Threads: the Stories

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Threads: the Stories

Hsuan-Tien Lin

Deptartment of CSIE, NTU

OOP Class, June 9, 2009

H.-T. Lin (NTU CSIE) Threads: the Stories OOP(even) 06/09/2009 0 / 19

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Story of a Bank: Part I

Once upon a time, a bank uses the following system to allow customers to spend in local stores easily

1 l o c a l c r e d i t = g e t C r e d i t ( customer ) ;

2 tospend = g e t P r i c e ( i t e m ) ;

3 i f ( tospend <= l o c a l c r e d i t ) {

4 n e w c r e d i t = l o c a l c r e d i t − tospend ;

5 n o t i f y N e w C r e d i t ( n e w c r e d i t ) ;

6 }

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Story of a Bank: Part II

Normally,

1 l o c a l c r e d i t 1 = g e t C r e d i t ( customer1 ) ; / / 10000

2 tospend1 = g e t P r i c e ( i te m1 ) ; / / 3000

3 i f ( tospend1 <= l o c a l c r e d i t 1 ) {

4 n e w c r e d i t 1 = l o c a l c r e d i t 1 − tospend1 ; / / 2000

5 n o t i f y N e w C r e d i t ( n e w c r e d i t 1 ) ;

6 }

12 }

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Story of a Bank: Part III

Unfortunately, customer 1 and 2 share the same account but go to different stores

7 }

12 }

13 g e t C r e d i t ( customer1 ) ; / / 8000

14 g e t C r e d i t ( customer2 ) ; / / 7000

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Story of a Bank: The End

Local copies are not trustworthy. Must update global copy atomically

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An Example with Counter Threads

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Story of a Couple: Part I

Once upon a time, a couple shares a credit card account. To prevent overdraft, they agreed on the following protocol for using the credit card:

1 tospend = g e t P r i c e ( i t e m ) ;

2 c u r r e n t l i m i t = c h e c k C r e d i t b y C e l l p h o n e ( ) ;

3 i f ( tospend <= c u r r e n t l i m i t )

4 d o _ t r a n s a c t i o n ( ) ; / / a t o m i c a l l y

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Story of a Couple: Part II

Normally,

1 tospend = g e t P r i c e ( i t e m ) ; / / by George : 50000

2 c u r r e n t l i m i t = c h e c k C r e d i t b y C e l l p h o n e ( ) ; / / 60000

3 i f ( tospend <= c u r r e n t l i m i t ) / / by Mary : yes

5 tospend = g e t P r i c e ( i t e m ) ; / / by Mary : 20000

7 i f ( tospend <= c u r r e n t l i m i t ) / / by Mary : no

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Story of a Couple: Part III

Unfortunately,

1 tospend = g e t P r i c e ( i t e m ) ; / / by George : 50000

3 / / George d r i v e s t o t h e s t o r e

4 tospend = g e t P r i c e ( i t e m ) ; / / by Mary : 20000

6 i f ( tospend <= c u r r e n t l i m i t ) / / by Mary : yes

7 d o _ t r a n s a c t i o n ( ) ;

8 i f ( tospend <= c u r r e n t l i m i t ) / / by George : yes

9 d o _ t r a n s a c t i o n ( ) ; / / OVERDRAFT ! !

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Story of a Couple: The End

Spent should happen immediately after checking

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An Example with Couple Threads

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Synchronization

synchronized: binds operations altogether (with respect to a lock) synchronized method: the lock is the class (for static method) or the object (for non-static method)

usually used to protect the variables within the class/object synchronized block: the lock is explicitly provided

flexible, fine-grained use

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More on the Lock

after getting the lock, can “use” any synchronized method/block that depends on the lock

lock releases after the method/block finishes (by return or exception)

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An Example with Counter Threads

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An Example with Couple Threads

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A Story of the Black and White Goats: Deadlock

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A Story of the Black and White Goats: Starvation

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A Story of the Black and White Goats: Livelock

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Ways to be Polite

1 s y n c h r o n i z e d v o i d make_payment (i n t amount ) {

2 w h i l e( no_money ( ) ) {

3 t h i s. complain ( 1 0 0 0 ) ;

4 }

5 }

3 Thread . s l e e p ( 1 0 0 0 ) ;

4 }

5 }

6 / / . . .

3 t h i s. w a i t ( 1 0 0 0 ) ;

4 }

5 }

6 / / . . .

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Wait until Notified

3 t h i s. w a i t ( ) ;

4 }

5 }

6

7 s y n c h r o n i z e d v o i d get_money (i n t amount ) {

8 money += amount ;

9 n o t i f y A l l ( ) ;

10 }