1 Shared Memory

2 processes

3 Types of Shared Variables Read/Write Test & Set Read-Modify-Write

4 Read/Write Variables Read(v)Write(v,a) return(v); v = a; Or simpler: x = v; v = x; (read v) (write v)

5 write 10

6 10

7 write read

8 write read 10

9 write 10 write 20 Simultaneous writes

10 write 10 write 20 Slower process Possibility 1 20 Simultaneous writes

11 write 10 write 20 Slower process Possibility 2 10 Simultaneous writes

12 write In general: write x Simultaneous writes

13 write Slower process: write Simultaneous writes

14 read Simultaneous Reads a

15 read Simultaneous Reads aa a All read the same value

16 Test&Set Variables Test&Set(v) temp = v; v = 1; return (temp); Reset(v) v = 0; v is a binary variable

17 0

18 test&set 0

19 test&set 0 1

20 test&set 1

21 test&set 11

22 1

23 reset 0

24 test&set 0 simultaneous accesses test&set

25 test&set 1 simultaneous accesses 0 test&set 1 1 Faster process

26 Read-Modify-Write Variables RMW(v, f) temp = v; v = f(v); return (temp); function on v

27 0

28 RMW(+5) 0

RMW(+5)

30 RMW(+8) 5

RMW(+8)

32 0 simultaneous accesses RMW(+5) RMW(+8)

33 13 simultaneous accesses RMW(+5) RMW(+8) 8 0 Faster process

34 0 simultaneous accesses RMW(+5) RMW(+8) RMW(+10)

35 23 simultaneous accesses RMW(+5) RMW(+8) RMW(+10) fastest middle slower

36 RMW(v, f) temp = v; v = f(v); return (temp); RMW simulate Test&Set Test&Set(v) temp = v; v = 1; return (temp); Reset(v) v = 0; (f(v) = 1) RMW(v, f) temp = v; v = f(v); return (temp); (f(v) = 0)

37 Mutual Exclusion

38 Process 1 - program V = 1; For (x = 10; x< y; x++) { cout << “hello”; } If (t > 10) then while (x < 100) m = 20; ……

39 Process 1 - program Remainder code Critical Section Remainder Code Process 2 - program Remainder code Critical Section Remainder Code Only one process can enter the critical section Critical Section

40 Process 1 - program Remainder code Critical Section Remainder Code Process 2 - program Remainder code Critical Section Remainder Code Processes may update the same variables in the critical section Critical Section v = v+10;

41 Process 1 Remainder code Entry code Critical Section Exit code Remainder Code Process 2 Remainder code Entry Code Critical Section Exit Code Remainder Code Mutual Exclusion Entry/Exit code guaranty that only one process is in the critical section

42 Process 1 Remainder code Entry code Critical Section Exit code Remainder Code Process 2 Remainder code Entry Code Critical Section Exit Code Remainder Code Mutual Exclusion

43 Process 1 Remainder code Entry code Critical Section Exit code Remainder Code Process 2 Remainder code Entry Code Critical Section Exit Code Remainder Code Mutual Exclusion

44 Process 1 Remainder code Entry code Critical Section Exit code Remainder Code Process 2 Remainder code Entry Code Critical Section Exit Code Remainder Code Mutual Exclusion

45 Process 1 Remainder code Entry code Critical Section Exit code Remainder Code Process 2 Remainder code Entry Code Critical Section Exit Code Remainder Code Mutual Exclusion

46 Process 1 Remainder code Entry code Critical Section Exit code Remainder Code Process 2 Remainder code Entry Code Critical Section Exit Code Remainder Code Mutual Exclusion

47 Mutual Exclusion Critical Exit Remainder Entry

48 Attributes of mutual exclusion algorithms No Deadlock: if some process is in the entry section then some process will enter the critical region No Lockout: if some process is in the entry section then the same process will enter the critical region

49 Mutual Exclusion test&set variables

50 While (test&set(v) = 1) Critical section Reset (v) Entry: Exit:

51 0 test&set entry v

52 1 critical section v 0

53 0 exit v reset

54 0 test&set entry v Two processors entry test&set

55 1 entry v critical section 0 1

56 1 entry v critical section 0 test&set

57 1 entry v critical section 0 1

58 0 entry v exit reset test&set

59 1 critical section v 0

60 0 exit v reset

61 1 entry v critical section test&set Problem: the algorithm doesn’t guaranty no lockout, a process may starve Example: may never enter the critical section

62 1 entry v critical section test&set Example: may never enter the critical section

63 1 entry v critical section test&set Example: may never enter the critical section : faster than

64 Mutual exclusion read-modify-write variables

65 v.first v.last Ticket of current process in critical section Ticket of last process waiting in entry section Shared variable v

66 p = RMW(v, (v.first, v.last +1)) Repeat q = RMW(v,v) Until q.first = p.last Critical section RMV(v, (v.first+1, v.last)) Entry: Exit: (p and q are local variables)

67 v.first 1 v.last 1

68 v.first 1 v.last 1 p.last

69 v.first 1 v.last 2 entry p.last1

70 v.first 1 v.last 2 critical section p.last1

71 v.first 2 v.last 2 exit p.last1

72 v.first 1 v.last 1 Four processes p.last

73 v.first 1 v.last 2 p.last1 entry

74 v.first 1 v.last 2 p.last1 entry

75 v.first 1 v.last 3 p.last1200 entry

76 v.first 1 v.last 3 p.last1200 entry

77 v.first 1 v.last 4 p.last1230 entry

78 v.first 1 v.last 4 p.last1230 entry

79 v.first 1 v.last 5 p.last1234 entry

80 v.first 1 v.last 5 p.last1234 critical section entry

81 v.first 2 v.last 5 p.last1234 entry exit

82 v.first 2 v.last critical section entry

83 v.first 3 v.last entry exit

84 v.first 3 v.last 4 34 entry

85 v.first i v.last-1 i+k i critical section entry i+1 entry The behavior is similar with a queue …… (head) (tail)

86 Good features of algorithm: Guarantees no lockout (any process will eventually enter the critical section) Uses only one shared variable (v)

87 A problem: values can grow unbounded ii+1 … i+k …

88 Solution: a circular queue …… 1 2 n (for n processes) v.firstv.last-1 Only n different values are needed