1. Operating Systems Principles Process Management and Coordination Lecture 3: Higher-Level Synchronization and Communication 主講人：虞台文

3. Operating Systems Principles Process Management and Coordination Lecture 3: Higher-Level Synchronization and Communication Motivation

6. Operating Systems Principles Process Management and Coordination Lecture 3: Higher-Level Synchronization and Communication Share Memory Methods

8. The Monitor Abstraction Shared among processes Processes cannot access them directly wait/signal primitives for processes communication or synchronization . Processes access the internal data only through these procedures. Procedure are mutually exclusive , i.e., only one process or thread may be executing a procedure within a given time. Internal Data Condition Variables Procedure 1 Procedure 2 Procedure 3

9. Example: Queue Handler Queue AddToQueue RemoveFromQueue

22. Example: Bounded Buffer Deposit Remove . . . . . . . . . 0 1 2 n  1 n  2

23. Example: Bounded Buffer Deposit Remove . . . . . . . . . 0 1 2 n  1 n  2 . . . nextin nextout count

24. Example: Bounded Buffer Deposit Remove count nextout nextin

27. Example: Alarm Clock 0 1 3 2 4 5 . . . The current time ( now ) of the alarm clock is increased periodically ( tick ). Wakeup Queue The wakeup queue is used to hold processes to be waken up orderly according to their wakeup time ( alarm ) .

28. Example: Alarm Clock p1: . . . AlarmClock.Wakeme(100); . . . Wakeup Queue p1(250) Call at time 150

29. Example: Alarm Clock p1: . . . AlarmClock.Wakeme(100); . . . Wakeup Queue p1(250) p2: . . . AlarmClock.Wakeme(150); . . . p1(250) p2(350) Call at time 150 Call at time 200

30. Example: Alarm Clock p1: . . . AlarmClock.Wakeme(100); . . . Wakeup Queue p1(250) p2: . . . AlarmClock.Wakeme(150); . . . p1(250) p2(350) p3: . . . AlarmClock.Wakeme(30); . . . p3(240) Call at time 150 Call at time 200 Call at time 210

31. Example: Alarm Clock p1: . . . AlarmClock.Wakeme(100); . . . Wakeup Queue p1(250) p2: . . . AlarmClock.Wakeme(150); . . . p1(250) p2(350) p3: . . . AlarmClock.Wakeme(30); . . . p3(240) p4: . . . AlarmClock.Wakeme(10); . . . p4(240) Call at time 150 Call at time 200 Call at time 210 Call at time 230

33. Example: Alarm Clock p1: . . . AlarmClock.Wakeme(100); . . . Wakeup Queue p1(250) p2: . . . AlarmClock.Wakeme(150); . . . p1(250) p2(350) p3: . . . AlarmClock.Wakeme(30); . . . p3(240) p4: . . . AlarmClock.Wakeme(10); . . . p4(240) Call at time 150 Call at time 200 Call at time 210 Call at time 230

35. Mesa and Java Monitors P1: . . . . . . . . if(!B1) c1.wait; . . . . . . . . P2: . . . . . . . . if(!B2) c2.wait; . . . . . . . . B1=FASLE; . . . . . . . .

36. Mesa and Java Monitors P1: . . . . . . . . if(!B1) c1.wait; . . . . . . . . P2: . . . . . . . . if(!B2) c2.wait; . . . . . . . . B1=FASLE; return; P3: . . . . . . . . B1=B2=TRUE; c1.notify; c2.notify; . . . . . . . . return; B1=FALSE

37. Mesa and Java Monitors P1: . . . . . . . . if(!B1) c1.wait; . . . . . . . . P2: . . . . . . . . if(!B2) c2.wait; . . . . . . . . B1=FASLE; return; P3: . . . . . . . . B1=B2=TRUE; c1.notify; c2.notify; . . . . . . . . return; B1=FALSE What action should P1 take? Continue or wait again?

38. Mesa and Java Monitors P1: . . . . . . . . if(!B1) c1.wait; . . . . . . . . P2: . . . . . . . . if(!B2) c2.wait; . . . . . . . . B1=FASLE; return; P3: . . . . . . . . B1=B2=TRUE; c1.notify; c2.notify; . . . . . . . . return; B1=FALSE What action should P1 take? Continue or wait again? 

39. Solution P1: . . . . . . . . if(!B1) c1.wait; . . . . . . . . P2: . . . . . . . . if(!B2) c2.wait; . . . . . . . . B1=FASLE; return; P3: . . . . . . . . B1=B2=TRUE; c1.notify; c2.notify; . . . . . . . . return; B1=FALSE What action should P1 take? Continue or wait again?    Replace if to while . while while

40. Example: Queue Handler Queue AddToQueue RemoveFromQueue

45. Operating Systems Principles Process Management and Coordination Lecture 3: Higher-Level Synchronization and Communication Distributed Synchronization and Communication

48. Types of Send/Receive broadcast message m broadcast message m wait until accepted implicit naming send message m to receiver r send message m to receiver r wait until accepted explicit naming nonblocking/asynchronous blocking/synchronous send if there is a message from any sender, then receive it; else proceed. wait message from any sender implicit naming if there is a message from sender s , then receive it; else proceed. wait message from sender s explicit naming nonblocking/asynchronous blocking/synchronous receive

49. Types of Send/Receive broadcast message m broadcast message m wait until accepted implicit naming send message m to receiver r send message m to receiver r wait until accepted explicit naming nonblocking/asynchronous blocking/synchronous send if there is a message from any sender, then receive it; else proceed. wait message from any sender implicit naming if there is a message from sender s , then receive it; else proceed. wait message from sender s explicit naming nonblocking/asynchronous blocking/synchronous receive if there is a message from any sender, then receive it; else proceed. wait message from any sender implicit naming if there is a message from sender s , then receive it; else proceed. wait message from sender s explicit naming nonblocking/asynchronous blocking/synchronous receive

50. Types of Send/Receive Little practical used Little practical used Little practical used  no use, e.g., Some debugging software may like it. broadcast message m broadcast message m wait until accepted implicit naming send message m to receiver r send message m to receiver r wait until accepted explicit naming nonblocking/asynchronous blocking/synchronous send if there is a message from any sender, then receive it; else proceed. wait message from any sender implicit naming if there is a message from sender s , then receive it; else proceed. wait message from sender s explicit naming nonblocking/asynchronous blocking/synchronous receive

51. Process Coordination Little practical used Little practical used Solving a variety of process coordination problems. broadcast message m broadcast message m wait until accepted implicit naming send message m to receiver r send message m to receiver r wait until accepted explicit naming nonblocking/asynchronous blocking/synchronous send if there is a message from any sender, then receive it; else proceed. wait message from any sender implicit naming if there is a message from sender s , then receive it; else proceed. wait message from sender s explicit naming nonblocking/asynchronous blocking/synchronous receive

52. Example:Printer Sever Little practical used Little practical used broadcast message m broadcast message m wait until accepted implicit naming send message m to receiver r send message m to receiver r wait until accepted explicit naming nonblocking/asynchronous blocking/synchronous send if there is a message from any sender, then receive it; else proceed. wait message from any sender implicit naming if there is a message from sender s , then receive it; else proceed. wait message from sender s explicit naming nonblocking/asynchronous blocking/synchronous receive

53. Implementation for Asynchronous Operations Little practical used Little practical used built-in buffers are required to hold messages broadcast message m broadcast message m wait until accepted implicit naming send message m to receiver r send message m to receiver r wait until accepted explicit naming nonblocking/asynchronous blocking/synchronous send if there is a message from any sender, then receive it; else proceed. wait message from any sender implicit naming if there is a message from sender s , then receive it; else proceed. wait message from sender s explicit naming nonblocking/asynchronous blocking/synchronous receive

55. Named Message Channels Named Pipe (Win32) P1: . . . . . . . . send(ch1, msg1); . . . . . . . . P2: . . . . . . . . send(ch2, msg2); . . . . . . . . P3: . . . . . . . . receive(ch1, x); . . . . . . . . receive(ch2, y); . . . . . . . . ch1 ch2

58. Bounded buffer with CSP Buffer Producer Consumer deposit request remove

59. Bounded buffer with CSP Buffer Producer Consumer deposit request remove send(deposit, data) send(request) receive(remove,data) receive(request) send(remove,data) receive(deposit, data)

60. Bounded buffer with CSP Buffer Producer Consumer deposit request remove receive(request) send(remove,data) receive(deposit, data) The Bounded Buffer uses the following three primitives for synchronization.

65. Ports and Mailboxes Port Mailboxes

67. Ports and Mailboxes

72. Rendezvous Caller Server q . f ( param ) accept f ( param ) S Similar syntax/semantics to RPC Name of the remote process (sever) Procedure name Procedure parameter Procedure body Keyword

74. Semantics of a Rendezvous p q Rendezvous p q Rendezvous q . f () accept f () S q . f () accept f () S

79. Example: Bounded Buffer BoundedBuffer .deposit(data) BoundedBuffer .remove(data)

81. Distributed Mutual Exclusion with Token Ring A practical and elegant compromise version of fully distributed approach.

82. Distributed Mutual Exclusion with Token Ring Controler[2]: P[2]: CS2; Program2; Controler[3]: P[3]: CS3; Program3; Controler[1]: P[1]: CS1; Program1; token token RequestCS ReleaseCS RequestCS ReleaseCS RequestCS ReleaseCS

84. Distributed Mutual Exclusion with Token Ring Controler[2]: P[2]: CS2; Program2; Controler[3]: P[3]: CS3; Program3; Controler[1]: P[1]: CS1; Program1; process controller[i] { while(1) { accept Token; select { accept Request_CS () {busy=1;} else null; } if (busy) accept Release_CS () {busy=0;} controller[(i+1) % n].Token; } } process p[i ] { while(1) { controller[i]. Request_CS (); CSi; controller[i]. Release_CS (); programi; } } token token RequestCS ReleaseCS RequestCS ReleaseCS RequestCS ReleaseCS

85. Operating Systems Principles Process Management and Coordination Lecture 3: Higher-Level Synchronization and Communication Other Classical Problems

86. Readers/Writers Problem Database

87. Readers/Writers Problem Database Writers can work only when no reader activated. One Writer can work at a time.

88. Readers/Writers Problem Database Readers can work only when no writer activated. Allows infinite number of readers.

90. Solution Using Monitor monitor Readers_Writers { int readCount=0, writing=0; condition OK_R, OK_W; start_read() { if (writing || !empty(OK_W)) OK_R.wait; readCount = readCount + 1; OK_R.signal; } end_read() { readCount = readCount - 1; if (readCount == 0) OK_W.signal; } start_write() { if ((readCount != 0)||writing) OK_W.wait; writing = 1; } end_write() { writing = 0; if (!empty(OK_R)) OK_R.signal; else OK_W.signal; } } Called by a reader that wishes to read. Called by a reader that has finished reading. Called by a writer that wishes to write. Called by a writer that has finished writing.

91. Solution Using Monitor monitor Readers_Writers { int readCount=0, writing=0; condition OK_R, OK_W; start_read() { if (writing || ! empty (OK_W)) OK_R.wait; readCount = readCount + 1; OK_R.signal; } end_read() { readCount = readCount - 1; if (readCount == 0) OK_W.signal; } start_write() { if ((readCount != 0)||writing) OK_W.wait; writing = 1; } end_write() { writing = 0; if (! empty (OK_R)) OK_R.signal; else OK_W.signal; } } Additional Primitive: empty ( c ) Return true if the associated queue of c is empty.

92. Dining Philosophers

97. The Problem of Clock Skewness File User (U) File Server (FS) e 1 e 2 e 3 e 4 C U C FS send send receive receive delta 1 delta 2 True causality of events: The log of events: 10 15 5 20 Impossible sequence!

98. Logical Clocks e i e s e k e r send receive p 1 p 2 e j

99. Logical Clocks in Action u v x y 4 5 13 15 6 14

101. The Elevator Algorithm

103. The Elevator Algorithm Using Priority Waits monitor elevator { int dir=1, up=1, down=-1, pos=1, busy=0; condition upsweep , downsweep ; request (int dest) { /*Called when button pushed*/ if (busy) { if ((pos<dest) || ((pos==dest) && (dir==up))) upsweep .wait(dest); else downsweep .wait(-dest); } busy = 1; pos = dest; } release () { /*Called when door closes*/ . . . . . . . . . . . . . . . } } Put the request into the proper priority queue and wait .

104. The Elevator Algorithm Using Priority Waits monitor elevator { int dir=1, up=1, down=-1, pos=1, busy=0; condition upsweep , downsweep ; request (int dest) { /*Called when button pushed*/ if (busy) { if ((pos<dest) || ((pos==dest) && (dir==up))) upsweep .wait(dest); else downsweep .wait(-dest); } busy = 1; pos = dest; } release () { /*Called when door closes*/ . . . . . . . . . . . . . . . } }

105. The Elevator Algorithm Using Priority Waits monitor elevator { int dir=1, up=1, down=-1, pos=1, busy=0; condition upsweep, downsweep; . . . . . . . . . . . . . . . release () { /*Called when door closes*/ busy = 0; if (dir==up) { if (!empty( upsweep )) upsweep .signal; else { dir = down; downsweep .signal; } } else { /*direction==down*/ if (!empty(downsweep)) downsweep .signal; else { dir = up; upsweep .signal; } } } }