What is cool in Java 8 and new in...
Transcript of What is cool in Java 8 and new in...
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What is cool in Java 8 and new in 9From a VM engineer‘s perspective
Tobias Hartmann
Compiler Group – Java HotSpot Virtual MachineOracle Corporation
March 2017
Copyright © 2017, Oracle and/or i ts affiliates. All rights reserved.
About me
• Software engineer in the HotSpot JVM Compiler Team at Oracle– Based in Baden, Switzerland
• Master’s degree in Computer Science from ETH Zurich
• Worked on various compiler-related projects
– Currently working on future Value Type support for Java
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Safe Harbor Statement
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.
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Outline
• Intro: The HotSpot Java Virtual Machine
• Part 1: What's cool in Java 8
– Background: JIT compilation in HotSpot
– Tiered Compilation
• Part 2: What's new in Java 9
– Segmented Code Cache
– Compact Strings
– Ahead-of-time Compilation
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A typical computing platform
5
Hardware
Operating system
Java Virtual Machine
User Applications
Java EEJava SE
Application Software
System Software
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A typical computing platform
6
Hardware
Operating system
Java Virtual Machine
User Applications
Java EEJava SE
Application Software
System Software
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A typical computing platform
7
Hardware
Operating system
Java Virtual Machine
User Applications
Java EEJava SE
Application Software
System Software
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Programming language implementation
C
Windows
Intel x86
8
Operatingsystem
Languageimplementation
Hardware
Programminglanguage
CompilerStandard librariesDebuggerMemory management
Linux
Intel x86
CompilerStandard librariesDebuggerMemory management
Linux
ARM
CompilerStandard librariesDebuggerMemory management
Solaris
SPARC
CompilerStandard librariesDebuggerMemory management
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(Language) virtual machine
Java
Windows
Intel x86
Operatingsystem
Virtual machine
Hardware
Programminglanguage
HotSpot VM
PPC ARM SPARC
Mac OS X SolarisLinux
JavaScript Scala Python
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The VM: An application developer’s view
Java source code
int i = 0;do {
i++;} while (i < f());
Bytecodes
0: iconst_01: istore_12: iinc5: iload_16: invokestatic f9: if_icmplt 2
12: return
compileHotSpotJava VMexecute
• Ahead-of-time• Using javac
• Instructions for an abstract machine• Stack-based machine (no registers)
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The VM: A VM engineer’s view
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Bytecodes
0: iconst_01: istore_12: iinc5: iload_16: invokestatic f9: if_icmplt 2
12: return
HotSpot Java VM
Garbage collector
manage
Interpreter
executeHeap
Stack
access
access
Compilation system
compile
C1
C2
Compiled methodproduce
Machine code
Debug info
Object maps
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Outline
• Intro: The HotSpot Java Virtual Machine
• Part 1: What's cool in Java 8
– Background: JIT compilation in HotSpot
– Tiered Compilation
• Part 2: What's new in Java 9
– Segmented Code
– Compact Strings
– Ahead-of-Time Compilation
• Conclusion
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Interpretation vs. compilation in HotSpot• Template-based interpreter
– Generated at VM startup (before program execution)
– Maps a well-defined machine code sequence to every bytecode instruction
• Compilation system
– Speedup relative to interpretation: ~100X
– Two just-in-time compilers (C1, C2)
– Aggressive optimistic optimizations
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Bytecodes0: iconst_01: istore_12: iinc5: iload_16: invokestatic f9: if_icmplt 2
12: return
Machine codemov -0x8(%r14), %eaxmovzbl 0x1(%r13), %ebxinc %r13mov $0xff40,%r10jpmq *(%r10, %rbx, 8)
Load local variable 1
Dispatch next instruction
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Ahead-of-time vs. just-in-time compilation
• AOT: Before program execution
• JIT: During program execution
– Tradeoff: Resource usage vs. performance of generated code
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Performance
Amount of compilationInterpretation Compile everything
Bad performancedue to interpretation
Bad performancedue to compilation overhead
Good performancedue to good selection of compiled methods and applied optimizations
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JIT compilation in HotSpot
• Resource usage vs. performance– Getting to the “sweet spot”
1. Selecting methods to compile
2. Selecting compiler optimizations
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1. Selecting methods to compile
• Hot methods (frequently executed methods)
• Profile method execution
– # of method invocations, # of backedges
• A method’s lifetime in the VM
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Interpreter Compiler (C1 or C2) Code cache
Gather profiling information Compile bytecode to native code Store machine code
# method invocations > THRESHOLD1
# of backedges > THRESHOLD2
Deoptimization
Compiler’s optimistic assumptionsproven wrong
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False
Control flow graph Generated code
Example optimization:
S1;S2;S3;if (x > 3)
S4; S5;S6;S7;
S8;S9;
10’000 0
guard(x > 3)S1;S2;S3;S4;S8;S9;
Deoptimize
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True
Hot path compilation
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Example optimization:
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class A {
void bar() {S1;
}
}
class B extends A {
void bar() {S2;
}
}
void foo() {
A a = create(); // return A or Ba.bar();
}
Class hierarchy Method to be compiled
loaded
not loaded
Compiler:Inline call?Yes.
Virtual call inlining
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Example optimization: Virtual call inlining
• Benefits of inlining
– Virtual call avoided
– Code locality
• Optimistic assumption: only A is loaded
– Note dependence on class hierarchy
– Deoptimize if hierarchy changes
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class A {
void bar() {S1;
}
}
class B extends A {
void bar() {S2;
}
}
void foo() {
A a = create(); // return A or BS1;
}
Class hierarchy Method to be compiled
loaded
not loaded
Compiler:Inline call?Yes.
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Example optimization: Virtual call inlining
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class A {
void bar() {S1;
}
}
class B extends A {
void bar() {S2;
}
}
void foo() {
A a = create(); // return A or Ba.bar();
}
Class hierarchy Method to be compiled
loaded
loaded
Compiler:Inline call?No.
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Deoptimization
• Compiler’s optimistic assumption proven wrong
– Assumptions about class hierarchy
– Profile information does not match method behavior
• Switch execution from compiled code to interpretation
– Reconstruct state of interpreter at runtime
– Complex implementation
• Compiled code
– Possibly thrown away
– Possibly reprofiled and recompiled
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Performance effect of deoptimization
• Follow the variation of a method’s performance
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Performance
Time
Interpreted Compiled Interpreted Compiled
Compilation DeoptimizationVM Startup Compilation
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Background: JIT compilation in HotSpot
• Resource usage vs. performance– Getting to the “sweet spot”
1. Selecting methods to compile
2. Selecting compiler optimizations
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2. Selecting compiler optimizations
• C1 compiler
– Limited set of optimizations
– Fast compilation
– Small footprint
• C2 compiler
– Aggressive optimistic optimizations
– High resource demands
– High-performance code
• Graal
– Experimental compiler
– Will be part of HotSpot for AOT in JDK 9
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Client VM
Server VM
Tiered Compilation(enabled since JDK 8)
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Outline
• Intro: The HotSpot Java Virtual Machine
• Part 1: What's cool in Java 8
– Background: JIT compilation in HotSpot
– Tiered Compilation
• Part 2: What's new in Java 9
– Segmented Code
– Compact Strings
– Ahead-of-Time Compilation
• Conclusion
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Tiered Compilation
• Introduced in JDK 7, enabled by default in JDK 8
• Combines the benefits of
– Interpreter: Fast startup
– C1: Fast compilation
– C2: High peak performance
• Within the sweet spot
– Faster startup
– More profile information
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Benefits of Tiered Compilation
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Performance
Time
VM Startup
Interpreted C1-compiled
warm-up time
Client VM (C1 only)
Compilation
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Benefits of Tiered Compilation
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PerformanceInterpreted C2-compiled
warm-up time
Server VM (C2 only)
Time
VM Startup Compilation
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Benefits of Tiered Compilation
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PerformanceInterpreted C1-compiled
warm-up time
Tiered compilation
C2-compiled
Time
VM Startup Compilation Compilation
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Additional benefit: More accurate profiling
time
Interpreter C1 (profiled) C2 (non-profiled)
Interpreter
Profiling without tiered compilation
Profiling with tiered compilation
C2 (non-profiled)
300 samples
100 samples 1000 samples
100 samples 200 samples
w/ tiered compilation: 1’100 samples gatheredw/o tiered compilation: 300 samples gathered
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Tiered Compilation
• Combined benefits of interpreter, C1, and C2
• Additional benefits
– More accurate profiling information
• Drawbacks
– Complex implementation
– Careful tuning of compilation thresholds needed
– More pressure on code cache
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A method’s lifetime (Tiered Compilation)
Interpreter C1 C2
Code cache
Collect profiling information Generate code quicklyContinue collectingprofiling information
Generate high-quality codeUse profiling information
Deoptimization
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Performance of a method (Tiered Compilation)
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Performance
Time
VM Startup
Interpreted C1 compiled C2 compiled
Compilation Compilation
Interpreted C2 compiled
Deoptimization Compilation
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Compilation levels (detailed view)
Interpreter
C1: no profiling
C1: limited profiling
C1: full profiling
C2
0
1
2
3
4
Co
mp
ilati
on
leve
lTypical compilation sequence
Associated thresholds:Tier3InvokeNotifyFreqLog
Tier3BackedgeNotifyFreqLogTier3InvocationThresholdTier3MinInvocationThreshold
Tier3BackEdgeThresholdTier3CompileThreshold
Associated thresholds:Tier4InvocationThreshold
Tier4MinInvocationThresholdTier4CompileThresholdTier4BackEdgeThreshold
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• Lambda Expressions
• Language Interop• Nashorn
• Parallel operations for core collections APIs
• Improvements in functionality
• Improved type inference
• Tiered Compilation
• PermGen removal• Performance improvements
• Profiles for constrained devices
• JSR 310: Date & Time APIs
• Non-Gregorian calendars
• Unicode 6.2
• ResourceBundle
• BCP47 locale matching
• Globalization & Accessibility
• JSR 308: Annotations on Java Type
• Native app bundling
• App Store Bundling tools
• jdeps
• Deployment enhancements
• JavaFX 8
• Public UI Control API
• Java SE Embedded support
• Enhanced HTML5 support
• 3D shapes and attributes
• Printing
• Limited doPrivilege
• NSA Suite B algorithm support
• SNI Server Side support
• DSA updated to FIPS186-3
• AEAD JSSE CipherSuites
• Mission Control
• Flight Recorder
• Usage Tracker
• Advanced Management Console
• MSI Enterprise JRE Installer
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InnovationCore libraries
Security
Java for Everyone
Client
ToolsJVM enhancements
Enterprise
JDK 8 features
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Outline
• Intro: The HotSpot Java Virtual Machine
• Part 1: What's cool in Java 8
– Background: JIT compilation in HotSpot
– Tiered Compilation
• Part 2: What's new in Java 9
• Conclusion
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• Segmented Code Cache• Ahead-of-Time Compilation
• Store Interned Strings in CDS Archives
• Improve Contended Locking• Compact Strings• Improve Secure Application Performance
• Leverage CPU Instructions for GHASH and RSA• Tiered Attribution for javac• Javadoc Search
• Marlin Graphics Renderer• HiDPI Graphics on Windows and Linux• Enable GTK 3 on Linux
• Update JavaFX/Media to Newer Version of GStreamer
• Jigsaw – Modularize JDK
• Enhanced Deprecation• Stack-Walking API• Convenience Factory Methods for Collections
• Platform Logging API and Service• jshell: The Java Shell (Read-Eval-Print Loop)• Compile for Older Platform Versions
• Multi-Release JAR Files• Platform-Specific Desktop Features• TIFF Image I/O
• Multi-Resolution Images
• Process API Updates
• Variable Handles• Spin-Wait Hints• Dynamic Linking of Language-
Defined Object Models• Enhanced Method Handles• More Concurrency Updates
• Compiler Control
Behind the scenes
New functionality
Specialized
• Parser API for Nashorn• Prepare JavaFX UI Controls & CSS APIs for Modularization
• Modular Java Application Packaging• New Version-String Scheme
• Reserved Stack Areas for Critical Sections• Segmented Code Cache• Ahead-of-Time Compilation
• Indify String Concatenation• Unified JVM Logging• Unified GC Logging
• Make G1 the Default Garbage Collector• Use CLDR Locale Data by Default• Validate JVM Command-Line Flag Arguments
• Java-Level JVM Compiler Interface• Disable SHA-1 Certificates• Deprecate the Applet API
• Process Import Statements Correctly• Annotations Pipeline 2.0• Elide Deprecation Warnings on Import Statements
• Filter Incoming Serialization Data
• Milling Project Coin• Simplified Doclet API
• Remove GC Combinations Deprecated in JDK 8
• Remove Launch-Time JRE Version Selection• Remove the JVM TI hprof Agent• Remove the jhat Tool
Housekeeping
Removed
• HTTP 2 Client
• Unicode 8.0• UTF-8 Property Files• ECMAScript 6 Features in Nashorn
• Datagram Transport Layer Security (DTLS)• OCSP Stapling for TLS• TLS Application-Layer Protocol
Negotiation Extension• SHA-3 Hash Algorithms• DRBG-Based SecureRandom Implementations
• Create PKCS12 Keystores by Default
• Merge Selected Xerces 2.11.0 Updates into JAXP
• HarfBuzz Font-Layout Engine
• XML Catalogs• HTML5 Javadoc
New standards
JDK 9 features
• Compact Strings
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Outline
• Intro: The HotSpot Java Virtual Machine
• Part1: What's cool in Java 8
– Background: JIT compilation in HotSpot
– Tiered Compilation
• Part2: What's new in Java 9
– Segmented Code
– Compact Strings
– Ahead-of-Time Compilation
• Conclusion
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What is the code cache?
• Stores code generated by JIT compilers
• Continuous chunk of memory
– Managed (similar to the Java heap)
– Fixed size
• Essential for performance
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Code cache usage: JDK 6 and 7
free space
VM internals
compiled code
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Code cache usage: JDK 8 (Tiered Compilation)
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free space
VM internals
C1 compiled (profiled)
C2 compiled (non-profiled)
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Code cache usage: JDK 9
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free space
VM internals
C1 compiled (profiled)
C2 compiled (non-profiled)
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Challenges
• Tiered compilation increases amount of code by up to 4X
• All code is stored in a single code cache
• High fragmentation and bad locality
• But is this a problem in real life?
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Code cache usage: Reality
profiled code
non-profiled code
free space
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Code cache usage: Reality
hotness
profiled code
non-profiled code
free space
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Design: Types of compiled code
Optimization level Size Cost Lifetime
Non-method code optimized small cheap immortal
Profiled code (C1) instrumented medium cheap limited
Non-profiled code (C2) highly optimized large expensive long
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• Without Segmented Code Cache • With Segmented Code Cache
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Design
Code Cache
non-profiled methods
profiled methods
non-methods
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non-profiled methodsprofiled methods
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Segmented Code Cache: Reality
profiled code
non-profiled code
free space
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Segmented Code Cache: Reality
non-profiled methodsprofiled methods hotness
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Evaluation: Responsiveness
• Sweeper (GC for compiled code)
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0
5
10
15
20
25
30
35
40
# full sweeps Cleanup pause time Sweep time
Re
du
ctio
n i
n %
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Evaluation: Performance
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0
2
4
6
8
10
12
14
SPECjbb2005 SPECjbb2013 JMH-Javac Octane (Typescript) Octane (Gbemu)
Imp
rove
me
nt
in %
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What we have learned
• Segmented Code Cache helps– To reduce the sweeper overhead and improve responsiveness
– To reduce memory fragmentation
– To improve code locality
• And thus improves overall performance
• To be released with JDK 9
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Outline
• Intro: The HotSpot Java Virtual Machine
• Part 1: What's cool in Java 8
– Background: JIT compilation in HotSpot
– Tiered Compilation
• Part 2: What's new in Java 9
– Segmented Code
– Compact Strings
– Ahead-of-Time Compilation
• Conclusion
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public class HelloWorld {public static void main(String[] args) {
String myString = "HELLO";System.out.println(myString);
}}
Java Strings
public final class String {private final char value[];...
}
char value[] =
H
0x0048 0x0045 0x004C 0x004C 0x004F
2 bytes
E L L O
UTF-16 encoded
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“Perfection is achieved, not when there is nothing more to add, but when there is nothing more to take away.”
– Antoine de Saint Exupéry
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There is a lot to take away here..
• UTF-16 encoded Strings always occupy two bytes per char
• Wasted memory if only Latin-1 (one-byte) characters used:
• But is this a problem in real life?
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char value[] =
H
0x0048 0x0045 0x004C 0x004C 0x004F
2 bytes
E L L O
0x0048 0x0045 0x004C 0x004C 0x004F
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Real life analysis: char[] footprint
• 950 heap dumps from a variety of applications– char[] footprint makes up 10% - 45% of live data
– Majority of characters are single byte
• Predicted footprint reduction of 5% - 10%
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Project Goals
• Memory footprint reduction by improving space efficiency of Strings
• Meet or beat performance of JDK 9
• Full compatibility with related Java and native interfaces
• Full platform support– x86/x64, SPARC, ARM
– Linux, Solaris, Windows, Mac OS X
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Design
• String class now uses a byte[] instead of a char[]
• Additional 'coder' field indicates which encoding is used
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public final class String {private final byte value[];private final byte coder;...
}
H E L L O
byte value[] = 0x00 0x48 0x00 0x45 0x00 0x4C 0x00 0x4C 0x00 0x4F
byte value[] = 0x48 0x45 0x4C 0x4C 0x4F
UTF-16 encoded
Latin-1 encoded
H E L L O
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Design
• If all characters have a zero upper byte→ String is compressed to Latin-1 by stripping off high order bytes
• If a character has a non-zero upper byte→ String cannot be compressed and is stored UTF-16 encoded
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byte value[] = 0x00 0x48 0x00 0x45 0x00 0x4C 0x00 0x4C 0x00 0x4F
byte value[] = 0x48 0x45 0x4C 0x4C 0x4F
UTF-16 encoded
Latin-1 encoded
InflationCompression
0x47
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Design
• Compression / inflation needs to fast
• Requires HotSpot support in addition to Java class library changes
– JIT compilers: Intrinsics and String concatenation optimizations
– Runtime: String object constructors, JNI, JVMTI
– GC: String deduplication
• Kill switch to enforce UTF-16 encoding (-XX:-CompactStrings)
– For applications that extensively use UTF-16 characters
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Evaluation: Performance
• SPECjbb2005
– 21% footprint reduction
– 27% less GCs
– 5% throughput improvement
• SPECjbb2015
– 7% footprint reduction
– 11% critical-jOps improvement
• Weblogic (startup)
– 10% footprint reduction
– 5% startup time improvement
• To be released with JDK 9
62
Copyright © 2017, Oracle and/or i ts affiliates. All rights reserved.
Outline
• Intro: The HotSpot Java Virtual Machine
• Part 1: What's cool in Java 8
– Background: JIT compilation in HotSpot
– Tiered Compilation
• Part 2: What's new in Java 9
– Segmented Code
– Compact Strings
– Ahead-of-Time Compilation
• Conclusion
63
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Ahead-of-Time Compilation
• Compile Java classes to native code prior to launching the VM
• AOT compilation is done by new jaotc tool
– Uses Java based Graal compiler as backend
– Stores code and metadata in shared object file
• Improves start-up time
– Limited impact on peak performance
• Sharing of compiled code between VM instances
64
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Revisit: Performance of a method (Tiered Compilation)
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Performance
Time
VM Startup
Interpreted C1 compiled C2 compiled
Compilation Compilation
Interpreted C2 compiled
Deoptimization Compilation
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Performance of a method (Tiered AOT)
66
Performance
Time
VM Startup
C2 compiled
Compilation Compilation
Interpreted C2 compiled
Deoptimization Compilation
AOT compiled C1 compiled
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Ahead-of-Time Compilation
• Experimental feature– Supported on Linux x64
– Limited to the java.base module
• Try with your own code - feedback is welcome!
• To be released with JDK 9
– More to come in future releases
67
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Summary
• Many cool features in Java 8– Tiered Compilation
• More features to come with Java 9 in July 2017– Segmented Code Cache, Compact Strings, Ahead-of-Time compilation
• Java – A vibrant platform
– Early access releases are available:
https://jdk9.java.net/download/
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"Our SaaS products are built on top of Java and the Oracle DB—that’s the platform.”Larry Ellison, Oracle CTO
Copyright © 2017, Oracle and/or i ts affiliates. All rights reserved.
Evaluation: Code locality
• Instruction Cache (ICache)– 14% less ICache misses
• Instruction Translation Lookaside Buffer (ITLB1)– 44% less ITLB misses
• Overall performance
– 9% speedup with microbenchmark
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1 caches virtual to physical address mappings to avoid slow page walks
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public class LogLineBench {int size;
String method = generateString(size);
public String work() throws Exceptions {return "[" + System.nanoTime() + "] " +
Thread.currentThread().getName() +"Calling an application method \"" + method +"\" without fear and prejudice.";
}
Microbenchmark: LogLineBench
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LogLineBench results
Performance ns/op Allocated b/op
1 10 100 1 10 100
Baseline 149 153 231 888 904 1680
CS disabled 152 150 230 888 904 1680
CS enabled 142 139 169 504 512 904
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• Kill switch works (no regression)
• 27% performance improvement and 46% footprint reduction