Владимир Иванов. Java 8 и JVM: что нового в HotSpot

4,089 views

Published on

Published in: Software, Technology, Education
0 Comments
0 Likes
Statistics
Notes
  • Be the first to comment

  • Be the first to like this

No Downloads
Views
Total views
4,089
On SlideShare
0
From Embeds
0
Number of Embeds
3,392
Actions
Shares
0
Downloads
29
Comments
0
Likes
0
Embeds 0
No embeds

No notes for slide

Владимир Иванов. Java 8 и JVM: что нового в HotSpot

  1. 1. 1 What’s New in HotSpot JVM 8 Vladimir Ivanov HotSpot JVM Compiler team Oracle Corp.
  2. 2. 2 What is Java?
  3. 3. 3 What is Java? §  Java –  The Language –  The Platform –  The Community
  4. 4. 4 What is Java? §  Java –  The Language –  The Platform –  The Community What I’ll talk about today
  5. 5. 5 What’s New in HotSpot JVM 8 §  Java 8 features support –  Project Lambda –  Nashorn JavaScript engine –  Type annotations §  Oracle implementation-specific improvements –  won’t be backported into 7 §  PermGen removal –  will be/already backported into 7 §  numerous functional & performance enhancements Categorization
  6. 6. 6
  7. 7. 7 Project Lambda x -> x+1 (s,i) -> s.substring(0,i) () -> System.out.print(“x”) Predicate<String> pred = s -> s.length() < 100; Lambda expressions in Java
  8. 8. 8 Project Lambda Function<Integer,Integer> f = x -> x+1 compiles to invokedynamic [ j.l.i.LambdaMetafactory.metafactory, MethodType(Function.apply), MethodHandle(lambda$0) ] () Lambda expressions in Java
  9. 9. 9 Project Lambda §  Lambda expressions §  Core Java API evolution –  new API §  Streams –  existing API «lambdafication» §  e.g. Collections API –  default methods
  10. 10. 10 Project Lambda public interface Iterable<T> { void forEach(Consumer<? super T> action); } API “lambdafication”
  11. 11. 11 Project Lambda interface I { default void m() { /* do smth */ } } class T implements I {} invokevirtual T.m()V => I.m Default methods interface I void m() {…} class T
  12. 12. 12 Project Lambda interface I { default void m() { /* do smth */ } } class T implements I { void m() { /* do smth */ } } invokevirtual T.m()V => T.m Default methods interface I void m() {…} class T void m() {…}
  13. 13. 13 Project Lambda interface I { default void m() { /* do smth */ } } class T1 implements I { void m() { /* do smth */ } } class T extends T1 implements I {} invokevirtual T.m()V => T1.m Superclass overrides superinterface interface I void m() {…} class T class T1 void m() {…}
  14. 14. 14 Project Lambda interface I { default void m() { /* do smth */ } } interface J extends I { default void m() { /* do smth */ } } class T implements I,J {} invokevirtual T.m()V => J.m Prefer most specific interface interface J void m() {…} class T class I void m() {…}
  15. 15. 15 Project Lambda interface I { default void m() { /* do smth */ } } interface J { default void m() { /* do smth */ } } class T implements I,J {} invokevirtual T.m()V => error Conflicting defaults interface J void m() {…} class T class I void m() {…}
  16. 16. 16 Project Lambda interface I { static void m() { /* do smth */ } } invokestatic I.m(); Static interface methods
  17. 17. 17 Project Lambda interface I { private ... void m() { /* do smth */ } } Not allowed in Java. Only on bytecode level. Private interface methods
  18. 18. 18 Project Lambda class A { A get() {} } class B extends A { B get() {} } Javac adds the following: synthetic bridge A get() { return ((B)this).get(); } Bridge methods: Covariant overrides
  19. 19. 19 Project Lambda §  Attempted to: –  use invokedynamic –  generate bridges by VM §  Finally: –  continue to generate bridge methods by javac Bridge methods: roads not taken
  20. 20. 20
  21. 21. 21 java.lang.OutOfMemoryError: PermGen space
  22. 22. 22 What was “PermGen”? §  “Permanent” Generation §  Region of Java heap for JVM Class Metadata §  Representation of Java classes –  Class hierarchy information, fields, names –  Method compilation information and bytecodes –  Vtables –  Constant pool and symbolic resolution –  Interned strings (moved out of PermGen in 7)
  23. 23. 23 Java Memory Layout with PermGen Copyright © 2013, Oracle and/or its affiliates. All rights reserved.6 Java Memory Layout with PermGen Eden Old Generation for older objects Permanent Generation for VM metadata Survivor Survivor
  24. 24. 24 Where did JVM Metadata go? Copyright © 2013, Oracle and/or its affiliates. All rights reserved.9 Where did JVM Metadata go? Eden Old Generation for older objects Permanent Generation for VM metadata Survivor Survivor Metaspace VM Metadata in native memory
  25. 25. 25 PermGen size §  Limited to MaxPermSize – default ~64M - 85M §  Contiguous with Java Heap §  Once exhausted throws OutOfMemoryError “PermGen space” §  Hard to predict the correct size
  26. 26. 26 Why was PermGen Eliminated? §  Fixed size at startup – applications ran out –  -XX:MaxPermSize=… §  Improve GC performance –  Special iterators for metadata –  Deallocate class data concurrently and not during GC pause §  Enable future improvements –  were limited by PermGen (e.g. G1 concurrent class unloading)
  27. 27. 27 Improving GC Performance §  During full collection, metadata to metadata pointers are not scanned –  A lot of complex code (particularly for CMS) for metadata scanning was removed §  Metaspace contains few pointers into the Java heap –  Pointer to java/lang/Class instance in class metadata –  A component java/lang/Class pointer in array class metadata §  No compaction costs for metadata
  28. 28. 28 Metaspace §  Take advantage of Java Language Specification property –  Class metadata lifetime same as their class loader’s §  Per loader storage area – Metaspace (collectively called Metaspace) –  Linear (bump) allocation only –  No individual reclamation (except for RedefineClasses and class loading failure) –  Not scanned by GC and not compacted –  Metaspace-allocated objects never relocate –  Reclamation en-masse when class loader found dead by GC
  29. 29. 29 Metaspace Allocation §  Multiple mmap/VirtualAlloc virtual memory spaces §  Allocate per-class loader chunk lists –  Chunk sizes depend on type of class loader –  Smaller chunks for sun/reflect/DelegatingClassLoader, JSR292 anonymous classes §  Return chunks to free chunk lists §  Virtual memory spaces returned when emptied §  Strategies to minimize fragmentation
  30. 30. 30 Metaspace Allocation Copyright © 2013, Oracle and/or its affiliates. All rights reserved.12 Metaspace Allocation ● Metachunks in virtual spaces (vs1, vs2, vs3...) Boot CL CL 3 CL 1 CL 2 vs1 vs3vs2 §  Metachunks in virtual spaces (vs1, vs2, vs3...)
  31. 31. 31 Java Object Memory Layout class Message { String text; void add(String s) { ...} … } Java Heap Layout Copyright © 2013, Oracle and/or its affiliates. All rights reserved.13 Java Object Memory Layout class Message { // JVM adds _mark and //_klass pointer String text; void add(String s) { …} String get() { … } } Java Heap Layout _mark _klass text _mark _klass count value Metaspace Klass Klass
  32. 32. 32 Java Object Memory Layout class Message { String text; void add(String s) { ...} … } with compressed class pointers Java Heap Layout
  33. 33. 33 How to tune Metaspace? §  -XX:MaxMetaspaceSize={unlimited} §  Limit the memory used by class metadata before excess swapping and native allocation failure occurs –  Use if suspected class loader memory leaks –  Use if on 32-bit
  34. 34. 34 How to tune Metaspace? §  -XX:MetaspaceSize={21M} §  Set to a higher limit if application loads more §  Possibly use same value set by PermSize to delay initial GC §  High water mark increases with subsequent collections for a reasonable amount of head room before next Metaspace GC
  35. 35. 35 How to tune Metaspace? §  -XX:CompressedClassSpaceSize={1G} §  Only valid if -XX:+UseCompressedClassPointers (default on 64 bit) §  Not committed until used
  36. 36. 36 Metaspace Monitoring and Management §  MemoryManagerMXBean with name MetaspaceManager §  MemoryPoolMXBeans –  “Metaspace” –  “Compressed Class Space” §  $ jmap -clstats <pid> §  $ jstat -gc <pid> §  $ jcmd <pid> GC.class_stats §  Java Mission Control (bundled with Oracle JDK 8)
  37. 37. 37 PermGen removal §  Hotspot metadata is now allocated in Metaspace –  Chunks in mmap spaces based on liveness of class loader §  Compressed class pointer space is still fixed size but large §  Tuning flags available but not required §  Change enables other optimizations and features in the future –  Application class data sharing –  Young collection optimizations, G1 class unloading –  Metadata size reductions and internal JVM footprint projects Summary
  38. 38. 38 Nashorn JavaScript engine for Java Platform
  39. 39. 39 Nashorn §  Nashorn is written completely in Java §  Extensively uses JSR292 §  Required numerous performance improvements –  LambdaForms (JEP 160) –  incremental inlining –  exact math intrinsics §  Math.addExact, Math.substractExact, etc –  improved type profiling & type speculation JVM support for dynamic languages
  40. 40. 40 JVM support for dynamic languages public static long addExact(long x, long y) { long r = x + y; if (((x ^ r) & (y ^ r)) < 0) { throw new ArithmeticException("long overflow"); } return r; } Exact Math intrinsics
  41. 41. 41 JVM support for dynamic languages Math.addExact(l1, Integer.MAX_VALUE) Compiled version: 0x0...5d: add $0x7fffffff,%r8 0x0...64: mov %r8,%r9 0x0...67: xor %r11,%r9 0x0...6a: mov %r8,%r11 0x0...6d: xor $0x7fffffff,%r11 0x0...74: and %r11,%r9 0x0...77: test %r9,%r9 0x0...7a: jl 0x0000000102c70e95 // slow path: throw exception Exact Math intrinsics
  42. 42. 42 JVM support for dynamic languages Math.addExact(l1, Integer.MAX_VALUE) Intrinsified version: 0x…1d: add $0x7fffffff,%rax 0x…24: jo 0x000000010c044b3f // slow path: overflow Exact Math intrinsics
  43. 43. 43 Smaller features §  JSR 308: Annotations on Java Types –  new class file attributes §  JEP 171: Fence Intrinsics –  sun.misc.Unsafe: loadFence, storeFence & fullFence §  JEP 136: Enhanced Verification Errors –  VerifyError with detailed error message §  JEP 142: Reduce Cache Contention on Specified Fields –  @Contended
  44. 44. 44 JSR 308: Annotations on Java Types Map<@Interned Key, @NonNull Value> = new HashMap<>();
  45. 45. 45 JSR 308: Annotations on Java Types §  New attributes: –  RuntimeVisibleTypeAnnotations –  RuntimeInvisibleTypeAnnotations §  Stored on the smallest enclosing class, field, method, or Code §  How to access: –  javax.lang.model –  javax.ide –  com.sun.source.tree Class File Attributes
  46. 46. 46 JEP 171: Fence Intrinsics §  How to express memory model: –  happens-before relations –  memory barriers/fences §  Happens-before in Java Memory Model (JMM) §  sun.misc.Unsafe.[load|store|full]Fence introduces memory barriers §  Why: –  some relations aren’t possible to express in JMM at the moment §  consider StoreStore
  47. 47. 47 JEP 142: Reduce Cache Contention on … §  What is cache contention and false sharing? –  adjacent memory accesses can produce unnecessary memory traffic on SMP systems
  48. 48. 48 JEP 142: Reduce Cache Contention on … §  How to avoid it? –  manually “pad” contended fields public class A { int x; int i01, i02, …, i16; int y; int i17, i18, …, i32; }
  49. 49. 49 JEP 142: Reduce Cache Contention on … §  How to avoid it? –  mark them @Contended public class A { int x; @Contended int y; }
  50. 50. 50 JEP 142: Reduce Cache Contention on … §  sun.misc.Contended §  How to use: -XX:-RestrictContended –  by default, has effect only in privileged context (on boot class path)
  51. 51. 51 JEP 136: Enhanced Verification Errors Before Exception in thread "main" java.lang.VerifyError: Bad instruction in method Test.main([Ljava/lang/String;)V at offset 0
  52. 52. 52 JEP 136: Enhanced Verification Errors Now Exception in thread "main" java.lang.VerifyError: Bad instruction Exception Details: Location: Test.main([Ljava/lang/String;)V @0: <illegal> Reason: Error exists in the bytecode Bytecode: 0000000: ff00 0200 004c 2b04 b800 03b9 0004 0200 0000010: 57b1
  53. 53. 53 Future work The following is intended to outline our general product direction. It is intended for information purposes only, and may not be incorporated into any contract. It is not a commitment to deliver any material, code, or functionality, and should not be relied upon in making purchasing decisions. The development, release, and timing of any features or functionality described for Oracle’s products remains at the sole discretion of Oracle.
  54. 54. 54 Future work §  Value Objects in Java –   http://openjdk.java.net/jeps/169 §  Project Sumatra: Java on GPU –  http://openjdk.java.net/projects/sumatra/ §  Project Panama: Native Interconnect for Java (JNI 2.0) –  http://mail.openjdk.java.net/pipermail/discuss/2014-March/003306.html Where does innovation take place?
  55. 55. 55 Graphic Section Divider

×