Java Concurrency, A(nother) Peek Under the Hood [Code One 2019]

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, timing, and pricing of any features or functionality described for Oracle’s products may change
and remains at the sole discretion of Oracle Corporation.
Statements in this presentation relating to Oracle’s future plans, expectations, beliefs, intentions and
prospects are “forward-looking statements” and are subject to material risks and uncertainties. A detailed
discussion of these factors and other risks that affect our business is contained in Oracle’s Securities and
Exchange Commission (SEC) filings, including our most recent reports on Form 10-K and Form 10-Q
under the heading “Risk Factors.” These filings are available on the SEC’s website or on Oracle’s website
at http://www.oracle.com/investor. All information in this presentation is current as of September 2019
and Oracle undertakes no duty to update any statement in light of new information or future events.
Safe Harbor
Copyright © 2019 Oracle and/or its affiliates.

Java Concurrency,
A(nother) Peek Under the Hood
[DEV4211]
Principal Member of Technical Staff
Java Platform Group
September 19, 2019
David Buck

IoT Microservices DevOps in the Cloud

JVM Sustaining Engineer
OpenJDK Update Project
Maintainer
JavaOne Rock Star
Co-author of Oracle WebLogic
Server 11g 構築・運用ガイド
@DavidBuckJP
https://blogs.oracle.com/buck/
Who am I? David Buck (left)

Program Agenda
Introduction
Background
Tangent 1: Special Relativity
History
Implementation
Tangent 2: HSDIS
Future
Conclusions
1
2
3
4
5
7
6

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/)]

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

picture: WTF Public License, Version 2

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;
}

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

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

Java Concurrency, A(nother) Peek Under the Hood [Code One 2019]

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

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)]

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

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

Original JMM Keywords
synchronized
mutual exclusion
happened-before

Original JMM Keywords
synchronized
mutual exclusion
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
mutual exclusion
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;
}
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++;
}

64-bit updates are not guaranteed atomic
• Reads and Writes to long and double values may not happen
atomically
• Possible to read a mix of two different writes
Old value MSB New value LSB

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.concurent
JSR-166
Doug Lea’s OSS multithreading library

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

Functional Programming (JDK 8)
Lambda Forms
Stream API
λ

Var Handles
• Lower-level than volatile / final
• Allows a value to be volatile only when we want it to be
• Explicate fence and acquire / release semantics
• Added in JDK 9 to replace several use cases previously only
supported with sun.misc.Unsafe

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

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

Safe Points
Methods proactively poll
JVM makes page unreadable to stop the 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

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

Avoid low-level API
java.util.concurent
synchronized
wait()/notify()
volatile / final
Var handles
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

Testing
As much variety as possible
Various hardware
Various JREs (including version）
Various 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)

[ JSR 133 (Java Memory Model) FAQ ]
https://www.cs.umd.edu/~pugh/java/
memoryModel/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/cook
book.html
[ PrintAssembly ]
https://wiki.openjdk.java.net/display/H
otSpot/PrintAssembly
[ BASIC DISASSEMBLER PLUGIN FOR
HOTSPOT DOWNLOADS ]
https://kenai.com/projects/base-
hsdis/downloads
References

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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, timing, and pricing of any features or functionality described for Oracle’s products may change
and remains at the sole discretion of Oracle Corporation.
Statements in this presentation relating to Oracle’s future plans, expectations, beliefs, intentions and
prospects are “forward-looking statements” and are subject to material risks and uncertainties. A detailed
discussion of these factors and other risks that affect our business is contained in Oracle’s Securities and
Exchange Commission (SEC) filings, including our most recent reports on Form 10-K and Form 10-Q
under the heading “Risk Factors.” These filings are available on the SEC’s website or on Oracle’s website
at http://www.oracle.com/investor. All information in this presentation is current as of September 2019
and Oracle undertakes no duty to update any statement in light of new information or future events.
Safe Harbor

Java Concurrency, A(nother) Peek Under the Hood [Code One 2019]

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