Programming Languages:Design, Specification, and

Implementation

G22.2210-001Rob Strom

November 30, 2006

Readings (week 1)

Theoretical and Historical Papers on Concurrency• The “Readers/Writers” Problem

http://www.md.chalmers.se/~ptrianta/SynC+Co/LIT/lamp77.pdf.gz• Shared Registers (Part II)

http://users.ece.gatech.edu/~dblough/6102/lamport_consistency.pdf• Wait-free synchronization

http://www.cs.brown.edu/~mph/Herlihy91/p124-herlihy.pdf• Partially-ordered time.

http://research.microsoft.com/users/lamport/pubs/time-clocks.pdf Concurrency models in object-oriented languages

• Ada Reference Manual Section 9• Java Reference Manual Chapter 17• Thinking in C++, chapter 11

• http://web.mit.edu/merolish/ticpp/TicV2.html#_Toc53985862

Readings (week 2) Problems with the Memory Model

• Java Memory model is Brokenhttp://www.cs.umd.edu/~pugh/java/broken.pdf

Thread-Safe Programming• Compiler-Enforced Thread-Safety (Guava):

http://researchweb.watson.ibm.com/people/d/dfb/papers/Bacon00Guava.ps • Safe Reader-Writer Parallelism (Optimistic Readers):

http://www.research.ibm.com/distributedmessaging/paper13.html

Programming Project on Concurrency – due Nov. 30 Extend the room world of the previous assignment so that multiple

people can enter rooms and do things• Each thing someone does is an atomic unit of work, e.g.

• Get things from factories and put them in the room• Pick up a bunch of objects and put them in a container• Pick up 2 containers and a funnel and transfer liquid from one container to the other

• Don’t deadlock• Anticipate contention – e.g. you’ve picked up object A and try to pick up object

B, but discover it’s gone because someone else picked up object B. You may need to give up your scripted action.

• The implementation should have the same realism properties as the non-concurrent implementation did: e.g. things shouldn’t spontaneously disappear or appear in multiple places.

You may use any one of C++, Java, or Ada (probably the same language you used for previous assignment, but this is not required)

Programming Languages Core Exam Syntactic issues: regular expressions, context-free grammars (CFG), BNF. Imperative languages: program organization, control structures, exceptions Types in imperative languages: strong typing, type equivalence, unions and

discriminated types in C and Ada. Block structure, visibility and scoping issues, parameter passing. Systems programming and weak typing: exposing machine characteristics, type

coercion, pointers & arrays in C. Run-time organization of block-structured languages: static scoping, activation

records, dynamic and static chains, displays. Programming in the large: abstract data types, modules, packages and namespaces

in Ada, Java, and C++. Functional programming: list structures, higher order functions, lambda expressions,

garbage collection, metainterpreters in Lisp and Scheme. Type inference and ML. Object-Oriented programming: classes, inheritance, polymorphism, dynamic

dispatching. Constructors, destructors and multiple inheritance in C++, interfaces in Java.

Generic programming: parametrized units and classes in C++, Ada and Java. Concurrent programming: threads and tasks, communication, race conditions and

deadlocks, protected methods and types in Ada and Java.

Monitors – Synchronized Methods Threads are active

• Started by calling start() method on a subclass of Thread

Objects are always passive A synchronized method has these properties:

• Critical section: locked before, unlocked after• Lock guaranteed to be released on termination• Causality respected: every action in method follows

the locking; every action after completion of method follows the unlocking.

Protected Objects in Adaprotected type Signal_Object is entry Wait; procedure Signal; function Is_Open return Boolean; private Open : Boolean := False;

end Signal_Object;

protected body Signal_Object is

entry Wait when Open is begin Open := False; end Wait;

procedure Signal is begin Open := True; end Signal;

function Is_Open return Boolean is begin return Open; end Is_Open;

end Signal_Object;

• Behaves like a task that repeatedly loops and accepts entries. But can be implemented without multiple threads.

• But multiple reads (functions) may execute concurrently• Only one write (procedure or entry call) may be active at

once. • But entry calls may queue up if their guard is false• Queued entry calls will run when their guard becomes

true. • They take priority over any calls that enter later.

Wait and Notify A caller invokes a synchronized method: but discovers some

essential condition is false. It issues wait() on the object (You must hold the lock to do this) This releases the lock and lets other callers call other methods When the condition is made true, notifyAll() is called on the

object (or notify() if you want Java to pick a waiter) That terminates the wait(), and lets all waiters (or the selected

waiter) try to compete for the lock again. Other callers may have made the condition false again, so it is

important to check the condition once again! Not guaranteed to be fair (same waiter can lose the battle to

get the lock each time)

Producer/consumer in Javapublic class ProducerConsumer {

int[] Q; // queue of integersint in = 0; // next free index in queueint out = 0; // first used index if count > 0int count = 0; // number of items on queue. // Invariant: in-out (mod Q.length) = count

ProducerConsumer(int size) {Q = new int[size];}

synchronized public void enq (int qe) throws InterruptedException{while (count >= Q.length) {wait();}Q[in] = qe;in = (in+1)%Q.length;if (count++ == 0)

notifyAll(); }synchronized public int deq

() throws InterruptedException {while (count == 0) {wait();}int r = Q[out];out = (out+1)%Q.length;if (count-- == Q.length)

notifyAll();return r;

}}

Memory Model in Java Defines what happens when multiple threads

access the same values without synchronizing How can this happen?

• Through static variables. Static variables are “considered harmful” but not forbidden.

• Through object references that are shared but not synchronized

The results are counter-intuitive, and attempts have been made to fix the model• Too weak a model breaks many programming idioms• Too strong a model inhibits many optimizations

The first sign of trouble: “prescient” stores

Assume thread 1 calls m1, thread 2 calls m2

What traces are possible?a=0, b=0a=0, b=2a=2, b=0? a=2, b=2

If allowed, breaks many assumptions.

If disallowed, prevents many compiler optimizations, and still may fail on some architectures unless barrier synchronizations are used

More bad news for optimizers: “Reads kill”

Suppose p.x = 2 A second thread updates q.x to 3 q is an alias of p Can I see i=2, j=3, k=2? If yes, this violates the original JMM and breaks some

programming idioms If no, this inhibits the ability to save values in registers

Worse news

Suppose P2 saw the new Foo.bar but the old Foo.bar.f!?!?!

Could see old values of things, or current values of previously stored things – a security exposure

Could see values of “final” fields change

What is Guava? Language proposal

• Guarantees thread-safety• Enforces Read/Update distinction

Small syntactic extensions Usually defaults to single-thread Java Translatable to existing JVM byte code Optimizations for efficient GVM No implementation yet

M1

V1O1

O2

O3

O5

O9

O10

O4O6

O8O7

V2

M2

X

Component Model

V11

X

Java and Guava types

Values• built in – primitive types• no classes/methods• no references/sharing• copy by value

Referenced Objects• classes-methods• access by reference• synchronized or not

Referenced Monitors• + always synchronized

Referenced Objects• + never synchronized• – restricted references

Values• + may be user-definable• + classes/methods• + move, copy-by-value

Guava changes: summary – Annotations•synchronized•volatile

+ Annotations•[read], update•[local], global•lent, kept, in Region, new

InstancetoString clone

hashCode equalsgetClass

ObjectMonitor

wait notifyfinalize notifyAll

Local ReferenceMobilecopy

Value

Monitors, Values, ObjectsM1

M2 M3

X: BucketList[]X[0]

X[1]

A 3 . D 9 . E .

B 3 . C 8 .

Y: BucketList[]Y[0]

Y[1]

A 3 . D 9 . E .

B 3 . C 8 .Y[1]Z B 3 . C 8 .

An object has An object has at most one at most one owningowning

monitor/valuemonitor/value

G 1 .

G 1 .G 1 .

Region Analysis: lent, kept

M2

M1

Y

Z

X

M2.foo(X);M2.foo(X);

this.N = P1;this.N = P1;Z.bar(P1,Y);Z.bar(P1,Y);

class M2type extends Monitor {class M2type extends Monitor { . . .. . . void foo (void foo (lentlent Bucket P1); Bucket P1); }} class Ztype extends Object {class Ztype extends Object { . . .. . . void bar (void bar (lentlent Bucket P1, Bucket P1, kept kept Bucket P2);Bucket P2); }}•lentlent = An unknown region = An unknown region

(Default for parameters of non-objects)(Default for parameters of non-objects)•keptkept = Same region as = Same region as thisthis

(Default for parameters of objects)(Default for parameters of objects)

this.A = P1;this.A = P1;this.A = P2;this.A = P2;P1.op();P1.op();P2.N = this;P2.N = this;

Z.bar(Y,P1);Z.bar(Y,P1);

M2

M1

Y

Z

X

M2.foo(X);M2.foo(X);

Region Analysis: new, in R

M2

M1 A

E = M2.m( E = M2.m( A,B,C,D); A,B,C,D);

P1.N = P2;P1.N = P2;P4.N = P3;P4.N = P3;P2.N = P4;P2.N = P4;return new Bucket ( return new Bucket ( 3, null);3, null);

class M2type extends Monitor {class M2type extends Monitor { . . .. . . new new Bucket m (Bucket P1 Bucket m (Bucket P1 in R1, in R1, Bucket P2 Bucket P2 in R1, in R1,

Bucket P3 Bucket P3 in R2, in R2, Bucket P4 Bucket P4 in R2in R2););

}}

•new new = No region= No region•in Rin R = Same region as other = Same region as other

parameters labeled parameters labeled in Rin R

B

C

D

EE

Update and Global A read method:

• Doesn’t update instance variables (region=this)• Doesn’t update anything synchronized• Otherwise, may update parameters

To check this, interfaces annotate• if a method isn’t a read method• if a parameter is modified• if a method is global (gets a lock)