程序代写代做代考 Erlang compiler go Java kernel algorithm Chapters 6 & 7: Synchronization Tools and Classic Problems

Chapters 6 & 7: Synchronization Tools and Classic Problems
Operating System Concepts – 9th Edition Silberschatz, Galvin and Gagne ©2013

Process Synchronization and Examples
■ Background
■ The Critical-Section Problem
■ Peterson’s Solution
■ Synchronization Hardware
■ Mutex Locks
■ Semaphores
■ Classic Problems of Synchronization
■ Monitors
■ Synchronization Examples
■ Alternative Approaches
Operating System Concepts – 9th Edition 5.2 Silberschatz, Galvin and Gagne ©2013

Objectives
■ To present the concept of process synchronization.
■ To introduce the critical-section problem, whose solutions
can be used to ensure the consistency of shared data
■ To present both software and hardware solutions of the critical-section problem
■ T o examine several classical process-synchronization problems
■ To explore several tools that are used to solve process synchronization problems
Operating System Concepts – 9th Edition 5.3 Silberschatz, Galvin and Gagne ©2013

Background
■ Processes can execute concurrently
● Maybeinterruptedatanytime,partiallycompleting
execution
■ Concurrent access to shared data may result in data inconsistency
■ Maintaining data consistency requires mechanisms to ensure the orderly execution of cooperating processes
■ Illustration of the problem: 
Suppose that we wanted to provide a solution to the consumer-producer problem that fills all the buffers. We can do so by having an integer counter that keeps track of the number of full buffers. Initially, counter is set to 0. It is incremented by the producer after it produces a new buffer and is decremented by the consumer after it consumes a buffer.
■ Simple, What could go wrong?! 😉
Operating System Concepts – 9th Edition 5.4 Silberschatz, Galvin and Gagne ©2013

Producer
while (true) {
/* produce an item in next produced */
}
while (counter == BUFFER_SIZE) ;
/* do nothing */
buffer[in] = next_produced;
in = (in + 1) % BUFFER_SIZE;
counter++;
Operating System Concepts – 9th Edition 5.5 Silberschatz, Galvin and Gagne ©2013

Consumer
while (true) {
while (counter == 0)
}
; /* do nothing */
next_consumed = buffer[out];
out = (out + 1) % BUFFER_SIZE;
counter–;
/* consume the item in next consumed */
Operating System Concepts – 9th Edition 5.6 Silberschatz, Galvin and Gagne ©2013

Race Condition
■ counter++ could be implemented as 
 
register1 = counter
register1 = register1 + 1
counter = register1
■ counter– could be implemented as 
■
 
register2 = counter
register2 = register2 – 1
counter = register2
Consider this execution interleaving with “count = 5” initially:
S0: producer execute register1 = counter
S1: producer execute register1 = register1 + 1 (interrupt)
S2: consumer execute register2 = counter
S3: consumer execute register2 = register2 – 1 (interrupt)
S4: producer execute counter = register1 (producer is back to waiting, consumer finishes)
S5: consumer execute counter = register2 Operating System Concepts – 9th Edition 5.7
{register1 = 5}  {register1 = 6}  
{register2 = 5}   {register2 = 4}  
{counter = 6 }   {counter = 4}
Silberschatz, Galvin and Gagne ©2013

What Can Happen If You Don’t Know This Lecture Cold
© John Day, All Rights Reserved, 2009
8

Critical Regions
Conditions required to avoid race condition
1. No two processes/threads may be simultaneously inside their critical regions.
2. No assumptions may be made about speeds or the number of CPUs.
3. No process running outside its critical region may block other processes/threads.
4. No process/thread should have to wait forever to enter its critical region.
Bottom Line: if two threads read/write the same variable, it is a critical region. Do something about it.
If one writes and the other only reads, it isn’t.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
9

Critical Section Problem
■ Consider system of n processes {p0, p1, … pn-1}
■ Each process has critical section segment of code
● Processmaybechangingcommonvariables,updating table, writing file, etc
● Whenoneprocessincriticalsection,noothermaybeinits critical section
■ Critical section problem is to design protocol to solve this
■ Each process must ask permission to enter critical section in entry section, may follow critical section with exit section, then remainder section
Operating System Concepts – 9th Edition 5.10 Silberschatz, Galvin and Gagne ©2013

Critical Section
■ General structure of process Pi
Operating System Concepts – 9th Edition 5.11 Silberschatz, Galvin and Gagne ©2013

do {
Algorithm for Process Pi critical section
while (turn == j);
turn = j;
remainder section
} while (true);
Operating System Concepts – 9th Edition 5.12 Silberschatz, Galvin and Gagne ©2013

1. 2.
Solution to Critical-Section Problem
Mutual Exclusion – If process Pi is executing in its critical section, then no other processes can be executing in their critical sections
Progress – If no process is executing in its critical section and there exist some processes that wish to enter their critical section, then the selection of the processes that will enter the critical section next cannot be postponed indefinitely
3. Bounded Waiting – A bound must exist on the number of times that other processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is granted
! Assume that each process executes at a nonzero speed ! No assumption concerning relative speed of the n
processes
Operating System Concepts – 9th Edition 5.13 Silberschatz, Galvin and Gagne ©2013

Critical-Section Handling in OS
Two approaches depending on if kernel is preemptive or non- preemptive
● Preemptive – allows preemption of process when running in kernel mode
● Non-preemptive – runs until exits kernel mode, blocks, or voluntarily yields CPU
! Essentially free of race conditions in kernel mode
Operating System Concepts – 9th Edition 5.14 Silberschatz, Galvin and Gagne ©2013

Peterson’s Solution
■ Good algorithmic description of solving the problem
■ Two process solution
■ Assume that the load and store machine-language instructions are atomic; that is, cannot be interrupted
■ The two processes share two variables: ● int turn;
● Boolean flag[2]
■ The variable turn indicates whose turn it is to enter the critical
section
■ The flag array is used to indicate if a process is ready to enter
the critical section. flag[i] = true implies that process Pi is ready!
Operating System Concepts – 9th Edition 5.15 Silberschatz, Galvin and Gagne ©2013

Algorithm for Process Pi do {
flag[i] = true;
turn = j;
while (flag[j] && turn = = j);
critical section
remainder section
} while (true);
/* Ready to enter */
/* Set flag */
/* Waiting my turn */
/* Leaving critical
section */
flag[i] = false;
Operating System Concepts – 9th Edition 5.16 Silberschatz, Galvin and Gagne ©2013

Peterson’s Solution (Cont.) ■ Provable that the three Critical Section requirement are met:
1. Mutual exclusion is preserved
Pi enters Critical Section only if:
either flag[j] = false or turn = i
2. Progress requirement is satisfied
3. Bounded-waiting requirement is met
Operating System Concepts – 9th Edition 5.17 Silberschatz, Galvin and Gagne ©2013

Synchronization Hardware
■ Many systems provide hardware support for implementing the critical section code.
■ All solutions below based on idea of locking ● Protectingcriticalregionsvialocks
■ Uniprocessors – could disable interrupts
● Currentlyrunningcodewouldexecutewithoutpreemption ● Generallytooinefficientonmultiprocessorsystems
! Operating systems using this not broadly scalable
■ Modern machines provide special atomic hardware instructions
! Atomic = non-interruptible
● Eithertestmemorywordandsetvalue ● Orswapcontentsoftwomemorywords
Operating System Concepts – 9th Edition 5.18 Silberschatz, Galvin and Gagne ©2013

Solution to Critical-section Problem Using Locks
do {
critical section
remainder section
} while (TRUE);
acquire lock
release lock
Operating System Concepts – 9th Edition 5.19 Silberschatz, Galvin and Gagne ©2013

test_and_set Instruction
Definition:
boolean test_and_set (boolean *target)
{
boolean rv = *target;
*target = TRUE;
return rv:
}
1. Executedatomically
2. Returnstheoriginalvalueofpassedparameter
3. Setthenewvalueofpassedparameterto“TRUE”.
If you are wondering where the “test” is . . .
Operating System Concepts – 9th Edition 5.20
Silberschatz, Galvin and Gagne ©2013

Solution using test_and_set()
■ Shared Boolean variable lock, initialized to FALSE ■ Solution:
do {
while (test_and_set(&lock)); /* do nothing */
/* critical section */
lock = false;
/* remainder section */
} while (true);
Operating System Concepts – 9th Edition 5.21 Silberschatz, Galvin and Gagne ©2013

Curious: Stallings’ Definition
• Test and Set Instruction
boolean testset (int i) {
if (i == 0) {
i = 1;
return true;
}
else {
return false;
} }
This looks like a test and set.
22

compare_and_swap Instruction
Definition:
int compare _and_swap(int *value, int expected, int new_value) {
int temp = *value;
if (*value == expected)
*value = new_value;
return temp;
}
1. Executedatomically
2. Returnstheoriginalvalueofpassedparameter“value”
3. Set the variable “value” the value of the passed parameter “new_value” but only if “value” ==“expected”. That is, the swap takes place only under this condition.
Operating System Concepts – 9th Edition 5.23 Silberschatz, Galvin and Gagne ©2013

Solution using compare_and_swap
■ Shared integer “lock” initialized to 0; ■ Solution:
do {
while (compare_and_swap(&lock, 0, 1) != 0)
; /* do nothing */
/* critical section */
lock = 0;
/* remainder section */
} while (true);
Operating System Concepts – 9th Edition 5.24 Silberschatz, Galvin and Gagne ©2013

Bounded-waiting Mutual Exclusion with test_and_set
do {
waiting[i] = true;
key = true;
while (waiting[i] && key)
key = test_and_set(&lock);
waiting[i] = false;
/* critical section */
j = (i + 1) % n;
while ((j != i) && !waiting[j])
j = (j + 1) % n;
if (j == i)
/*i is in */
/*finds the next one to run */
lock = false; /* No one else to run */
else
waiting[j] = false; /* let j go next */
/* remainder section */
} while (true);
Operating System Concepts – 9th Edition 5.25 Silberschatz, Galvin and Gagne ©2013

Mutex Locks
■ Previous solutions are complicated and generally inaccessible to application programmers
■ OS designers build software tools to solve critical section problem
■ Simplest is mutex lock
■ Protectacriticalsection byfirstacquire()alockthen
release() the lock
● Booleanvariableindicatingiflockisavailableornot
■ Calls to acquire() and release() must be atomic ● Usuallyimplementedviahardwareatomicinstructions
■ But this solution requires busy waiting
■ This lock therefore called a spinlock
Operating System Concepts – 9th Edition 5.26 Silberschatz, Galvin and Gagne ©2013

■
acquire() and release()
acquire() {
while (!available)
; /* busy wait */
available = false;;
}
release() {
available = true;
}
■
■ do{
critical section
remainder section
} while (true);
acquire lock
release lock
Operating System Concepts – 9th Edition 5.27
Silberschatz, Galvin and Gagne ©2013

Semaphore
■ Synchronization tool that provides more sophisticated ways (than Mutex locks) for process to synchronize their activities.
■ Semaphore S – integer variable
■ Can only be accessed via two indivisible (atomic) operations
● wait()andsignal()
! Originally called P() and V() Also called up and down
■ Definition of the wait() operation
wait(S) {
while (S <= 0) ; // busy wait S--; } ■ Definition of the signal() operation signal(S) { S++; } Operating System Concepts – 9th Edition 5.28 Silberschatz, Galvin and Gagne ©2013 Semaphore Usage ■ Counting semaphore – integer value can range over an unrestricted domain ■ Binary semaphore – integer value can range only between 0 and 1 ● Same as a mutex lock ■ Can solve various synchronization problems ■ Consider P1 and P2 that require S1 to happen before S2 Create a semaphore “synch” initialized to 0 P1: S1; signal(synch); P2: wait(synch); S2; ■ Can implement a counting semaphore S as a binary semaphore Operating System Concepts – 9th Edition 5.29 Silberschatz, Galvin and Gagne ©2013 Semaphore Implementation ■ Must guarantee that no two processes can execute the wait() and signal() on the same semaphore at the same time ■ Thus, the implementation becomes the critical section problem where the wait and signal code are placed in the critical section ● Couldnowhavebusywaitingincriticalsection implementation ! But implementation code is short ! Little busy waiting if critical section rarely occupied ■ Note that applications may spend lots of time in critical sections and therefore this is not a good solution Operating System Concepts – 9th Edition 5.30 Silberschatz, Galvin and Gagne ©2013 Semaphore Implementation with no Busy waiting ■ With each semaphore there is an associated waiting queue ■ Each entry in a waiting queue has two data items: ● value (of type integer) ● pointer to next record in the list ■ Two operations: ● block–placetheprocessinvokingtheoperationonthe appropriate waiting queue ● wakeup–removeoneofprocessesinthewaitingqueue and place it in the ready queue ■ typedefstruct{ int value; struct process *list; } semaphore; Operating System Concepts – 9th Edition 5.31 Silberschatz, Galvin and Gagne ©2013 Implementation with no Busy waiting (Cont.) wait(semaphore *S) { S->value–;
if (S->value < 0) { add this process to S->list;
block(); }
}
signal(semaphore *S) {
S->value++;
if (S->value <= 0) { remove a process P from S->list;
wakeup(P); }
}
Operating System Concepts – 9th Edition 5.32 Silberschatz, Galvin and Gagne ©2013

Struct Semaphore; Count: Integer; Queue:DoubleLinkList;
Word me; %points at current stack;
Procedure P(S);
Semaphore S;
Begin
Lockout(
S.count := S.count – 1;
If S < 0 then Queueme(Semaphore(Stacklist[s])); %If so, queue and run another Fi) End P; Procedure V(S); Semaphore S; Begin Lockout ( S.count := S.count + 1; If S ≤ 0 Then %increment count of semaphore %Did we unblock somebody Makeready(Qdelink(Semaphore[S])); %If so, make ready Fi) End V; Semaphores Really How to Implement %decrement count of semaphore %Do we block © John Day, All Rights Reserved, 2009 33 Procedure Makeready(readystack) Value readystack; Word readystack; Begin Stack_status:=Stackrunning; Entermereadyq(me); Switchstackto(readystack); Else Enterreadyq(readystack); Fi End makeready; Procedure Queueme(Q); Word Q; Begin Mystack.status:= [Q]; Stackpriorityqenter(Q, me); Switchstackto(netxtguytorun); %run somebody else End queueme; How It Works (1) %set status field in Stack If Stack.Priority(readystack) GTR mystack.Priority Then %Should we switch %Put current stack in readyq %and run this guy %otherwise just put new guy in readyq %for dump readers, tells what q stack is in %Queue by software priority © John Day, All Rights Reserved, 2009 34 How It Works (2) Procedure Switchstackto(newstack); Value newstack; Word newstack; Begin Move Stack Pointer of me to bottom of stack; Me := newstack; %Point Processor at newstack Move Stack Pointer of me from bottom of stack to Stack Pointer Register; End switchstackto; The context switch happens here © John Day, All Rights Reserved, 2009 35 Deadlock and Starvation ■ Deadlock – two or more processes are waiting indefinitely for an event that can be caused by only one of the waiting processes ■ Let S and Q be two semaphores initialized to 1 P0 wait(S); wait(Q); ... signal(S); signal(Q); P1 wait(Q); wait(S); ... signal(Q); signal(S); ■ Starvation – indefinite blocking ● A process may never be removed from the semaphore queue in which it is suspended ■ Priority Inversion – Scheduling problem when lower-priority process holds a lock needed by higher-priority process ● Solved via priority-inheritance protocol Operating System Concepts – 9th Edition 5.36 Silberschatz, Galvin and Gagne ©2013 Classical Problems of Synchronization ■ Classical problems used to test newly-proposed synchronization schemes ● Bounded-BufferProblem ● ReadersandWritersProblem ● Dining-PhilosophersProblem Operating System Concepts – 9th Edition 5.37 Silberschatz, Galvin and Gagne ©2013 Bounded-Buffer Problem ■ 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 Operating System Concepts – 9th Edition 5.38 Silberschatz, Galvin and Gagne ©2013 Bounded Buffer Problem (Cont.) ■ The structure of the producer process do { ... /* produce an item in next_produced */ ... wait(empty); wait(mutex); ... /* add next produced to the buffer */ ... signal(mutex); signal(full); } while (true); Operating System Concepts – 9th Edition 5.39 Silberschatz, Galvin and Gagne ©2013 Bounded Buffer Problem (Cont.) ■ The structure of the consumer process Do { wait(full); wait(mutex); ... /* remove an item from buffer to next_consumed */ ... signal(mutex); signal(empty); ... /* consume the item in next consumed */ ... } while (true); Operating System Concepts – 9th Edition 5.40 Silberschatz, Galvin and Gagne ©2013 Readers-Writers Problem ■ A data set is shared among a number of concurrent processes ● Readers–onlyreadthedataset;theydonotperformanyupdates ● Writers – can both read and write ■ Problem – allow multiple readers to read at the same time ● Onlyonesinglewritercanaccesstheshareddataatthesametime ■ Several variations of how readers and writers are considered – all involve some form of priorities ■ Shared Data ● Dataset ● Semaphore rw_mutex initialized to 1 ● Semaphoremutexinitializedto1 ● Integerread_countinitializedto0 Operating System Concepts – 9th Edition 5.41 Silberschatz, Galvin and Gagne ©2013 Readers-Writers Problem (Cont.) ■ The structure of a writer process do { wait(rw_mutex); ... /* writing is performed */ ... signal(rw_mutex); } while (true); Operating System Concepts – 9th Edition 5.42 Silberschatz, Galvin and Gagne ©2013 Readers-Writers Problem (Cont.) ■ The structure of a reader process do { wait(mutex); read_count++; if (read_count == 1) wait(rw_mutex); signal(mutex); ... /* reading is performed */ ... wait(mutex); read count--; if (read_count == 0) signal(rw_mutex); signal(mutex); } while (true); Operating System Concepts – 9th Edition 5.43 Silberschatz, Galvin and Gagne ©2013 Readers-Writers Problem Variations ■ First variation – no reader kept waiting unless writer has permission to use shared object ■ Second variation – once writer is ready, it performs the write ASAP ■ Both may have starvation leading to even more variations ■ Problem is solved on some systems by kernel providing reader-writer locks Operating System Concepts – 9th Edition 5.44 Silberschatz, Galvin and Gagne ©2013 Dining-Philosophers Problem ■ Philosophers spend their lives alternating thinking and eating ■ Don’t interact with their neighbors, occasionally try to pick up 2 chopsticks (one at a time) to eat from bowl ● Need both to eat, then release both when done ■ In the case of 5 philosophers ● Shared data ! Bowl of rice (data set) ! Semaphore chopstick [5] initialized to 1 Operating System Concepts – 9th Edition 5.45 Silberschatz, Galvin and Gagne ©2013 ■ Dining-Philosophers Problem Algorithm The structure of Philosopher i: do { wait (chopstick[i] ); wait (chopStick[ (i + 1) % 5] ); // eat signal (chopstick[i] ); signal (chopstick[ (i + 1) % 5] ); // think } while (TRUE); What is the problem with this algorithm? ■ Operating System Concepts – 9th Edition 5.46 Silberschatz, Galvin and Gagne ©2013 Dining-Philosophers Problem Algorithm (Cont.) ■ Deadlock handling ● ● ● Allow at most 4 philosophers to be sitting simultaneously at the table. Allow a philosopher to pick up the forks only if both are available (picking must be done in a critical section. Use an asymmetric solution -- an odd-numbered philosopher picks up first the left chopstick and then the right chopstick. Even-numbered philosopher picks up first the right chopstick and then the left chopstick. Operating System Concepts – 9th Edition 5.47 Silberschatz, Galvin and Gagne ©2013 ■ Problems with Semaphores Incorrect use of semaphore operations:  ● signal (mutex) .... wait (mutex)  ● wait (mutex) ... wait (mutex) ● Omitting of wait (mutex) or signal (mutex) (or both) ● Old Saying: If you don’t do it right, it won’t work! Deadlock and starvation are possible. ■ Operating System Concepts – 9th Edition 5.48 Silberschatz, Galvin and Gagne ©2013 Monitors ■ A high-level abstraction that provides a convenient and effective mechanism for process synchronization ■ Abstract data type, internal variables only accessible by code within the procedure ■ Only one process may be active within the monitor at a time ■ But not powerful enough to model some synchronization schemes monitor monitor-name { // shared variable declarations procedure P1 (...) { .... } procedure Pn (...) {......} Initialization code (...) { ... } } } Operating System Concepts – 9th Edition 5.49 Silberschatz, Galvin and Gagne ©2013 Schematic view of a Monitor Operating System Concepts – 9th Edition 5.50 Silberschatz, Galvin and Gagne ©2013 Condition Variables ■ condition x, y; ■ Two operations are allowed on a condition variable: ● x.wait() – a process that invokes the operation is suspended until x.signal()There are no restrictions on where the x.wait occurs ! Meanwhile, other processes may enter the critical section. The process waiting will resume where it executed the wait. ● x.signal() – resumes one of processes (if any) that invoked x.wait() ! If no x.wait() on the variable, then it has no effect on the variable ■ Warning: Condition Wait MUST be the first statement of the block or else there is a potential for very bad things to happen. ! Suppose while waiting for condition, variables are changed such waiting process shouldn’t be here? – Per Brinch Hansen knew what he was doing, it is not clear these guys do. Operating System Concepts – 9th Edition 5.51 Silberschatz, Galvin and Gagne ©2013 Monitor with Condition Variables Operating System Concepts – 9th Edition 5.52 Silberschatz, Galvin and Gagne ©2013 Condition Variables Choices ■ If process P invokes x.signal(), and process Q is suspended in x.wait(), what should happen next? ● Both Q and P cannot execute in parallel. If Q is resumed, then P must wait ● Still the same problem. P shouldn’t signal until it leaves the critical section. ■ Options include ● Signal and wait – P waits until Q either leaves the monitor or it waits for another condition ● Signal and continue – Q waits until P either leaves the monitor or it waits for another condition ● Both have pros and cons – language implementer can decide ● Monitors implemented in Concurrent Pascal compromise ! P executing signal immediately leaves the monitor, Q is resumed ● Implemented in other languages including Mesa, C#, Java Operating System Concepts – 9th Edition 5.53 Silberschatz, Galvin and Gagne ©2013 Why This Characterization of Condition Wait is Incompetent • In the original, Condition Wait is a language construct of the form: Condition_Wait begin
end;
• With this there is no question what should happen:
• If process A attempts to enter the critical section, A might block on the . Other processes could enter and leave the critical section. While the other processes are in the critical section no other process may enter.
• When the condition is met, A’s entry to the critical section behaves normally, i.e. if someone is in the critical section, A waits; otherwise A enters.
• Searching much of what Hansen wrote on this, I have found nothing where he says condition-wait must be the first statement but it is always used as the first statement.
• I think Hansen didn’t think anyone would be dumb enough to use condition_wait anywhere else.
• He hadn’t allowed for the current crop of ‘brilliant’ systems programmers.
54

Monitor Solution to Dining Philosophers
monitor DiningPhilosophers
{
enum { THINKING; HUNGRY, EATING) state [5] ;
condition self [5];
void pickup (int i) {
state[i] = HUNGRY;
}
}
test(i);
if (state[i] != EATING) self[i].wait;
void putdown (int i) {
state[i] = THINKING;
// test left and right neighbors
test((i + 4) % 5);
test((i + 1) % 5);
Operating System Concepts – 9th Edition 5.55 Silberschatz, Galvin and Gagne ©2013

Solution to Dining Philosophers (Cont.)
void test (int i) {
if ((state[(i + 4) % 5] != EATING) &&
(state[i] == HUNGRY) &&
(state[(i + 1) % 5] != EATING) ) {
state[i] = EATING ;
self[i].signal () ;
} }
initialization_code() {
for (int i = 0; i < 5; i++) state[i] = THINKING; } } Operating System Concepts – 9th Edition 5.56 Silberschatz, Galvin and Gagne ©2013 Solution to Dining Philosophers (Cont.) ■ Each philosopher i invokes the operations pickup() and putdown() in the following sequence: DiningPhilosophers.pickup(i); EAT DiningPhilosophers.putdown(i); ■ No deadlock, but starvation is possible Operating System Concepts – 9th Edition 5.57 Silberschatz, Galvin and Gagne ©2013 Monitor Implementation Using Semaphores ■ Variables semaphore mutex; // (initially = 1) semaphore next; // (initially = 0) int next_count = 0; ■ Each procedure F will be replaced by wait(mutex); ... body of F; ... if (next_count > 0)
signal(next)
else
signal(mutex);
■ Mutual exclusion within a monitor is ensured
Operating System Concepts – 9th Edition 5.58 Silberschatz, Galvin and Gagne ©2013

Monitor Implementation – Condition Variables
■ For each condition variable x, we have:
semaphore x_sem; // (initially = 0)
int x_count = 0;
■ The operation x.wait can be implemented as:
x_count++;
if (next_count > 0)
signal(next);
else
signal(mutex);
wait(x_sem);
x_count–;
Operating System Concepts – 9th Edition
5.59 Silberschatz, Galvin and Gagne ©2013

Monitor Implementation (Cont.)
■ The operation x.signal can be implemented as: 
if (x_count > 0) {
next_count++;
signal(x_sem);
wait(next);
next_count–;
}
Operating System Concepts – 9th Edition 5.60 Silberschatz, Galvin and Gagne ©2013

Resuming Processes within a Monitor
■ If several processes queued on condition x, and x.signal() executed, which should be resumed?
■ FCFS frequently not adequate
■ conditional-wait construct of the form x.wait(c)
● Wherecisprioritynumber
● Processwithlowestnumber(highestpriority)is
scheduled next
Operating System Concepts – 9th Edition 5.61 Silberschatz, Galvin and Gagne ©2013

Single Resource allocation
■ Allocate a single resource among competing processes using priority numbers that specify the maximum time a process plans to use the resource
R.acquire(t); …
access the resource;
…
R.release;
■ Where R is an instance of type ResourceAllocator
Operating System Concepts – 9th Edition 5.62 Silberschatz, Galvin and Gagne ©2013

A Monitor to Allocate Single Resource
monitor ResourceAllocator
{
boolean busy;
condition x;
void acquire(int time) {
if (busy)
x.wait(time);
busy = TRUE;
}
void release() {
busy = FALSE;
x.signal();
}
initialization code() {
busy = FALSE;
}
}
Operating System Concepts – 9th Edition 5.63 Silberschatz, Galvin and Gagne ©2013

■ Solaris ■ Windows ■ Linux
■ Pthreads
Synchronization Examples
Operating System Concepts – 9th Edition 5.64 Silberschatz, Galvin and Gagne ©2013

Solaris Synchronization
■ Implements a variety of locks to support multitasking, multithreading (including real-time threads), and multiprocessing
■ Uses adaptive mutexes for efficiency when protecting data from short code segments
● Starts as a standard semaphore spin-lock
● If lock held, and by a thread running on another CPU, spins
● If lock held by non-run-state thread, block and sleep waiting for signal of lock being released
■ Uses condition variables
■ Uses readers-writers locks when longer sections of code need
access to data
■ Uses turnstiles to order the list of threads waiting to acquire either an adaptive mutex or reader-writer lock
● Turnstiles are per-lock-holding-thread, not per-object
■ Priority-inheritance per-turnstile gives the running thread the highest of
the priorities of the threads in its turnstile
Operating System Concepts – 9th Edition 5.65 Silberschatz, Galvin and Gagne ©2013

Windows Synchronization
■ Uses interrupt masks to protect access to global resources on uniprocessor systems
■ Uses spinlocks on multiprocessor systems
● Spinlocking-threadwillneverbepreempted
■ Also provides dispatcher objects user-land which may act mutexes, semaphores, events, and timers
● Events
! An event acts much like a condition variable
● Timersnotifyoneormorethreadwhentimeexpired
● Dispatcherobjectseithersignaled-state(objectavailable)
or non-signaled state (thread will block)
Operating System Concepts – 9th Edition 5.66 Silberschatz, Galvin and Gagne ©2013

Linux Synchronization
■ Linux:
● PriortokernelVersion2.6,disablesinterruptsto
implement short critical sections
● Version2.6andlater,fullypreemptive
■ Linux provides: ● Semaphores
● atomicintegers
● spinlocks
● reader-writerversionsofboth
■ On single-cpu system, spinlocks replaced by enabling and disabling kernel preemption
Operating System Concepts – 9th Edition 5.67 Silberschatz, Galvin and Gagne ©2013

Pthreads Synchronization
■ Pthreads API is OS-independent
■ It provides:
● mutexlocks
● conditionvariable
■ Non-portable extensions include:
● read-writelocks ● spinlocks
Operating System Concepts – 9th Edition 5.68 Silberschatz, Galvin and Gagne ©2013

Alternative Approaches
■ Transactional Memory 
■ OpenMP 
■ Functional Programming Languages
Operating System Concepts – 9th Edition 5.69 Silberschatz, Galvin and Gagne ©2013

Transactional Memory
■ A memory transaction is a sequence of read-write operations to memory that are performed atomically.
void update()
{
/* read/write memory */
}
Operating System Concepts – 9th Edition
5.70 Silberschatz, Galvin and Gagne ©2013

OpenMP
■ OpenMP is a set of compiler directives and API that support parallel progamming.
void update(int value)
{
#pragma omp critical
{
count += value
}
}
The code contained within the #pragma omp critical directive
is treated as a critical section and performed atomically.
Operating System Concepts – 9th Edition 5.71 Silberschatz, Galvin and Gagne ©2013

Functional Programming Languages
■ Functional programming languages offer a different paradigm than procedural languages in that they do not maintain state.  
■ Variables are treated as immutable and cannot change state once they have been assigned a value. 
■ There is increasing interest in functional languages such as Erlang and Scala for their approach in handling data races.
Operating System Concepts – 9th Edition 5.72 Silberschatz, Galvin and Gagne ©2013

What Can Happen If You Don’t Know This Lecture Cold
© John Day, All Rights Reserved, 2009
73

Conjecture:
Critical Sections Are Unnecessary
• An artifact of trying to solve a non-sequential problem with a sequential programming model.
– If every “resource” that must be modified by multiple actors (thread, process, task, etc.) is controlled by a single actor that makes all changes to the resource,
– Any actor wishing to modify the resource sends a non-blocking request (message) to the controlling thread, and
– The controlling thread processes a given request to completion, before processing another request.
• Order of processing is policy.
• The actor never blocks, it may send requests to other actors and note its state
but the actor goes on to do other work for others. – The only critical sections then are in IPC.
© John Day, All Rights Reserved, 2009
74

A Request
■ Look at the rest of these slides from Tanenbaum’s book. They cover the same material.
■ Which ones are easier to understand?
■ Send me an email with your opinion.
Operating System Concepts – 9th Edition 5.76 Silberschatz, Galvin and Gagne ©2013

Mutual Exclusion with Busy Waiting (2)
Entering and leaving a critical region using the TSL instruction
77

Mutual Exclusion with Busy Waiting (3) Solution using Swap
• Shared Boolean variable lock initialized to FALSE; Each process has a local Boolean variable key.
• Solution:
while (true) {
key = TRUE;
while ( key == TRUE)
Swap (&lock, &key ); // critical section
lock = FALSE;
// remainder section
}
78

Mutual Exclusion with Busy Waiting (4)
Peterson’s solution for achieving mutual exclusion
79

Semaphores
The producer-consumer problem using semaphores
80

Mutexes
Implementation of mutex_lock and mutex_unlock Basically a semaphore with a count of 1.
81

Mutexes in Pthreads (2)
Figure 2-31. Some of the Pthreads calls relating to condition variables.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
83

Mutexes in Pthreads (4)
.. .
.. .
Figure 2-32. Using threads to solve the producer-consumer problem.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
85

Mutexes in Pthreads (5)
.. .
Figure 2-32. Using threads to solve the producer-consumer problem.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
86

Monitors do 2 things:
• Only access monitor data via procedures in the
monitor. (You should be doing this anyway.)
• Essentially inhibits interrupts within a reduced scope, i.e. the users of the monitor.
Monitors (1)
Example of a monitor
87

Monitors (2)
• Outline of producer-consumer problem with monitors
– only one monitor procedure active at one time
– buffer has N slots
88

Monitors (3)
Solution to producer-consumer problem in Java (part 1)
89

Monitors (4)
Solution to producer-consumer problem in Java (part 2)
90

Dining Philosophers (1)
• Philosophers eat/think
• Eating needs 2 forks
• Pick one fork at a time
• How to prevent deadlock
91

Dining Philosophers (2)
A non-solution to the dining philosophers problem
92

Dining Philosophers (3)
Solution to dining philosophers problem (part 1)
93

Dining Philosophers (4)
Solution to dining philosophers problem (part 2)
94

The Readers and Writers Problem
A solution to the readers and writers problem
95

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