Java Concurrency, A(nother) Peek Under the Hood [JavaOne 2016 CON1497]

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Java Concurrency
A(nother) Peek Under the Hood
David Buck
Principal Member of Technical Staff
Java SE Sustaining Engineering
September, 2016

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.
4

About Me
David Buck
• Java SE Sustaining Engineering
• Mostly JRockit fixes
• OpenJDK 8 Updates
Project Maintainer
• Hobbies: Programming

Program Agenda
Introduction
Background
Tangent 1: Special Relativity
History
Implementation
Tangent 2: HSDIS
Future
Conclusions
1
2
3
4
5
6
6

Introduction

Motivation
“But I don’t write multithreaded code…”

Web Server
picture: Coolcaesar at the English language Wikipedia [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]

ＧＵＩ

Libraries
picture: David Vignoni / ICON KING (http://icon-king.com) [LGPL (http://www.gnu.org/licenses/lgpl.html) or LGPL (http://www.gnu.org/licenses/lgpl.html)]

Batch Processing
picture: US Social Security Administration (http://www.ssa.gov/history/acalcs.html) [Public domain]

“But I don’t write multithreaded code…”

Race Conditions
picture: Sakurambo at English Wikipedia [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]

Heisenbugs
Bug Heisenberg
Heisenbug
picture: Bundesarchiv, Bild 183-R57262 / Unknown / CC-BY-SA 3.0 [CC BY-SA 3.0 de (http://creativecommons.org/licenses/by-sa/3.0/de/deed.en)]

Observer Effect
picture: Christian Schirm (Own work) [CC0]

Avoiding Heisenbugs
• Java Memory Model
• synchronized keyword
• java.util.concurrent

Avoiding Heisenbugs
• Java Memory Model
• synchronized keyword
• java.util.concurrent
• Don’t do ＸＸＸ
• You must do ＹＹＹ

a² + b² == c²

Background

Memory Model

Memory Model
Definition 1
Specification that explains when we can guarantee that a written value may
be read correctly

void hogeMethod1() {
int localA = 42;
assert localA == 42;
}

static int staticA;
void hogeMethod2() {
staticA = 42;
assert staticA == 42;
}

int data = 0;
boolean ready = false;
void hoge3() {
while (!ready) {};
assert data == 42;
}
void hoge4() {
data = 42;
ready = true;
}

Culprit #1

Culprit #2

Memory Ordering
void hoge5() {
a = 1;
b = 2;
c = 3;
d = 4;
e = 5;
a = a + 1;
}

Memory Ordering
void hoge5() {
a = 1;
b = 2;
c = 3;
d = 4;
e = 5;
a = a + 1;
}
void hoge5() {
a = 2;
b = 2;
c = 3;
d = 4;
e = 5;
}

Memory Ordering
void hoge5() {
a = 1;
b = 2;
c = 3;
d = 4;
e = 5;
a = a + 1;
}
void hoge5() {
b = 2;
c = 3;
d = 4;
e = 5;
a = 2;
}

Rule
Single threaded behavior must never change

Out of Order Execution
picture: Amit6, original version (File:Superscalarpipeline.png) by User:Poil (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)]

CPU Differences
Type Alpha ARMv7
PA-
RISC
POWER
SPARC
RMO
SPARC
PSO
SPARC
TSO
x86
x86
oostore
AMD64 IA-64 zSeries
load-
load
Y Y Y Y Y Y Y
load-
store
Y Y Y Y Y Y Y
store-
store
Y Y Y Y Y Y Y Y
store-
load
Y Y Y Y Y Y Y Y Y Y Y Y
Atomic
(loads)
Y Y Y Y Y
Atomic
(stores)
Y Y Y Y Y Y
Depend
ent
loads
Y
instruct
ion
cache
Y Y Y Y Y Y Y Y Y Y
chart source: https://en.wikipedia.org/wiki/Memory_ordering

CPU Differences
Loose Strict
Alpha
X86

CPU Differences
Loose Strict

Memory Barriers
int data = 0;
boolean ready = false;
void hoge3() {
while (!ready) {};
assert data == 42;
}
void hoge4() {
data = 42;
ready = true;
}

Memory Barriers Types
• store-store
• store-load
• load-store
• load-load

Compiler Barriers
• ＧＣＣ
– __asm__ __volatile__("":::"memory");
• ＶＣ＋＋
– _ReadBarrier
– _WriteBarrier
– _ReadWriteBarrier

picture: Sakurambo (Own work) [Public domain]

A
B
C

A
B
C
A, B, C

A
B
C
A, B, C C, B, A

Observed order depends on viewer's frame of reference
A
B
C
A, B, C C, B, A

No single “correct” timeline
• Special Relativity
– Observed order of events depends on frame of reference

No single “correct” timeline
• Special Relativity
– Observed order of events depends on frame of reference
• Multithreaded Code
– Observed order of events depends on thread

Memory Model
Definition 2
• Clarify what multithreaded behavior developers may depend on

Memory Model
Definition 2
• Clarify what multithreaded behavior developers may depend on
• Limits what types of optimizations the runtime may do

History

Java Memory Model (JMM)
• Write once, run anywhere
• 1995
• Part of the formal language
specification
• Happened-before
picture: Peter Campbell [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 4.0-3.0-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/4.0-3.0-2.5-2.0-1.0)]

happened-before

Leslie Lamport
• LaTex
• happened-before
picture: Leslie Lamport [GFDL (http://en.wikipedia.org/wiki/GFDL]

happened-before
void hoge5() {
a = 1;
b = 2;
c = 3;
d = 4;
e = 5;
a = a + 1;
}

happened-before
void hoge4() {
data = 42;
ready = true;
}
void hoge3() {
while (!ready) {};
assert data == 42;
}

happened-before
void synchronized hoge4()
{
data = 42;
ready = true;
}
void synchronized hoge3()
{
while (!ready) {};
assert data == 42;
}
Warning: if hoge3 is executed first, the above code will deadlock

JMM Specification

Original JMM Keywords
• synchronized
– mutual exclusion
– happened-before

Original JMM Keywords
• synchronized
– happened-before
• volatile
– Visibility

Problems with the Original JMM
• volatile does not establish a happened-before relationship
• final values can change
• Many important runtime optimization were not allowed

Doug Lea
• Author of the Java Multithreaded
bible
• Introduced his own OSS high-level
multithreaded library

Java SE 5.0
• Modern JMM introduced
• Ｌｅａ’s library added to official JDK
picture: Zvi Roger [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)]

Modern JMM
• JSR-133
• Fixed shortcomings in original JMM
– volatile does establish happens-before
– final values will never change

Modern JMM Keywords
• synchronized
– happened-before
• volatile
– visibility
– happened-before
• final
– immutability

JSR-133 の volatile
void hoge4() {
data = 42;
ready = true;
}
void hoge3() {
while (!ready)
{};
assert data ==
42;
}
Warning: if hoge3 is executed first, the above code will deadlock
volatile boolean ready = false;

volatile != atomic
volatile int id = 0;
int incID() {
return id++;
}

volatile != atomic
int incID() {
return id++;
}
reg = [id]
reg++
[id] = reg

volatile != atomic
synchronized int incID() {
return id++;
}

happened-before
• Monitor (acquisition / release)
• Volatile (read / write)
• final “freeze” (constructor)
• JNI
– System.out.println（）
– Input / Output
• thread start()/join()
• operations on j.u.concurrent collections

java.util.concurency
• JSR-166
• Doug Lee’s OSS multithreading library

Multithreaded Control
• java.util.concurency
• synchronized
– wait()/notify()
• volatile / final
Abstract
Low-level

Fork/Join (JDK 7)

Functional Programming (JDK 8)
• Lambda Forms
• Stream API
λ

Implementation

Tangent 2: HSDIS (HotSpot Disassembler)
• Lets us see code generated by JIT
• Requires a disassembler plugin
– GNU binutils-based plugin
– base-hsdis
• Command line options
– +PrintAssembly
– +CompileCommand=print,*MyClass.myMethod

Demos

ARM Assembly Crash Course

Op Target Source

MOV r02 #42

MOV r02 #42
r02 = 42

Op Target Source
Exception
STR Source Target

MOV
Copies registers
MOV r02 r03
Writes Literals
MOV r02 #42

LDx
Loads data from memory to a register
ldr r03, [r06]
r03 = *r06

CMP / BEQ
Compare and branch if equal
cmp r0, #0
beq 0x74178d08
if (r0 == 0) goto 0x74178d08

DMB
Data Memory Barrier
dmb st
dmb sy

One more thing…

Safe Points
• Methods proactively poll
• JVM makes page unreadable to stop world
movw ip, #45056 ; 0xb000
movt ip, #30461 ; 0x76fd
ldr ip, [ip] ; {poll_return}

Long Value
static long val = 0;
public static void main(String[] args) {
new Thread(() -> { while(true) val = -1L; } ).start();
new Thread(() -> { while(true) val = 0L; } ).start();
while (true) {
long temp = val;
if ( temp != -1 && temp != 0 ) {
System.out.println("temp (DEC): " + temp);
System.out.println("temp (BIN): " + Long.toBinaryString(temp));
System.exit(-1);
}
}
}
}

Object Initialization
public class ObjInit {
static ObjInit globalRef = new ObjInit();
long number;
public ObjInit() {
number = 42;
}

Object Initialization
new Thread(()
-> { while(true) {
long temp = globalRef.number;
if (temp != 42) {
System.out.println(temp);
System.exit(-1);
}
}
} ).start();
new Thread(()
-> { while(true) globalRef = new ObjInit(); } ).start(); }}

public class Loop extends Thread {
static boolean done = false;
Thread childThread = new Loop();
childThread.start();
System.out.println("Starting loop");
spinWait();
System.out.println("Exited loop");
}
private static void spinWait() {
while (!done) ;
}
public void run() {
try {
Thread.sleep(5 * 1000);
} catch (Exception e) {}
done = true;
}
} // class Loop
Loop

Future

JEP-188/JMM9
• Not planned for JDK 9 (JMM9 != JDK 9)
• JVM-level specification
• Compatibility with C11/C++11
• Cover new features added since ＪＤＫ 5
– (e.g. AtomicX.weakCompareAndSet())

Conclusions

Avoid low-level API
• java.util.concurency
• synchronized
– wait()/notify()
• volatile / final
Abstract
Low-level

Code against the specification
• Intermittent bugs are common
• Behavior changes with JRE / platform
• Stuff to avoid
– synchronized (new Object()) { }
– Randomly adding volatile declarations until it magically starts working

Testing
• As close to production environment as possible
– Same hardware
– Same JRE (including version）
– Same settings

Java Concurrency in Practice
• Java Concurrency Bible
• Covers JSR-133 / JSR-166

Summary
• Avoid low-level APIs
• Code against the standard, not the implementation
• Testing is always indispensible
• Must read Java Concurrency in Practice (JCiP)

Thank You!!!

• [ JSR 133 (Java Memory Model) FAQ ]
https://www.cs.umd.edu/~pugh/java/memoryMo
del/jsr-133-faq.html
• [ Java Concurrency in Practice ]
http://jcip.net/
• [ Concurrent Programming in Java ]
http://gee.cs.oswego.edu/dl/cpj/
• [ The JSR-133 Cookbook for Compiler Writers ]
http://gee.cs.oswego.edu/dl/jmm/cookbook.html
• [ PrintAssembly ]
https://wiki.openjdk.java.net/display/HotSpot/Pri
ntAssembly
• [ BASIC DISASSEMBLER PLUGIN FOR HOTSPOT
DOWNLOADS ]
https://kenai.com/projects/base-
hsdis/downloads
References

Safe Harbor Statement
The preceding 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.
108

Java Concurrency, A(nother) Peek Under the Hood [JavaOne 2016 CON1497]

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