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Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Chapter 6: ProcessSynchronization
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6.2 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Module 6: Process Synchronization
Background
The Critical-Section Problem Semaphores
Classic Problems of Synchronization
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6.3 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Objectives
To introduce the critical-section problem, whose solutions can be used to ensure the consistency ofshared data
To present both software and hardware solutions of the critical-section problem
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6.4 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Background
Concurrent access to shared data may result in data inconsistency
Maintaining data consistency requires mechanisms to ensure the orderly execution of cooperatingprocesses
Suppose that we wanted to provide a solution to the consumer-producer problem that fills allthe buffers.We can do so by having an integer countthat keeps track of the number of full buffers. Initially, count isset to 0. It is incremented by the producer after it produces a new buffer and is decremented by theconsumer after it consumes a buffer.
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5/416.5 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Producer
while (true) {
/* produce an item and put in nextProduced */
while (counter == BUFFER_SIZE)
; // do nothing
buffer [in] = nextProduced;
in = (in + 1) % BUFFER_SIZE;
counter++;
}
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6/416.6 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Consumer
while (true) {while (counter == 0)
; // do nothing
nextConsumed = buffer[out];
out = (out + 1) % BUFFER_SIZE;
counter--;
/* consume the item in nextConsumed
}
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7/416.7 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Race Condition
counter++ could be implemented as
register1 = counterregister1 = register1 + 1counter = register1
counter-- could be implemented as
register2 = counter
register2 = register2 - 1count = register2
Consider this execution interleaving with count = 5 initially:
S0: producer execute register1 = counter {register1 = 5}S1: producer execute register1 = register1 + 1 {register1 = 6}S2: consumer execute register2 = counter {register2 = 5}S3: consumer execute register2 = register2 - 1 {register2 = 4}
S4: producer execute counter = register1 {count = 6 }S5: consumer execute counter = register2 {count = 4}
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8/416.8 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Critical Section Problem
Consider system of n processes {p0, p1, pn-1}
Each process has critical section segment of code
Process may be changing common variables, updating table, writing file, etc
When one process in critical section, no other may be in its 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 exisection, then remainder section
Especially challenging with preemptive kernels
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Critical Section
General structure of process pi is
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Solution to Critical-Section Problem
1. Mutual Exclusion - If process Pi is executing in its critical section, then no other processes can beexecuting in their critical sections
2. Progress - If no process is executing in its critical section and there exist some processes that wish to entertheir critical section, then the selection of the processes that will enter the critical section next cannot bepostponed indefinitely
3. Bounded Waiting - A bound must exist on the number of times that other processes are allowed to entertheir critical sections after a process has made a request to enter its critical section and before thatrequest is granted
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Semaphore
Synchronization tool
Semaphore S integer variable Two standard operations modify S: wait() and signal()
Originally called P() andV()
Less complicated
Can only be accessed via two indivisible (atomic) operations
wait (S) {
while S
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12/416.12 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Semaphore asGeneral Synchronization Tool
Countingsemaphore integer value can range over an unrestricted domain
Binary semaphore integer value can range only between 0and 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);
// remainder section
} while (TRUE);
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6.13 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Semaphore Implementation
Must guarantee that no two processes can execute wait () and signal () on the same semaphore at thesame time
Thus, implementation becomes the critical section problem where the wait and signal code are placed inthe crtical section
Could now have busy waitingin 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
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6.14 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Semaphore Implementationwith 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
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6.15 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Semaphore Implementation withno Busy waiting (Cont.)
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 list;
wakeup(P);
}
}
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6.16 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Deadlock and Starvation
Deadlock two or more processes are waiting indefinitely for an event that can be caused by only one ofthe waiting processes
Let S and Q be two semaphores initialized to 1P0 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 Solved via priority-inheritance protocol
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6.17 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Classical Problems of Synchronization
Classical problems used to test newly-proposed synchronization schemes
Bounded-Buffer Problem
Readers and Writers Problem
Dining-Philosophers Problem
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6.18 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Bounded-Buffer Problem
Nbuffers, 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
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6.19 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Bounded Buffer Problem (Cont.)
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);
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6.20 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Bounded Buffer Problem (Cont.)
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);
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6.21 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
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
Several variations of how readers and writers are treated all involve priorities
Shared Data
Data set
Semaphore mutex initialized to 1
Semaphore wrt initialized to 1
Integer readcount initialized to 0
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6.22 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Readers-Writers Problem (Cont.)
The structure of a writer process
do {
wait (wrt) ;
// writing is performed
signal (wrt) ;
} while (TRUE);
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6.23 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Readers-Writers Problem (Cont.)
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);
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6.24 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Readers-Writers Problem Variations
Firstvariation no reader kept waiting unless writer has permission to use shared object
Secondvariation once writer is ready, it performs write asap
Both may have starvation leading to even more variations
Problem is solved on some systems by kernel providing reader-writer locks
Di i Phil h P bl
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6.25 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Dining-Philosophers Problem
Philosophers spend their lives thinking and eating
Dont 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
Di i Phil h P bl Al ith
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6.26 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
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?
Th D dl k P bl
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6.27 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
The Deadlock Problem
A set of blocked processes each holding a resource and
waiting to acquire a resource held by another process in theset
Example
System has 2 disk drives
P1 and P2 each hold one disk drive and each needs another one Example
semaphores A and B, initialized to 1
P0 P1
wait (A); wait(B)
wait (B); wait(A)
B id C i E l
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6.28 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Bridge Crossing Example
Traffic only in one direction
Each section of a bridge can be viewed as a resource
If a deadlock occurs, it can be resolved if one car backs up (preemptresources and rollback)
Several cars may have to be backed up if a deadlock occurs
Starvation is possible
Note Most OSes do not prevent or deal with deadlocks
System Model
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6.29 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
System Model
Resource types R1, R2, . . ., RmCPU cycles, memory space, I/O devices
Each resource type Ri has Wi instances.
Each process utilizes a resource as follows:
request
use
release
Deadlock Characterization
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6.30 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Deadlock Characterization
Mutual exclusion: only one process at a time can use a resource
Hold and wait: a process holding at least one resource is waiting toacquire additional resources held by other processes
No preemption: a resource can be released only voluntarily by theprocess holding it, after that process has completed its task
Circular wait: there exists a set {P0, P1, , Pn} of waiting processes suchthat P0 is waiting for a resource that is held by P1, P1 is waiting for aresource that is held by
P2, , Pn1 is waiting for a resource that is held by Pn, and Pn is waiting fora resource that is held by P0.
Deadlock can arise if four conditions hold simultaneously
Resource Allocation Graph
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6.31 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Resource-Allocation Graph
V is partitioned into two types:
P= {P1, P2, , Pn}, the set consisting of all the processes inthe system
R= {R1, R
2, , R
m}, the set consisting of all resource types
in the system
request edge directed edge PiRj
assignment edge directed edge RjPi
A set of vertices Vand a set of edges E
Resource Allocation Graph (Cont )
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6.32 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Resource-Allocation Graph (Cont.)
Process
Resource Type with 4 instances
Pirequests instance of Rj
Pi is holding an instance of Rj Pi
Pi
Rj
Rj
Example of a Resource Allocation Graph
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6.33 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Example of a Resource Allocation Graph
Resource Allocation Graph With A Deadlock
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6.34 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Resource Allocation Graph With A Deadlock
Graph With A Cycle But No Deadlock
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6.35 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Graph With A Cycle But No Deadlock
Basic Facts
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6.36 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Basic Facts
If graph contains no cycles no deadlock
If graph contains a cycle
if only one instance per resource type, then deadlock
if several instances per resource type, possibility of deadlock
Methods for Handling Deadlocks
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6.37 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Methods for Handling Deadlocks
Ensure that the system will never enter a deadlock state
Allow the system to enter a deadlock state and then recover
Ignore the problem and pretend that deadlocks never occur
in the system; used by most operating systems, includingUNIX
Deadlock Prevention
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6.38 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Deadlock Prevention
Mutual Exclusion not required for sharableresources; must hold for nonsharable resources
Hold and Wait must guarantee that whenever a
process requests a resource, it does not hold any otherresources
Require process to request and be allocated all itsresources before it begins execution, or allow process torequest resources only when the process has none
Low resource utilization; starvation possible
Restrain the ways request can be made
Deadlock Prevention (Cont.)
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6.39 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Deadlock Prevention (Cont.)
No Preemption
If a process that is holding some resources requests anotherresource that cannot be immediately allocated to it, then allresources currently being held are released
Preempted resources are added to the list of resources forwhich the process is waiting
Process will be restarted only when it can regain its oldresources, as well as the new ones that it is requesting
Circular Wait impose a total ordering of all resourcetypes, and require that each process requests resources in
an increasing order of enumeration
Deadlock Avoidance
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6.40 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
Deadlock Avoidance
Simplest and most useful model requires that eachprocess declare the maximum numberof resources ofeach type that it may need
The deadlock-avoidance algorithm dynamically examinesthe resource-allocation state to ensure that there cannever be a circular-wait condition
Resource-allocation stateis defined by the number ofavailable and allocated resources, and the maximumdemands of the processes
Requires that the system has some additional a prioriinformationavailable
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Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition
End of Chapter 6