OS Process Synchronization, semaphore and Monitors

Lecture Plan UNIT-III
Lecture Topic Slide
No. No.
1 Process synchronization 2-5
2 critical- section prob, Peterson’s Solution 6-8

3 synchronization Hardware 9-15
4 Semaphores 16-27
5 Monitors 28-31
6 atomic transactions 32-40
7 Case study 41-42
8 REVISION

Process synchronization
• Concurrent access to shared data may result in
data inconsistency
• Maintaining data consistency requires mechanisms
to ensure the orderly execution of cooperating
processes
• 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 count that keeps
track of the number of full buffers. Initially, count 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.

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++;
}

Consumer
while (true) {
while (count == 0)
; // do nothing
nextConsumed = buffer[out];
out = (out + 1) % BUFFER_SIZE;
count--;

/* consume the item in
nextConsumed
}

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 = 5}
S1: 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}

Solution to Critical-Section
Problem
1.Mutual Exclusion - If process P is executing in its critical section,
i
then no other processes can be executing in their critical
sections
2. 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

Peterson’s Solution
• Two process solution
• Assume that the LOAD and STORE
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!

Algorithm for Process Pi
do {
flag[i] = TRUE;
turn = j;
while (flag[j] && turn == j);
critical section
flag[i] = FALSE;
remainder section
} while (TRUE);

Synchronization Hardware
• Many systems provide hardware support for
critical section code
• Uniprocessors – could disable interrupts
– Currently running code would execute without
preemption
– Generally too inefficient on multiprocessor
systems
• Operating systems using this not broadly scalable
• Modern machines provide special atomic
hardware instructions
• Atomic = non-interruptable
– Either test memory word and set value
– Or swap contents of two memory words

Solution to Critical-section Problem
Using Locks

do {
acquire lock
critical section
release lock
remainder section
} while (TRUE);

Test And Set Instruction
• Definition:

boolean TestAndSet (boolean *target)
{
boolean rv = *target;
*target = TRUE;
return rv:
}

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);

Swap Instruction

• Definition:

void Swap (boolean *a, boolean *b)
{
boolean temp = *a;
*a = *b;
*b = temp:
}

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;


} while (TRUE);

Bounded-waiting Mutual Exclusion with TestandSet()
do {
waiting[i] = TRUE;
key = TRUE;
while (waiting[i] && key)
key = TestAndSet(&lock);
waiting[i] = FALSE;
// critical section
j = (i + 1) % n;
while ((j != i) && !waiting[j])
j = (j + 1) % n;
if (j == i)
lock = FALSE;
else
waiting[j] = FALSE;
} while (TRUE);

Semaphore
• Synchronization tool that does not require busy waiting
• Semaphore S – integer variable
• Two standard operations modify S: wait() and signal()
– Originally called P() and V()
• Less complicated
• Can only be accessed via two indivisible (atomic)
operations
– wait (S) {
while S <= 0
; // no-op
S--;
}
– signal (S) {
S++;

Semaphore as General Synchronization Tool
• Counting semaphore – integer value can range over an unrestricted
domain
• Binary semaphore – integer value can range only between 0
and 1; can be simpler to implement
– Also known as mutex locks
• Can implement a counting semaphore S as a binary semaphore
• Provides mutual exclusion
Semaphore mutex; // initialized to 1
do {
wait (mutex);
// Critical Section
signal (mutex);
} while (TRUE);

Semaphore Implementation
• Must guarantee that no two processes can execute wait () and
signal () on the same semaphore at the same time
• Thus, implementation becomes the critical section problem
where the wait and signal code are placed in the crtical section.
– Could now have busy waiting in critical section
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.

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 – place the process invoking the operation
on the appropriate waiting queue.
– wakeup – remove one of processes in the
waiting queue and place it in the ready queue.

Semaphore Implementation with no Busy waiting
• Implementation of wait:
wait(semaphore *S) {
S->value--;
if (S->value < 0) {
add this process to S->list;
block();
}
}
• Implementation of signal:

signal(semaphore *S) {
S->value++;
if (S->value <= 0) {
remove a process P from S->list;
wakeup(P);
}
}

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 P1
wait (S); wait (Q);
wait (Q); wait (S);
. .
. .
. .
signal (S); signal (Q);
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

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.

Bounded Buffer Problem
• The structure of the producer process

do {
// produce an item in nextp
wait (empty);
wait (mutex);
// add the item to the buffer
signal (mutex);
signal (full);
} while (TRUE);
The structure of the consumer process
do {
wait (full);
wait (mutex);
// remove an item from buffer to nextc
signal (mutex);
signal (empty);
// consume the item in nextc
} while (TRUE);

Readers-Writers Problem
• A data set is shared among a number of concurrent processes
– Readers – only read the data set; they do not perform any
updates
– Writers – can both read and write

• Problem – allow multiple readers to read at the same time.
Only one single writer can access the shared data at the same
time
Shared Data
– Data set
– Semaphore mutex initialized to 1
– Semaphore wrt initialized to 1
– Integer readcount initialized to 0

Readers-Writers Problem
• The structure of a writer process
do {
wait (wrt) ; // writing is performed
signal (wrt) ;
} while (TRUE);
The structure of a reader process
do {
wait (mutex) ;
readcount ++ ;
if (readcount == 1)
wait (wrt) ;
signal (mutex) //reading is performed wait (mutex) ;
readcount - - ;
if (readcount == 0)
signal (wrt) ;
signal (mutex) ;
} while (TRUE);

Dining-Philosophers Problem
• Shared data
– Bowl of rice (data set)
– Semaphore chopstick [5] initialized to 1
do {
wait ( chopstick[i] );
wait ( chopStick[ (i + 1) % 5] );

// eat

signal ( chopstick[i] );
signal (chopstick[ (i + 1) %
5] );

// think

} while (TRUE);

Monitors
• A high-level abstraction that provides a convenient
and effective mechanism for process synchronization
• Only one process may be active within the monitor at
a time
monitor monitor-name
{
// shared variable declarations
procedure P1 (…) { …. }
…

procedure Pn (…) {……}

Initialization code ( ….) { … }
…
}
}

Solution to Dining Philosophers
monitor DP
{ 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);
}
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;
} }

• Each philosopher I invokes the operations pickup() and putdown() in the
following sequence:
DiningPhilosophters.pickup (i);
EAT
DiningPhilosophers.putdown (i);
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.

Monitor Implementation
• 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--;

Atomic Transactions
• System Model
• Log-based Recovery
• Checkpoints
• Concurrent Atomic Transactions

System Model
• Assures that operations happen as a single logical unit of
work, in its entirety, or not at all
• Related to field of database systems
• Challenge is assuring atomicity despite computer system
failures
• Transaction - collection of instructions or operations that
performs single logical function
– Here we are concerned with changes to stable storage –
disk
– Transaction is series of read and write operations
– Terminated by commit (transaction successful) or abort
(transaction failed) operation
– Aborted transaction must be rolled back to undo any
changes it performed

Log-Based Recovery
• Record to stable storage information about all modifications
by a transaction
• Most common is write-ahead logging
– Log on stable storage, each log record describes single
transaction write operation, including
• Transaction name
• Data item name
• Old value
• New value
– <Ti starts> written to log when transaction T i starts
– <Ti commits> written when Ti commits
• Log entry must reach stable storage before operation on data
occur

Checkpoints
• Log could become long,
and recovery could take
long
• Checkpoints shorten log
and recovery time.
• Checkpoint scheme:
1. Output all log records currently in volatile storage to stable
storage
2. Output all modified data from volatile to stable storage
3. Output a log record <checkpoint> to the log on stable
storage
• Now recovery only includes
Ti, such that Ti started
executing before the most
recent checkpoint, and all

Concurrent Transactions

• Must be equivalent to serial execution –
serializability
• Could perform all transactions in critical
section
– Inefficient, too restrictive
• Concurrency-control algorithms provide
serializability

Serializability
• Consider two data items A and B
• Consider Transactions T0 and T1
• Execute T0, T1 atomically
• Execution sequence called schedule
• Atomically executed transaction order called
serial schedule
• For N transactions, there are N! valid serial
schedules

Locking Protocol
• Ensure serializability by associating lock with
each data item
– Follow locking protocol for access control
• Locks
– Shared – Ti has shared-mode lock (S) on item Q, Ti
can read Q but not write Q
– Exclusive – Ti has exclusive-mode lock (X) on Q, Ti
can read and write Q
• Require every transaction on item Q acquire
appropriate lock
• If lock already held, new request may have to

Two-phase Locking Protocol
• Generally ensures conflict serializability
• Each transaction issues lock and unlock
requests in two phases
– Growing – obtaining locks
– Shrinking – releasing locks
• Does not prevent deadlock

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
• Uses condition variables and 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

Windows XP Synchronization
• Uses interrupt masks to protect access to
global resources on uniprocessor systems
• Uses spinlocks on multiprocessor systems
• Also provides dispatcher objects which
may act as either mutexes and
semaphores
• Dispatcher objects may also provide
events
– An event acts much like a condition variable

Linux Synchronization
• Linux:
– Prior to kernel Version 2.6, disables interrupts
to implement short critical sections
– Version 2.6 and later, fully preemptive

• Linux provides:
– semaphores
– spin locks

OS Process Synchronization, semaphore and Monitors

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