?????????????????????? Process Synchronization

1
??????????????????????Process Synchronization

• ???????? ?????? ??????? ???????

2
Objectives

• ??????????????????????????????????????????????????
  ???? ??????????????????????????????????????????
• ??????????????????????????????????????????????????
  ??????????????????????????? ??????????
  ??????????????????? ????????
• ??????????????????????????????????????
  ?????????????????????????????????? ?????????????

3
Agenda

• Basic Concepts
• Scheduling Criteria
• Scheduling Algorithms
• Multiple-Processor Scheduling
• Real-Time Scheduling
• Algorithm Evaluation

4
Agenda Process Synchronization

• Background
• The Critical-Section Problem
• Petersons Solution
• Synchronization Hardware
• Semaphores
• Classic Problems of Synchronization
• Monitors
• Synchronization Examples
• Atomic Transactions

5
Background

• ??????????????????????? ??????????????????????????
  ?????????????????? ?????????????????????
• ??????????????? (Multiprogramming)
• ?????????????? (Multiprocessing)
• ???????????????????? (Distributed processing)
  ???? ????????? (cluster)
• ??????????????????????
• ????????????????????? (concurrency)
• ????????????? (synchronization)

6
Background (2)

• ??????????????????????????????????????????????????
  ??? ??????
• ??????????????????
• ??????????????????????????????????????????????????
  ?????????????????????????????????????
• ???????????????????????
• ??????????????????????????????????????????????????
  ???? ?????????????????????????????????????????????
  ???????????????????????????????????
• ???????????????????????
• ??????????????????????????????????????????????????
  ?????????????????????? ??????

7
Background (3)

• ???????????????????????????? ?????????????????????
  ??????????????????????????
• ??????????????????????????????????????????????????
  ??????????????????????????????????????????????????
  ????????????
• ???????????????????????? consumer-producer
  problem ????????????????????? ????????????????????
  ??? integer count ????????????????? track
  ?????????????????????????
• ??????????????????????? 0 ????????????????????
  producer ?????????????????????????????
• ??????????????? consumer ??????????????????

8
Producer

• while (true)
• / produce an item
  and put in nextProduced
• while (count BUFFER_SIZE)
• // do nothing
• buffer in nextProduced
• in (in 1) BUFFER_SIZE
• count

9
Consumer

• while (1)
• while (count 0)
• // do nothing
• nextConsumed bufferout
• out (out 1) BUFFER_SIZE
• count--
• / consume the item in nextConsumed

10
???????????????(race condition)

• ??????????????????????????????????????????????????
  ???????????????????????????????????? ????????
  ??????????????? (race condition)
• ???????????????????????????????????????????
  (scheduling policy) ??????
• ??????????????????????????? ??????????????????????
  ????????? ? ??????????????????????????????????????
  ???? counter ??? ?????????????????????????????????
  ???????? (synchronized) ??????????

11
Race Condition

• count could be implemented as register1
  count register1 register1 1 count
  register1
• count-- could be implemented as register2
  count register2 register2 - 1 count
  register2
• Consider this execution interleaving with count
  5 initially
• S0 producer execute register1 count
  register1 5S1 producer execute register1
  register1 1 register1 6 S2 consumer
  execute register2 count register2 5 S3
  consumer execute register2 register2 - 1
  register2 4 S4 producer execute count
  register1 count 6 S5 consumer execute
  count register2 count 4
• count ??????? 4 ???? 6 ????? ???????????????????
  5

12
??????????Critical Section

• ?????????????????????????????????????????????? n
  T0, T1, , Tn-1
• ??????????????????????????????????????????????????
  ????????????????????????????????
  ??????????????????????????????????????????????????
  ??????? ?????????? (critical section)
  ??????????????????????????????????????,
  ?????????????, ????????? ????????
• ????????????????????????????? ????????????????????
  ????????????????????????? ??????????????????
  ?????????????????????????????????????????

13
?????????????????????

• 1. ?????????????? (mutual exclusion)
• ????????? Ti ????????????????????????????????????
  ? ???????????????????????????????????????????????
  ???
• 2. ???????????? (Progress)
• ??????????????????????????????????????????
  ???????????? ???????????????????
  ??????????????????????????????????????????????????
  ????????????????????????????
• 3. ???????????????? (Bounded waiting)
• ???????????? ?????????????????????
  ??????????????????????????????????????????????????
  ??????????????????????????????????????????
  ??????????????????????????????????????????
  ??????? ???????????????????????? (starvation)

14
Initial Attempts to Solve Problem

• Only 2 processes, P0 and P1
• General structure of process Pi (other process
  Pj)
• do
• entry section
• critical section
• exit section
• reminder section
• while (1)
• ??????????????????????????????????????????????????
  ??????????
• ???????????????????????????????????? 3 ???

15
Algorithm 1

• Shared variables
• int turninitially turn 0
• turn - i ? Pi can enter its critical section
• Process Pi
• do
• while (turn ! i)
• critical section
• turn j
• reminder section
• while (1)
• ????????????????????????????????? (Mutual
  Exclusion) ???????????????????????? (Progress)

16
Algorithm 2

• Shared variables
• boolean flag2initially flag 0 flag 1
  false.
• flag i true ? Pi ready to enter its critical
  section
• Process Pi
• do
• flagi true while (flagj)
  critical section
• flag i false
• remainder section
• while (1)
• ????????????? ???????????????????? (Mutual
  Exclusion) ???????????????????????? (Progress)

17
Algorithm 3

• Combined shared variables of algorithms 1 and 2.
• Process Pi
• do
• flag i true turn j while (flag j
  and turn j)
• critical section
• flag i false
• remainder section
• while (1)
• ????????????? ????????????????????? 3 ???
  ????????????????????????????????? 2 ????????????

18
??????????????????? Synchronization Hardware

• ???????????????????????????????????????
  interrupts
• ??????????????????????????????????????????????????
  ??????????????????????????????????????
• ?????????????????? ??????????????????
• ??????????????????????????????????????????????????
  ????????????????????????????????????????
• ??????????????????????????????????????????????????
  ??????????????????????????????????????????????????
  ???
• ??????????????????????????????????????????????????
  ????????????????????????????????????

19
??????????????????? Synchronization Hardware

• ??????????????????????????????????????????????????
  ?????????????????????????????????????
  ?????????????????????????? 2 ?????????????????
  (atomic instruction)
• Atomic non-interruptable
• ??????????????????????????????????????????????????
  ???????????????????
• Test-and-Set Instruction
• Swap Instruction

20
Test-and-Set Instruction

• Definition
• boolean TestAndSet (boolean target)
• boolean rv target
• target TRUE
• return rv

21
Solution using TestAndSet

• Shared boolean variable lock., initialized to
  false.
• Solution
• do
• while ( TestAndSet (lock ))
• / do nothing
• // critical section
• lock FALSE
• // remainder section
• while ( TRUE)

22
Swap Instruction

• Definition
• void Swap (boolean a, boolean b)
• boolean temp a
• a b
• b temp

23
Solution using Swap

• Shared Boolean variable lock initialized to
  FALSE Each process has a local Boolean variable
  key.
• Solution
• do
• key TRUE
• while ( key TRUE)
• Swap (lock, key )
• // critical section
• lock FALSE
• // remainder section
• while ( TRUE)

24
????????Semaphore

• ??????????????????????????????????????????????
• ???????? S ??????????????????? (integer)
  ????????????????? ????????????????????????????????
  ??????????? 2 ???????????????????????????????
  (atomic operation) ??????
• wait ??? signal
• ??????????????????????????????? (atomic
  operation)
• ????????????????????????????????????????
  ??????????????????????????????????????????????????
  ????????????????????????
• ??????? wait(S) ??????????????????????? S (S ? 0)
  ?????????????? (S--) ?????????????????????????????
  ????

25
???????? (2)Semaphore

• S wait() and signal()
• Originally called P() and V()
• wait (S)
• while S lt 0
• // no-op
• S--
• signal (S)
• S

26
Semaphore as General Synchronization Tool

• ????????????????????????? 2 ???? ??????
• ??????????? (Counting semaphore)
  ?????????????????????????
• ?????????????? (Binary semaphore)
  ?????????????????? 0 ??? 1 ????????????????
  ????????????? mutex locks
• ??????????????????????????????????
  ?????????????????????????????????????????? 0 ???
  1 ?????????????????????
• ????????? counting semaphore S ??????????????????
• Provides mutual exclusion
• Semaphore S // initialized to 1
• wait (S)
• Critical Section
• signal (S)

27
Semaphore Implementation

• ??????????????????????????????????????????????????
  ???????????????????????????? ?????????????????????
  ????????????????????????????????????? ?
  ?????????????
• ??????????????????????????????????????????????????
  ??????????????????????????????????????
• ??????????????????????????????????????????????????
  ????? ????????????????????????????????????????????
  ??????????????????? P ??????????
  (??????????????????)
• ??????????????????????????????????????????????????
  ???? V (????????????????)
• ????????????????????????????????????? 0
  ??????????????????????????????????????
• ???????????? ?????????????????????????????????????
  ??????????????????????????????? 0

28
Semaphore Implementation with no Busy waiting

• ???????????????????????????????????????????
  (waiting queue) ???????????????????????????? 2
  ??????
• value (of type integer)
• pointer (????????????????????????????????)
• Two operations
• block ??????????????????????
• wakeup(P) ??????????????????????? P
  ???????????????? (?????????????????????????????)

29
Semaphore Implementation with no Busy waiting
(Cont.)

• Implementation of wait
• wait (S)
• value--
• if (value lt 0)
• add this process to waiting
  queue
• block()
• Implementation of signal
• Signal (S)
• value
• if (value lt 0)
• remove a process P from the
  waiting queue
• wakeup(P)

30
?????????????????????Deadlock and Starvation on
Semaphore

• ?????????? (Deadlock) ???????????????? 2
  ??????????????????????????????????????????????????
  ????????????? ?????????????????????????????????
  ??????????????????????????????????????????????????
  ???? ????????????????????????????????????????
• Let S and Q be two semaphores initialized to 1
• P0 P1
• wait (S)
  wait (Q)
• wait (Q)
  wait (S)
• . .
• . .
• . .
• signal (S)
  signal (Q)
• signal (Q)
  signal (S)
• ???????????? (starvation) ???? ???????????????????
  ????? (indefinite blocking) ??????????????????????
  ??????????????????????????????????????????????????
  ? ?????????????????????????????????????????

31
???????????????????????????? Classical Problems
of Synchronization

• ??????????????????????????????????????????????????
  ?
• Bounded-Buffer problem (Producer/Consumer
  problem)
• Readers and Writers problem
• Dining-Philosophers problem

32
Bounded-Buffer Problem

• ?????????????????????????-?????????
  (Producer/Consumer problem)
• ??????????????????????????????????????????????????
  ????????????
• ?????????????????????????????????????????????????
  (???????, ??????) ??????????????????
  ??????????????????????????????????????????????????
  ????????????????????????????????????
• ??????????????????????????????????????????????????
  ?????????????????????????????????????
  ??????????? ? ????????????? ????
  (????????????????????) ???????????????????????????
  ????????????????????

33
Bounded-Buffer Problem (Cont.)

• N buffers, each can hold one item
• Semaphore mutex initialized to the value 1
• Semaphore full initialized to the value 0
• Semaphore empty initialized to the value N.

34
Bounded Buffer Problem (Cont.)

• The structure of the producer process
• do
• // produce an item
• wait (empty)
• wait (mutex)
• // add the item to the
  buffer
• signal (mutex)
• signal (full)
• while (true)

35
Bounded Buffer Problem (Cont.)

• The structure of the consumer process
• do
• wait (full)
• wait (mutex)
• // remove an item from
  buffer
• signal (mutex)
• signal (empty)
• // consume the removed item
• while (true)

36
Readers-Writers Problem

• ????????????-?????????????????????????????????????
  ????????????????????????????????????????????
• ??????????????????????????????????????????????????
  ?????????????
• ??????? ?????????????????????????????, ????????
  update
• ???????? ???????????????????????????
• ??????????? ??????????????????????????????????????
  ????????????????? ????????????????????????????????
  ? (readers) ??????????????????????????????
  ????????????????????????????? (writers)
  ??????????????????????????????????????????????????

37
Readers-Writers Problem (Cont.)

• ???????????????????????????????????????????????
• 1. ??????????????????????????????????????
• 2. ???????????????????????????????????????????????
  ???? ? ???????????????
• 3. ?????????????????????????????
  ???????????????????????????????????????????
• ?????????????????????????????????????????????
• ??????? ??????????????????????????????????????
• ???????? ?????????????????????????????????????
• Shared Data
• Data set
• Semaphore mutex initialized to 1.
• Semaphore wrt initialized to 1.
• Integer readcount initialized to 0.

38
Readers-Writers Problem (Cont.)

• The structure of a writer process
• do
• wait (wrt)
• // writing is performed
• signal (wrt)
• while (true)

39
Readers-Writers Problem (Cont.)

• The structure of a reader process
• do
• wait (mutex)
• readcount
• if (readercount 1) wait
  (wrt)
• signal (mutex)
• // reading is
  performed
• wait (mutex)
• readcount - -
• if redacount 0) signal
  (wrt)
• signal (mutex)
• while (true)

40
Dining-Philosophers Problem

• Shared data
• Bowl of rice (data set)
• Semaphore chopstick 5 initialized to 1

41
Dining-Philosophers Problem (Cont.)

• ?????????????? 5 ?????????????????????????????
• ?????????????????? 5 ????????????????
• ?????????????????? 5 ?????????????????????????????
  ????????????????????????
• ??????????????????????????????????????????????????
  ???????????????????????????????????????????
  ??????????????????????????????????????????????????
  ?????????????????????
• ??????????????????????????????????????????????????
  ???????????? ?????????????????????????????????????
  ???????????????????????

42
Dining-Philosophers Problem (Cont.)

• The structure of Philosopher i
• Do
• wait ( chopsticki )
• wait ( chopStick (i 1) 5 )
• // eat
• signal ( chopsticki )
• signal (chopstick (i 1) 5 )
• // think
• while (true)

43
Problems with Semaphores

• ??????????????????????????????????????????????????
  ????????????????????????????? ????
  ??????????????????????????????????????????????????
  ??????????????????????????????????????????????????
  ??????????????????????
• ????????????????????????????? ???
• ?????????????????????????????????
• ??????????????????????????????????????????????????
  ???????? P ??????????
• ?????????????????????????????????????? V
  ????????????????????

44
Problems with Semaphores (Cont.)

• Correct use of semaphore operations
• signal (mutex) . wait (mutex)
• wait (mutex) wait (mutex)
• Omitting of wait (mutex) or signal (mutex) (or
  both)

45
Problems with Semaphores (Cont.)

• ?????????????????????????????????????
  ??????????????????? ???????????
• ??????????????????????? 4 ????????????
  ????????????????????
• ??????????????????????????????????????????????????
  ??????????????????????? (?????????????????????????
  ??????????????)
• ????????????????????????? ????????????
  ????????????????????????????????????????
  ???????????????????? ?????????????????????????????
  ??????????????????????????????????????????????????
  
• ??????????????????????????????????????????????????
  ??????????????????????????????????????
  ?????????????????????????????????????????????

46
????????????? Monitors

• ??????????????????????????????????????????????????
  ??????????????????? (procedure)
  ???????????????????????? ???????????????????????
  
• ?????????????????????????????????
  ???????????????????????????????????????????????
• ????????????????????????????????????????
• ??????????????????????????????????????????????????
  ????????????????????????????????
  ??????????????????????????????????????? ?????????
• ??????????????????????????????????????????????????
  ??????????????????????????????????
• ??????????????????????????????????????????????????
  ??????????????????????????? ?????????????????????
  ??????????????????????????????????????????????????
  ????????????
• ? ??????????????? ????????????????????????????????
  ?? active ??????????????????????

47
????????????? Monitors

• monitor monitor-name
• // shared variable declarations
• procedure P1 () .
• procedure Pn ()
• Initialization code ( .)

48
Schematic view of a Monitor
49
Condition Variables

• condition x, y
• Two operations on a condition variable
• x.wait () a process that invokes the operation
  is
• suspended.
• x.signal () resumes one of processes (if any)
  tha
• invoked x.wait ()

50
Monitor with Condition Variables
51
Solution to Dining Philosophers

• ????????????????????????????????????
  ???????????????????????????????????????????????
• ??????????????????????????????????????????????????
  ??????????? pickUp() ?????????????????????????????
  ?????
• ????????????????????????????????? putDown()
  ????????????????
• ??????? ????????? i ???????????????????? pickUp()
  ??? putDown() ????????

52
Solution to Dining Philosophers (cont.)

• monitor DP
• enum THINKING HUNGRY, EATING) state 5
• condition self 5
• void pickup (int i)
• statei HUNGRY
• test(i)
• if (statei ! EATING) self i.wait
• void putdown (int i)
• statei THINKING
• // test left and right
  neighbors
• test((i 4) 5)
• test((i 1) 5)

53
Solution to Dining Philosophers (cont.)

• void test (int i)
• if ( (state(i 4) 5 ! EATING)
• (statei HUNGRY)
• (state(i 1) 5 ! EATING) )
• statei EATING
• selfi.signal ()
• initialization_code()
• for (int i 0 i lt 5 i)
• statei THINKING

54
Synchronization Examples

• Solaris
• Windows XP
• Linux
• Pthreads

55
Solaris Synchronization

• ?????? lock ??????? ?????????????????
  multitasking ??? multithreading (???????
  real-time threads) ??? multiprocessing
• ??? adaptive mutexes ?????????????????????????????
  ???????????????????????? short code segments
• ??? lock ??? condition variables and
  readers-writers ??????????????????????????????????
  ???????????
• ??? turnstiles ???????????????????????????????????
  ? ?????????? adaptive mutex or reader-writer lock

56
Windows XP Synchronization

• ??? interrupt ????????????????????????????????????
  ???????????????????????? (uniprocessor systems)
• ??? spinlocks ?? multiprocessor systems
• ??????????? dispatcher objects ????????????????
  mutexes ??? semaphores
• Dispatcher objects ?????????????????????????
  (events)
• ?????????????????????????????????????????????????
  (condition variable)

57
Linux Synchronization

• Linux
• ??????????? interrupts ????????????????????????
  (short critical sections) ???
• Linux provides
• semaphores
• spin locks

58
Pthreads Synchronization

• Pthreads API is OS-independent
• It provides
• mutex locks
• condition variables
• Non-portable extensions include
• read-write locks
• spin locks

59
End of Process Synchronization
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