Concurrent Programming

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Ronald Nichols
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1 Cncurrent Prgramming Cncurrency: Crrectly and efficiently managing access t shared resurces frm mul7ple pssibly-simultaneus clients Requires crdina(n, par7cularly synchrniza7n t avid incrrect simultaneus access: make smebdy blck Want t blck un7l anther thread is dne using what we need nt cmpletely dne execu7ng Even crrect cncurrent applica7ns are usually highly nn-determinis7c: hw threads are scheduled affects what pera7ns frm ther threads they see when nn-repeatability cmplicates tes7ng and debugging 1

2 Examples Mul7ple threads: 1. Prcessing different bank-accunt pera7ns What if 2 threads change the same accunt at the same 7me? 2. Using a shared cache f recent files (e.g., hashtable) What if 2 threads insert the same file at the same 7me? 3. Crea7ng a pipeline (think assembly line) with a queue fr handing wrk t next thread in sequence? What if enqueuer and dequeuer adjust a circular array queue at the same 7me? 2

3 Java Cncurrency The java.lang.runnable interface vid run(); The java.lang.thread class Thread(Runnable r); vid start(); sta7c vid sleep(lng millis); vid jin(); blean isalive(); sta7c Thread currentthread();

4 Atmicity An ac7n is atmic if it is indivisible Effec7vely, it happens all at nce N effects f the ac7n are visible un7l it is cmplete N ther ac7ns have an effect during the ac7n In Java, integer increment (i++) is nt atmic 1. Lad data frm variable 2. Increment data by 1 3. Stre data t variable i

5 Race Cndi7n A race cndi7n is when mul7ple threads access shared data and unexpected results ccur depending n the rder f their ac7ns

6 Why threads? Unlike parallelism, nt abut implemen7ng algrithms faster But threads s7ll useful fr: Cde structure fr respnsiveness Example: Respnd t GUI events in ne thread while anther thread is perfrming an expensive cmputa7n Prcessr u(liza(n (mask I/O latency) If 1 thread ges t disk, have smething else t d Failure isla(n Cnvenient structure if want t interleave mul7ple tasks and d nt want an excep7n in ne t stp the ther 6

7 Sharing resurces It is cmmn in cncurrent prgrams that: Different threads might access the same resurces in an unpredictable rder r even at abut the same 7me Prgram crrectness requires that simultaneus access be prevented using synchrniza7n Simultaneus access is rare Makes tes7ng difficult Must be much mre disciplined when designing / implemen7ng a cncurrent prgram Will discuss cmmn idims knwn t wrk 7

8 Alice, Bb Bill and the Bank A. Alice t pay Bb $30 Bank actins A1. Des Alice have $30? A2. Give $30 t Bb A3. Take $30 frm Alice B. Alice t pay Bill $30 Bank actins B1. Des Alice have $30? B2. Give $30 t Bill B3. Take $30 frm Alice If Alice starts with $40, can Bb and Bill bth get $30?

9 Alice, Bb Bill and the Bank A. Alice t pay Bb $30 Bank actins A1. Des Alice have $30? A2. Give $30 t Bb A3. Take $30 frm Alice B. Alice t pay Bill $30 Bank actins B1. Des Alice have $30? B2. Give $30 t Bill B3. Take $30 frm Alice A.1 A.2 B.1 B.2 A.3 B.3 If Alice starts with $40, can Bb and Bill bth get $30?

10 Cannical example Crrect cde in a single-threaded wrld class BankAccunt { private int balance = 0; int getbalance() { return balance; vid setbalance(int x) { balance = x; vid withdraw(int amunt) { int b = getbalance(); if(amunt > b) thrw new WithdrawTLargeExceptin(); setbalance(b amunt); // ther peratins like depsit, etc. 10

11 Interleaving Suppse: Thread T1 calls x.withdraw(100) Thread T2 calls y.withdraw(100) If secnd call starts befre first finishes, we say the calls interleave Culd happen even with ne prcessr since a thread can be pre-empted at any pint fr 7me-slicing If x and y refer t different accunts, n prblem Yu ck in yur kitchen while I ck in mine But if x and y alias, pssible truble 11

12 A bad interleaving Interleaved withdraw(100) calls n the same accunt Assume ini7al balance == 150 Time Thread 1 Thread 2 int b = getbalance(); if(amunt > b) thrw new ; setbalance(b amunt); int b = getbalance(); if(amunt > b) thrw new ; setbalance(b amunt); Lst withdraw unhappy bank 12

13 Incrrect fix It is temp7ng and almst always wrng t fix a bad interleaving by rearranging r repea7ng pera7ns, such as: vid withdraw(int amunt) { if(amunt > getbalance()) thrw new WithdrawTLargeExceptin(); // maybe balance changed setbalance(getbalance() amunt); This fixes nthing! Narrws the prblem by ne statement (Nt even that since the cmpiler culd turn it back int the ld versin because yu didn t indicate need t synchrnize) And nw a nega7ve balance is pssible why? 13

14 Mutual exclusin Sane fix: Allw at mst ne thread t withdraw frm accunt A at a 7me Exclude ther simultaneus pera7ns n A t (e.g., depsit) Called mutual exclusin: One thread using a resurce (here: an accunt) means anther thread must wait a.k.a. cri7cal sec7ns, which technically have ther requirements Prgrammer must implement cri7cal sec7ns The cmpiler has n idea what interleavings shuld r shuld nt be allwed in yur prgram yu need language primi7ves t d it! 14

15 Wrng! Why can t we implement ur wn mutualexclusin prtcl? It s technically pssible under certain assump7ns, but wn t wrk in real languages anyway class BankAccunt { private int balance = 0; private blean busy = false; vid withdraw(int amunt) { while(busy) { /* spin-wait */ busy = true; int b = getbalance(); if(amunt > b) thrw new WithdrawTLargeExceptin(); setbalance(b amunt); busy = false; // depsit wuld spin n same blean 15

16 Just mved the prblem! Time Thread 1 Thread 2 while(busy) { while(busy) { busy = true; busy = true; int b = getbalance(); if(amunt > b) thrw new ; setbalance(b amunt); int b = getbalance(); if(amunt > b) thrw new ; setbalance(b amunt); Lst withdraw unhappy bank 16

17 What we need We need help frm the language One basic slu7n: Lcks Nt Java yet, thugh Java s apprach is similar and slightly mre cnvenient An ADT with pera7ns: new: make a new lck, ini7ally nt held acquire: blcks if this lck is already currently held ü Once nt held, makes lck held [all at nce!] release: makes this lck nt held ü If >= 1 threads are blcked n it, exactly 1 will acquire it 17

18 Why that wrks An ADT with pera7ns new, acquire, release The lck implementa7n ensures that given simultaneus acquires and/r releases, a crrect thing will happen Example: Tw acquires: ne will win and ne will blck Hw can this be implemented? Need t check if held and if nt make held all-at-nce Uses special hardware and O/S supprt ü See cmputer-architecture r pera7ng-systems curse Here, we take this as a primi7ve and use it 18

19 Almst-crrect pseudcde class BankAccunt { private int balance = 0; private Lck lk = new Lck(); vid withdraw(int amunt) { lk.acquire(); // may blck int b = getbalance(); if(amunt > b) thrw new WithdrawTLargeExceptin(); setbalance(b amunt); lk.release(); // depsit wuld als acquire/release lk 19

20 Sme mistakes A lck is a very primi7ve mechanism S7ll up t yu t use crrectly t implement cri7cal sec7ns Incrrect: Use different lcks fr withdraw and depsit Mutual exclusin wrks nly when using same lck balance field is the shared resurce being prtected Pr perfrmance: Use same lck fr every bank accunt N simultaneus pera7ns n different accunts Incrrect: Frget t release a lck (blcks ther threads frever!) Previus slide is wrng because f the excep7n pssibility! if(amunt > b) { lk.release(); // hard t remember! thrw new WithdrawTLargeExceptin(); 20

21 Other pera7ns If withdraw and depsit use the same lck, then simultaneus calls t these methds are prperly synchrnized But what abut getbalance and setbalance? Assume they are public, which may be reasnable If they d nt acquire the same lck, then a race between setbalance and withdraw culd prduce a wrng result If they d acquire the same lck, then withdraw wuld blck frever because it tries t acquire a lck it already has 21

22 Re-entrant lck A re-entrant lck (a.k.a. recursive lck) Remembers the thread (if any) that currently hlds it a cunt When the lck ges frm nt-held t held, the cunt is set t 0 If (cde running in) the current hlder calls acquire: it des nt blck it increments the cunt On release: if the cunt is > 0, the cunt is decremented if the cunt is 0, the lck becmes nt-held 22

23 Re-entrant lcks wrk int setbalance(int x) { lk.acquire(); balance = x; lk.release(); vid withdraw(int amunt) { lk.acquire(); setbalance1(b amunt); lk.release(); This simple cde wrks fine prvided lk is a reentrant lck Okay t call setbalance directly Okay t call withdraw (wn t blck frever) 23

24 Nw sme Java Java has built-in supprt fr re-entrant lcks Several differences frm ur pseudcde Fcus n the synchrnized statement synchrnized (expressin) { statements 1. Evaluates expressin t an bject Every bject (but nt primi7ve types) is a lck in Java 2. Acquires the lck, blcking if necessary If yu get past the {, yu have the lck 3. Releases the lck at the matching Even if cntrl leaves due t thrw, return, etc. S impssible t frget t release the lck 24

25 Java versin #1 (crrect but nn-idima7c) class BankAccunt { private int balance = 0; private Object lk = new Object(); int getbalance() { synchrnized (lk) { return balance; vid setbalance(int x) { synchrnized (lk) { balance = x; vid withdraw(int amunt) { synchrnized (lk) { int b = getbalance(); if(amunt > b) thrw setbalance(b amunt); // depsit wuld als use synchrnized(lk) 25

26 Imprving the Java As wrinen, the lck is private Might seem like a gd idea But als prevents cde in ther classes frm wri7ng pera7ns that synchrnize with the accunt pera7ns Mre idima7c is t synchrnize n this Als mre cnvenient: n need t have an extra bject 26

27 Java versin #2 class BankAccunt { private int balance = 0; int getbalance() { synchrnized (this){ return balance; vid setbalance(int x) { synchrnized (this){ balance = x; vid withdraw(int amunt) { synchrnized (this) { int b = getbalance(); if(amunt > b) thrw setbalance(b amunt); // depsit wuld als use synchrnized(this) 27

28 Syntac7c sugar Versin #2 is slightly pr style because there is a shrter way t say the same thing: Pupng synchrnized befre a methd declara7n means the en7re methd bdy is surrunded by synchrnized(this){ Therefre, versin #3 (next slide) means exactly the same thing as versin #2 but is mre cncise 28

29 Java versin #3 (final versin) class BankAccunt { private int balance = 0; synchrnized int getbalance() { return balance; synchrnized vid setbalance(int x) { balance = x; synchrnized vid withdraw(int amunt) { int b = getbalance(); if(amunt > b) thrw setbalance(b amunt); // depsit wuld als use synchrnized 29

30 Mre Java ntes Class java.util.cncurrent.lcks.reentran tlck wrks much mre like ur pseudcde Oqen use try { finally { t avid frgepng t release the lck if there s an excep7n Als library and/r language supprt fr readers/ writer lcks and cndi(n variables (future lecture) Java prvides many ther features and details. See, fr example: Java Cncurrency in Prac7ce by Getz et al 30

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Programmazione di sistemi multicore

Programmazione di sistemi multicore A.A. 2015-2016 LECTURE 12 IRENE FINOCCHI http://wwwusers.di.uniroma1.it/~finocchi/ Shared-memory concurrency & mutual exclusion TASK PARALLELISM AND OVERLAPPING MEMORY