https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation A practical approach to Java Memory Model A practical approach to Java Memory Model Andrzej Czarny https://www.ocadotechnology.com/ 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Ocado Technology Senior Software Engineer - Simulation Algorithm Development 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Contents • Out of order execution • Introduction to JCStress • Basic JMM concepts and definitions • Examples/Puzzles • Future of JMM 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Out of order execution • Compiler optimizations • Hardware CPU cache optimization 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation public class JITOptimization { private static boolean flag = true; public static void main(String ... args) throws InterruptedException { new Thread(JITOptimization::loop).start(); Thread.sleep(1000); new Thread(JITOptimization::terminate).start(); } public static void terminate() { flag = false; System.out.println("Flag has been changed"); } public static void loop() { boolean printOnce = true; while (getFlag()) { if (printOnce) { printOnce = false; System.out.println("I'm in the loop"); } } System.out.println("I'm outside of the loop"); } public static boolean getFlag() { return flag;} } JIT optimization 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation public class JITOptimization { private static boolean flag = true; public static void main(String ... args) throws InterruptedException { new Thread(JITOptimization::loop).start(); Thread.sleep(1000); new Thread(JITOptimization::terminate).start(); } public static void terminate() { flag = false; System.out.println("Flag has been changed"); } public static void loop() { boolean flag = getFlag(); while (flag) { } } public static boolean getFlag() { return flag;} } JIT optimization 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Demo 1 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Processor Symmetric multiprocessing Memory C-1 C-2 C-3 C-4 C-5 Thread 2 C-0 Thread 1 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Processor OutOfOrder execution C-0 Execution Engine Store Buffer L1 L2 L3 C-5 L S Load Buffer Memory r2 = x y:524 Coherence Protocol x = 168 store r1 = y load 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Processor OutOfOrder execution C-0 Execution Engine Store Buffer L1 L2 L3 C-5 L S Load Buffer Memory r2 = x y:524 x = 168 store r1 = y load Coherence Protocol 168 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Processor OutOfOrder execution C-0 Execution Engine Store Buffer L1 L2 L3 C-5 L S Load Buffer Memory r2 = x x = 168 store r1 = y load Coherence Protocol 168 168 168 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Hardware CPU cache optimization Type Alpha ARMv7 POWER SPARC x86 LoadsLoads mb dmb lwsync LoadsStores mb dmb hwsync StoresStores wmb dmb-st lwsync StoresLoads mb dmb hwsync membar mfence locked insn Weak memory model Strong memory model 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation JCStress Tutorial The Java Concurrency Stress tests (jcstress) is an experimental harness and a suite of tests to aid the research in the correctness of concurrency support in the JVM, class libraries, and hardware. http://openjdk.java.net/projects/code-tools/jcstress/ 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Test Example public class SomeTest { @JCStressTest @State @Outcome(id="1", expect=Expect.ACCEPTABLE_INTERESTING, desc="special case") public static class Test1 { @Actor public void actor1(II_Result r) {} @Arbiter public void arbiter(II_Result r) {} } @JCStressTest(Mode.Termination) @State public static class Test2 { @Actor public void actor1(II_Result r) {} @Signal public void signal() {} } } 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation StoreLoad reordering @JCStressTest @State public class StoreLoad { int x, y; @Actor public void actor1(II_Result r) { x = 1; //store r.r1 = y; //load } @Actor public void actor2(II_Result r) { y = 1; //store r.r2 = x; // load } } 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Demo 2 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation “The Java Memory Model was the first attempt to provide a comprehensive threading memory model for a popular programming language.” https://en.wikipedia.org/wiki/Memory_model_(programming) Java Memory Model 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation JMM history • Before JSR133 – Not possible to create thread-safe immutable objects – Reordering volatile and non volatile fields • JSR133 – Java 1.5 – Thread-safe immutable object – More powerful volatile 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Synchronized-with relations unlock action on monitor x write to a volatile variable x action that starts a thread final action in a thread all subsequent lock actions on x all subsequent reads of x first action in a thread any action that detects that a thread has terminated Thread A Thread B 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Happens-before Order Thread A Operation 1 Operation 2 Operation 3 synchronized(y) Operation 4 Thread B Operation write volatile z Operation 2 read volatile x synchronized(v) write volatile x Operation 3 Operation 4 Operation 5 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Happens-before Order Order for Thread B Operation 1, 2 results Operation 1 write volatile z Operation 2 read volatile x synchronized(v) Operation 3 write volatile x Thread A Operation 1 Operation 2 Operation 3 synchronized(y) Operation 4 Thread B Operation 1 write volatile z Operation 2 read volatile x synchronized(v) write volatile x Operation 3 Operation 4 Operation 5 Operation 4 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Happens-before Order Order for Thread B Operation 1 write volatile z Operation 2 read volatile x synchronized(v) Operation 3 write volatile x Thread A Operation 1 Operation 2 Operation 3 synchronized(y) Operation 4 Thread B Operation 1 write volatile z Operation 2 read volatile x synchronized(v) write volatile x Operation 3 Operation 4 Operation 5 Operation 4 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation final Fields Semantics 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Thread-safe immutable object @JCStressTest @State public class AmIImmutable { static AmIImmutable i = new AmIImmutable(); int x, y, z; AmIImmutable() { x = 1; y = 2; z = 3; } @Actor public void actor1() { i = new AmIImmutable(); } @Actor public void actor2(III_Result r) { AmIImmutable tmp = i; r.r1 = tmp.x; r.r2 = tmp.y; r.r3 = tmp.z; } } 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Demo 3 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Thread-safe immutable object @JCStressTest @State public class Escape { volatile static IntSupplier supplier; static Escape escape = new Escape(); final int x; public Escape() { supplier = () -> getX(); x = 1; } int getX(){return x;} @Actor public void actor1() { escape = new Escape(); } @Actor public void actor2(II_Result r) { r.r1 = supplier.getAsInt(); r.r2 = escape.getX(); } } 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Demo 4 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation volatile 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Thread-safe immutable object @JCStressTest @State public class AmISynchronized { int y = 1; volatile int x = 1; @Actor public void actor1() { y = 2; x = 3; } @Actor public void actor2(I_Result r) { r.r1 = y * x; } } 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Demo 5 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation What next • JEP 188: Java Memory Model Update – Improve formalization – JVM coverage – Extend scope (AtomicX.weakCompareAndSet) – C11/C++11 compatibility – Implementation guidance. JVM implementors – Testing support – Tool support (static analysis) 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Summary Never ignore concurrency issues! Use highest level of abstraction Check framework requirements regarding concurrency Code against specification not implementation Always use final on immutable classes 

https://github.com/amczarny/JMMPresentation https://github.com/amczarny/JMMPresentation Q & A