Courses: concurrency #2

Java Concurrency #2Sergey Lukjanov, 2012

[email protected]

Sunday, November 4, 12

mailto:[email protected]

mailto:[email protected]

JMM / what & why?


JMM / what & why?• JVM observes within-thread as-if-serial semantics;


JMM / what & why?• JVM observes within-thread as-if-serial semantics;• JMM defines how threads interact through memory;


JMM / what & why?• JVM observes within-thread as-if-serial semantics;• JMM defines how threads interact through memory;• JMM answers the main question: • “When the thread will see changes done in the

other one?”


JMM / what & why?• JVM observes within-thread as-if-serial semantics;• JMM defines how threads interact through memory;• JMM answers the main question: • “When the thread will see changes done in the

other one?”

• why we need it?• write correct applications;• avoid over synchronizations.


Variable visibility

x = y = 0

Thread 1 Thread 2a = xy = 1

b = yx = 1

a = ?b = ?


Variable visibility

x = y = 0

Thread 1 Thread 2a = xy = 1

b = yx = 1

a = 0; b = 0;a = 0; b = 1;a = 1; b = 0;a = 1; b = 1;


Reordering


Reordering• compiler can optimize ops order (w/o changing

application within-thread as-if-serial semantics);



application within-thread as-if-serial semantics);• CPU can execute commands in the other order in

some cases;




some cases;• cache can store values back to the main memory in

not the same order as it was written by application;




some cases;• cache can store values back to the main memory in

not the same order as it was written by application;• etc.


Happens-before relationship


Happens-before relationship• it is a guarantee that memory written to by statement

A is visible to statement B, that is, that A completes its write before statement B starts its read;



A is visible to statement B, that is, that A completes its write before statement B starts its read;• A happens-before B := hb(A, B);



A is visible to statement B, that is, that A completes its write before statement B starts its read;• A happens-before B := hb(A, B);• transitive: hb(x, y) & hb (y, z) => hb(x, z).


Will T2 see any changes done by T1? #1

Thread 1⇓

y = 2⇓

lock M1⇓

x = 1⇓

unlock M1⇓

z = 3⇓

Thread 2⇓

lock M2⇓

temp = x⇓

unlock M2⇓

temp2 = y⇓

temp3 = z⇓



Thread 1⇓

y = 2⇓

lock M1⇓

x = 1⇓

unlock M1⇓

z = 3⇓

Thread 2⇓

lock M1⇓

temp = x⇓

unlock M1⇓

temp2 = y⇓

temp3 = z⇓



Thread 1⇓

y = 2⇓

volatile M1⇓

x = 1⇓

write M1⇓

z = 3⇓

Thread 2⇓

read M1⇓

temp = x⇓

temp2 = y⇓

temp3 = z⇓



Thread 1⇓

y = 2⇓

lock M1⇓

x = 1⇓

unlock M1⇓

z = 3⇓

Thread 2⇓

if(!t1.isAlive())⇓

temp = x⇓

temp2 = y⇓

temp3 = z⇓



Thread 1⇓

y = 2⇓

x = 1⇓

t2.interrupt()⇓

z = 3⇓

Thread 2⇓

Thread.interrupted()⇓

temp = x⇓

temp2 = y⇓

temp3 = z⇓


Happens-before rules


Happens-before rules• each action A in thread happens-before each action B (that placed after A in code) in the same thread;



• unlock monitor happens-before any following lock on the same monitor;




• write into volatile var happens-before any following read from the same volatile var;





• `Thread.start()` happens-before first action in the started thread;





• `Thread.start()` happens-before first action in the started thread;

• last action in thread1 happens-before any action in thread2 if thread2 detects that thread1 ends by invoking `thread1.join()` or if `thread1.isAlive()` invoked and `false` returned;


Happens-before rules #2


Happens-before rules #2• constructor happens-before finalizer;



• thread interruption happens-before place, where thread detects that it has been interrupted;



• thread interruption happens-before place, where thread detects that it has been interrupted;

• some more rules...


Reordering: synchronized

...synchronized (this) {

...

...} // end of synch...


Reordering: volatile

...volatile write...

...volatile read...


Volatile guarantees


Volatile guarantees• happens-before;



• reordering;



• reordering;

• atomic read/write (including Long and Double at i386)


Examples


Courses: concurrency #2

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