Monitors. Lecture 6. A Typical Monitor State. wait(c) Signal and Continue. Signal and What Happens Next?

Transcription

1 Monitos Lectue 6 Monitos Summay: Last time A combination of data abstaction and mutual exclusion Automatic mutex Pogammed conditional synchonisation Widely used in concuent pogamming languages and libaies Java, ptheads, C#, Today Moe monito synchonisation Poblem solving using monitos in Java 2 A Typical Monito State wait(c) bounday (mutex) queue max 1 pocess executing code p c p c q q d pocesses blocked on c s queue d blocks calle on c s queue Running pocess Blocked pocess 3 4 Signal and What Happens Next? Signal and Continue bounday queue B SC: B=W<S waiting queue W unblocked pocess joins the bounday queue at the end p c p c q q d d signaling pocess S calls signal(c) signaling pocess continues 5 6 1

2 Java and Monitos Baie Monito Uneliable The essence of a monito is the combination of data abstaction class mutual exclusion synchonized condition vaiables default: implicit, one pe object opeations fo blocking and unblocking on condition vaiables included in Object Simple but not 100% eliable solution Spuious wakeup possible public synchonized void await() aived++; if (aived < N) else { aived = 0; 7 8 Baie Monito Outline Baie Monito One-Shot Dealing with spuious wakeup Waiting must be always inside a while loop public synchonized void await() aived++; if (aived < N) { while (...) else { aived = 0; public synchonized void await() aived++; if (aived < N) { while (!done) else { done = tue; aived = 0; 9 10 Baie Monito Taking Tuns Baie Monito Init public synchonized void await() aived++; if (aived < N) { final Tun mytun = tun; while (mytun == tun) else { tun = new Tun(); aived = 0; public class CyclicBaie { pivate int aived = 0; pivate int N; pivate static class Tun { pivate Tun tun; public CyclicBaie(int N) { this.n = N; tun = new Tun(); //pevious slide

3 Java 5 and Monitos Reades/Wites Poblem The essence of a monito is the combination of data abstaction class mutual exclusion explicit i locking package java.util.concuent.locks condition vaiables unlimited opeations fo blocking and unblocking on condition vaiables Eithe one wite o multiple eades, but not both Which tains ae Reades Wites Semaphoes Passing the baton Reades/Wites Poblem Reades/Wites Monito Anothe classic synchonisation poblem Two kinds of pocesses shae access to a database Reades examine the contents Multiple eades allowed concuently Wites examine and modify A wite must have mutex Invaiant (n==0 nw==0) nw<=1 Database is globally accessible Cannot be intenal to monito (citical section!) Encapsulate only the access potocol public inteface ReadesWites { public void statread() thows InteuptedException; public void endread(); public void statwite() thows InteuptedException; public void endwite(); Reades/Wites Monito Notation Maco Stat with an easie non-fai solution public class RWContolle implements RW { pivate final Lock lock = new ReentantLock(); pivate final Condition oktoread = lock.newcondition(); pivate final Condition oktowite = lock.newcondition(); public method( ) thows { lock.lock(); Thead ct = Thead.cuentThead(); ty { //Nomal main code hee finally { lock.unlock(); pivate int n = 0; pivate int nw = 0; //next slides SYNC method( ) thows { //Nomal main code hee

4 Reades/Wites Reading Signal a wite afte all eades left SYNC void statread() thows IE { while (nw > 0) oktoread.a n++; Reades/Wites Witing On leave: signal a wite and all eades SYNC void statwite() thows IE { while (n > 0 nw > 0) oktowite.a nw++; SYNC void endread() { n--; if (n == 0) oktowite.signal(); n==nw==0 SYNC void endwite() { nw--; oktowite.signal(); oktoread.signalall(); n==nw== Analysis Fainess Consideations Stavation Reades can continuously ead Waiting wites will not be woken R R R R R w Suitable policy? Fo example: No new eades when a wite is waiting Change tuns in some way Stict ode of aival Pefomance Fainess often equies moe book-keeping Depends highly on platfom Java signalall() might be inevitable fo condition echecking Fai Reades/Wites Reades/Wites Reading No new eades when a wite is waiting Avoids stavation Spuious wakeup can spoil fainess Quite efficient Count delayed eades and wites Simila to semaphoe passing the baton solution pivate int d = 0; pivate int dw = 0; We only need to modify statread() One exta fainess condition endread() is exactly the same SYNC void statread() thows IE { if (nw > 0 dw >0) { d++; do oktoread.a while (nw > 0); d--; n++;

5 Reades/Wites Witing Reades/Wites Witing One exta fainess condition SYNC void statwite() thows IE { if (n > 0 nw > 0 d > 0) { dw++; do oktowite.a while (n > 0 nw > 0); dw--; nw++; On leave: Faily signal a eades, o One wite SYNC void endwite() { nw--; if (d > 0) oktoread.signalall(); else oktowite.signal(); Fai Reades/Wites Special Pais Stict ode of aival Faiest Least efficient at least in Java spuious wakeup Maintain queue of theads as they aive We also need to know thei type enum Type {Reade, Wite; class Pai { Queue<Pai> w = new AayDeque<Pai>(); pivate class Pai { public Type type; public Thead thead; public Pai(Type type, Thead thead) { this.type = type; this.thead = thead; public boolean equals(object obj) { if (obj instanceof Pai) etun thead.equals( ((Pai)obj).thead); else etun false; Reades/Wites Reading Reades/Wites Reading SYNC void statread() thows IE { while (nw > 0 (!w.isempty() &&!ct.equals(w.peek().thead))) { if (!w.contains( new Pai(Type.Reade,ct))) w.add(new Pai(Type.Reade,ct)); oktoread.a w.poll(); n++; if (!w.isempty() && w.peek().type == Type.Reade) oktoread.signalall(); Signal all wites afte all eades left Since thee ae no moe eades thee must be a waiting wite Wake up all to find the one waiting longest SYNC void endread() { n--; if (n == 0) oktowite.signalall();

6 Reades/Wites Witing Reades/Wites Witing SYNC void statwite() thows IE { while (n > 0 nw > 0 (!w.isempty() &&!ct.equals(w.peek().thead))) { if (!w.contains( new Pai(Type.Wite,ct))) w.add(new Pai(Type.Wite,ct)); oktowite.a w.poll(); nw++; On leave: wake up only the appopiate pocess type to find the fist SYNC void endwite() { nw--; if (!w.isempty() && w.peek().type == Type.Reade) oktoread.signalall(); else oktowite.signalall(); Resouce Allocation Single Resouce Allocato PtC A contolle contols access to copies of some esouce Clients make equests to take (acquie) o etun (elease) one esouce A equest should only succeed if thee is a esouce available, Othewise the equest must block monito ResouceAllocato<E> { pivate Condition fee; pivate int avail = N; pivate Queue<E> units = public E acquie() { if (avail == 0) wait(fee); else avail--; etun units.emove(); public void elease(e e) { units.add(e); if (empty(fee)) avail++; else signal(fee); Resouce Allocation Java Resouce Allocation Multiple public class ResouceAllocato<E> { pivate Queue<E> units = ; public synchonized E allocate() while (units.size() == 0) etun units.emove(); public synchonized void elease(e e) { units.add(e); notify(); Clients equiing multiple esouces should not ask fo esouces one at a time Why would this be bad? A contolle contols access to copies of some esouce Clients make equests to take o etun any numbe of the esouces A equest should only succeed if thee ae sufficiently many esouces available, Othewise the equest must block

7 Resouce Allocation Multiple Synchonisation Shootout public class ResouceAllocato<E> { pivate Queue<E> units = ; public sync Set<E> allocate(int n) while (units.size() < n) etun take(n); public sync void elease(set<e> et) { units.addall(et); Semaphoes vs Monitos Semaphoes Efficient Expessive: any synchonisation (await-statement) Easy to implement Monitos Can monitos implement semaphoes? Impotant theoetical question An illustative example, but not nomal pactice Implementing a low-level language constuct in a high-level language is not nomally a good idea Can semaphoes implement monitos? Implementing Monitos Monito Enty public class MonitoImpl { pivate Semaphoe e = new Semaphoe(1, tue); pivate Map<Thead,Semaphoe> semmap = new HashMap<Thead,Semaphoe>(); potected void lock(); potected void unlock(); potected CV newcv(); potected void wait(cv cv) thows IE; potected void signal(cv cv); Binay semaphoe (lock) fo enty potected void lock() { e.acquieuninteuptibly(); potected void unlock() { e.elease(); Condition Vaiables wait(cv) A condition vaiable is a queue of theads Pivate semaphoe fo blocking public inteface CV extends Queue<Thead> { pivate class CVImpl extends AayDeque<Thead> implements CV { potected CV newcv() { etun new CVImpl(); potected void wait(cv cv) thows IE { cv.add(thead.cuentthead()); Semaphoe e wait = new Semaphoe(0); e(0); semmap.put(thead.cuentthead(), wait); unlock(); ty { wait.acquie(); finally { lock();

8 signal(cv) Monitos vs Semaphoes Signal the fist waiting thead on its pivate semaphoe Semaphoes can implement monitos Monitos can implement semaphoes potected void signal(cv cv) { if (cv.size() > 0) { Thead waite = cv.emove(); Semaphoe wait = semmap.get(waite); wait.elease(); The same expessive powe Can implement any await statement Impotant theoetical esult Nested Monito Calls Nested Monito Calls What happens if monito A calls monito B? A Fou appoaches Ban nested calls Release A s lock when enteing B Moe concuency B Fou appoaches continued: Maintain lock on A while in B; wait( ) in B eleases both locks Maintain lock on A while in B; etun to A on leaving B wait( ) in B eleases only B s lock less concuency can lead to deadlock easie to eason about safety popeties Odeing access The Java Case Recusion The Java Case Fist a special case What if A and B ae the same object? Reentant lock can be e-locked safely public class Reentant { public synchonized void a() { b(); System.out.pintln(TN+" in a()"); public synchonized void b() { System.out.pintln(TN+" in b()");... This woks in a simila way acoss multiple objects Theads collect the locks as they go If they aleady have the lock on the object then they ypoceed A wait( ) opeation eleases the lock fo the cuent object only. Othe locks ae still held. Note: this means that you will block while holding locks a good chance to deadlock!

9 The Java Case GUI Famewoks The same ules apply to eentant locks in package java.util.concuent.locks Note: collecting locks means that you will block while holding locks A good chance to deadlock! Pogamming discipline helps Remembe the dining philosophes? Odeing access/calls can help avoiding cicula waiting AWT Attempted to be thead-safe Result: deadlocks possible Swing Abandons thead-safety in geneal One main event-dispatching thead uns all Swing activity Some thead-safe methods ae povided Fo example: epaint() Swing Swing Example Thead-safe Swing Opeations modifying Swing components must un in the event-dispatching thead SwingUtilities.invokeLate(Runnable invokelate(runnable dorun) SwingUtilities.invokeAndWait(Runnable dorun) thows InteuptedException, InvocationTagetException Cemona JFlash window class JFlash public JFlash(Sting asting) { supe(asting); public void un() { // Pepae all the intenal components pack(); setvisible(tue); main thead SwingUtilities.invokeLate(window); Summay Java Next Time Monito based Signal and Continue semantics Native Java synchonized methods One implicit condition vaiable Java 5 Fully fledged monitos But moe explicit pogamming Shaed-memoy pogamming Only fo the insane pogamme? Message passing Could this be the holy gail of concuent pogamming? Fist look at the possibilities