home/q/java/jdk1.6.0_20/bin/java core.26408 > core_stack_1013.txt www.qunar.com
  • 43. www.qunar.com