1

Shared Memory

2

Shared Memory

1p

3p

2p

4p

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

1p write 10

6

1p write 1010

7

1p write 1010

2pread

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1p write 1010

2pread

10

9

1p write 10 2pwrite 20

Simultaneous writes

10

1p write 10 2pwrite 20

Slower process

Possibility 1

20

Simultaneous writes

11

1p write 10 2pwrite 20Slower process

Possibility 2

10

Simultaneous writes

12

1p write 2pwrite

In general:

1a2a

3pwrite 3a

ipwrite ia

x

Simultaneous writes

13

1p write 2pwrite

Slower process:

1a2a

3pwrite 3a

ipwrite ia

ka

kpSimultaneous writes

14

1p read 2pread

Simultaneous Reads

a

15

1p read 2pread

Simultaneous Reads

aa a

All read the same value

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Test&Set Variables

Test&Set(v)temp = v;v = 1;return (temp);

Reset(v)v = 0;

v is a binary variable

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0

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1p test&set0

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1p test&set0 1

20

2ptest&set

1

21

2ptest&set

1 1

22

1

23

3p reset0

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1p test&set0

2ptest&set

simultaneous accesses

3ptest&set

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1p test&set1

simultaneous accesses

0

2ptest&set

3ptest&set

11

Faster process

26

Read-Modify-Write Variables

RMW(v, f)temp = v;v = f(v);return (temp);

function on v

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0

28

1p RMW(+5)0

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1p0 5

RMW(+5)

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2pRMW(+8)

5

31

2p

13 5

RMW(+8)

32

1p0

2p

simultaneous accesses

RMW(+5) RMW(+8)

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1p13

2p

simultaneous accesses

RMW(+5) RMW(+8)

8 0

Faster process

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1p0

2p

simultaneous accesses

RMW(+5) RMW(+8)

2pRMW(+10)

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1p23

2p

simultaneous accesses

RMW(+5) RMW(+8)

2pRMW(+10)

08 13

fastestmiddle

slower

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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)

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Mutual Exclusion

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Process 1 - program

V = 1;For (x = 10; x< y; x++) { cout << “hello”;}

If (t > 10) then while (x < 100) m = 20;

……

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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

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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; v = v+10;

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Process 1

Remainder codeEntry codeCritical SectionExit codeRemainder Code

Process 2

Remainder codeEntry CodeCritical SectionExit CodeRemainder Code

Mutual Exclusion

Entry/Exit code guaranty that onlyone process is in the critical section

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Process 1

Remainder codeEntry codeCritical SectionExit codeRemainder Code

Process 2

Remainder codeEntry CodeCritical SectionExit CodeRemainder Code

Mutual Exclusion

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Process 1

Remainder codeEntry codeCritical SectionExit codeRemainder Code

Process 2

Remainder codeEntry CodeCritical SectionExit CodeRemainder Code

Mutual Exclusion

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Process 1

Remainder codeEntry codeCritical SectionExit codeRemainder Code

Process 2

Remainder codeEntry CodeCritical SectionExit CodeRemainder Code

Mutual Exclusion

45

Process 1

Remainder codeEntry codeCritical SectionExit codeRemainder Code

Process 2

Remainder codeEntry CodeCritical SectionExit CodeRemainder Code

Mutual Exclusion

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Process 1

Remainder codeEntry codeCritical SectionExit codeRemainder Code

Process 2

Remainder codeEntry CodeCritical SectionExit CodeRemainder Code

Mutual Exclusion

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Mutual Exclusion

Critical

Exit

Remainder

Entry

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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

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Mutual Exclusion

test&set variables

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While (test&set(v) = 1)

Critical section

Reset (v)

Entry:

Exit:

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0

1p test&setentry

v

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1

1pcritical section

v0

53

0

1pexit

vreset

54

0

1p test&setentry

v

Two processors

2pentry

test&set

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1

1pentry

v 2p

critical section

01

56

1

1pentry

v 2p

critical section

0test&set

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1

1pentry

v 2p

critical section

01

58

0

1pentry

v 2pexit

resettest&set

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1

1p

criticalsection

v0

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0

1pexit

vreset

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1

1pentry

v 2p

critical section

test&set

Problem: the algorithm doesn’t guaranty no lockout, a process may starve

Example: may never enter the critical section1p

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1

1pentry

v

3p

critical section

test&set

Example: may never enter the critical section1p

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1

1pentry

v 4p

critical section

test&set

Example: may never enter the critical section1p

432 ,, ppp : faster than 1p

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Mutual exclusion

read-modify-write variables

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v.first

v.last

Ticket of current process in critical section

Ticket of last processwaiting in entry section

Shared variable v

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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)

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v.first 1

v.last1

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v.first 1

v.last1

p.last 1p

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v.first 1

v.last2

entry

p.last 1 1p

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v.first 1

v.last2

critical section

p.last 1 1p

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v.first 2

v.last2

exit

p.last 1 1p

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v.first 1

v.last1

Four processes

p.last 1p 2p 3p 4p

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v.first 1

v.last2

p.last 1 1p 2p 3p 4pentry

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v.first 1

v.last2

p.last 1 1p 2p 3p 4pentry

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v.first 1

v.last3

p.last 1 1p 2 2p 0 3p 0 4pentry entry

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v.first 1

v.last3

p.last 1 1p 2 2p 0 3p 0 4pentry entry

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v.first 1

v.last4

p.last 1 1p 2 2p 3 3p 0 4pentry entry entry

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v.first 1

v.last4

p.last 1 1p 2 2p 3 3p 0 4pentry entry entry

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v.first 1

v.last5

p.last 1 1p 2 2p 3 3p 4 4pentry entry entry entry

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v.first 1

v.last5

p.last 1 1p 2 2p 3 3p 4 4p

criticalsection entry entry entry

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v.first 2

v.last5

p.last 1 1p 2 2p 3 3p 4 4pentry entry entryexit

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v.first 2

v.last5

2 2p 3 3p 4 4p

criticalsection entry entry

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v.first 3

v.last5

2 2p 3 3p 4 4pentry entryexit

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v.first 3

v.last4

3 3p 4 4pentry entry

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v.first i

v.last-1i+k

i 1ip

i+k 1kip

criticalsection entry

i+1 2ip

entry

The behavior is similar with a queue

……

(head) (tail)

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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

i i+1 … i+k …

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Solution: a circular queue

……1 2 n

(for n processes)

v.first v.last-1

Only n different values are needed