Multiprocess Synchronization Algorithms ( ) Lecturer: Danny Hendler

2 Grade structure 3 (theoretic) problem sets –Up to 30% of grade Take-home exam –At least 70% of grade

3 Adminstrative details -Office 218 -Office hours: Sundays, 3pm-5pm (or make an appointment) -Course site: autumn-2007/course-main.html (or simply enter thru my home page)

4 Books Distributed Computing: Fundamentals, Simulations, and Advanced Topics, Hagit Attiya and Jennifer Welch, John Wiley and Sons Synchronization Algorithms and Concurrent Programming, Gadi Taubenfeld, Pearson/Prentice Hall. Book site:

5 Distributed Systems A distributed system is a collection of individual computing devices that communicate with one another. E.g.: –VLSI chips –Shared-memory multiprocessor –Local-area network –The Internet

6 One sequential model, MANY distributed models Shared-memory / Message-passing –Which operations are allowed (read, write, read- modify-write)? Synchronous, timing-conditions, asynchronous Are failures allowed? –Fail-stop, Byzantine failures, message omission Is the number of processes known?

7 Types of distributed systems Multi-core computers The Internet Peer-to-peer systems Grid computers Wireless networks Sensor networks Ad-hoc networks …

8 Motivating examples of Synchronization

9 The Too-Much-Milk Problem TimeAliceBob 5:00Arrive Home 5:05Look in fridge; no milk 5:10Leave for grocery 5:15Arrive home 5:20Arrive at groceryLook in fridge; no milk 5:25Buy milkLeave for grocery 5:30Arrive home; put milk in fridge 5:40Buy milk 5:45Arrive home; too much milk! Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2006

10 Solving the Too-Much-Milk Problem Required properties  Mutual Exclusion: Only one person buys milk at a time  Progress: Someone always buys milk if it is needed Communication primitives  Leave a note (set a flag)  Remove a note (reset a flag)  Read a note (test the flag) Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2006

11 Important Distinction  Safety Property –Nothing bad happens ever –Example: mutual exclusion  Liveness Property –Something good happens eventually –Example: progress  Safety Property –Nothing bad happens ever –Example: mutual exclusion  Liveness Property –Something good happens eventually –Example: progress Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2006

12 Solution 1 Does it work? Alice if (no milk) { if (no note) { leave Note buy milk remove note } Bob if (no milk) { if (no note) { leave Note buy milk remove note } No, may end up with two milk.  mutual exclusion progress Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2006

13 Solution 2 Alice leave note A if (no note B) { if (no milk) {buy milk} } remove note A Bob leave note B if (no note A) { if (no milk) {buy milk} } remove note B Using labeled notes Does it work? No, may end up with no milk. mutual exclusion  progress Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2006

14 Solution 3 Does it work? Alice leave note A while (note B) {skip} if (no milk) {buy milk} remove note A Bob leave note B if (no note A) { if (no milk) {buy milk} } remove note B No ! a timing assumption is needed mutual exclusion  progress Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2006

15 Is solution #3 a good solution? No  A timing assumption is needed!  Unfair to one of the processes  Alice is busy waiting – consuming CPU cycles without accomplishing useful work  A timing assumption is needed!  Unfair to one of the processes  Alice is busy waiting – consuming CPU cycles without accomplishing useful work Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2006

16 Solution 4 Using 4 labeled notes A1A2B2B1 the fridge’s door Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2006

17 Alice leave A1 if B2 {leave A2} else {remove A2} while B1 and ((A2 and B2) or (no A2 and no B2)) {skip} if (no milk) {buy milk} remove A1 Bob leave B1 if (no A2) {leave B2} else {remove B2} while A1 and ((A2 and no B2) or (no A2 and B2)) {skip} if (no milk) {buy milk} remove B1 Solution 4 A correct, fair, symmetric algorithm! Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2006

18 Solution 4 Notations color: there is a note A1A2B2 Alice’s turn B2 dotted line: there is no note solid line without color: we do not care if there is a note who will buy milk ? XX Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2006

19 A1A2B2 Alice’s turn A1A2B2B1 Alice’s turn A2B2B1 Bob’s turn A1A2B2B1 Alice’s turn A1A2B2B1 Bob’s turn A1A2B2B1 Bob’s turn Solution 4 B2 XX XX Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2006

20 A variant of Solution 4 The order of the first two statements is replaced Is it correct ? No, may end up with two milk. Alice if B2 {leave A2} else {remove A2} leave A1 while B1 and ((A2 and B2) or (no A2 and no B2)) {skip} if (no milk) {buy milk} remove A1 Bob if (no A2) {leave B2} else {remove B2} leave B1 while A1 and ((A2 and no B2) or (no A2 and B2)) {skip} if (no milk) {buy milk} remove B1 MILK Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2006 B2 B1A1

21 A variant of Solution 4 The order of the first two statements is replaced Is it correct ? No, may end up with two milk.  mutual exclusion progress Alice if B2 {leave A2} else {remove A2} leave A1 while B1 and ((A2 and B2) or (no A2 and no B2)) {skip} if (no milk) {buy milk} remove A1 Bob if (no A2) {leave B2} else {remove B2} leave B1 while A1 and ((A2 and no B2) or (no A2 and B2)) {skip} if (no milk) {buy milk} remove B1 MILK Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2006 B2 B1A1

22 The coordinated attack problem Blue army wins if both blue camps attack simultaneously Design an algorithm to ensure that both blue camps attack simultaneously Enemy 12 The problem is due to by Jim Gray (1977) Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2006

23 The coordinated attack problem Communication is done by sending messengers across valley Messengers may be lost or captured by the enemy  Enemy 12 Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2006

24 The coordinated attack problem Theorem: There is no algorithm that solves this problem ! Proof: Assume to the contrary that such an algorithm exits. Let P be an algorithm that solves it with the fewest # of messages, when no message is lost. P should work even when the last messenger is captured. So P should work even if the last messenger is never sent. But this is a new algorithm with one less message. A contradiction.  The model is too weak! Synchronization Algorithms and Concurrent Programming Gadi Taubenfeld © 2006