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5 process synchronization
- 2. 5.2 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
Chapter 5: Process Synchronization
Background
The Critical-Section Problem
Peterson’s Solution
Synchronization Hardware
Mutex Locks
Semaphores
Classic Problems of Synchronization
Monitors
Synchronization Examples
Alternative Approaches
- 3. 5.3 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
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
To examine several classical process-synchronization
problems
To explore several tools that are used to solve process
synchronization problems
- 4. 5.4 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
Background
Processes can execute concurrently
May be interrupted at any time, partially completing
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.
- 5. 5.5 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
Critical Section Problem
The critical section is a code segment where the shared
variables can be accessed. An atomic action is required in
a critical section i.e. only one process can execute in
its critical section at a time.
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 exit section,
then remainder section
- 6. 5.6 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
Critical Section
General structure of process Pi
- 7. 5.7 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
Solution to Critical-Section Problem
1. Mutual Exclusion - If process Pi is executing in its critical
section, 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
- 8. 5.8 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
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
- 9. 5.9 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
Synchronization Hardware
Many systems provide hardware support for implementing the
critical section code.
All solutions below based on idea of locking
Protecting critical regions via locks
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-interruptible
- 10. 5.10 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
Solution to Critical-section Problem Using Locks
do {
acquire lock
critical section
release lock
remainder section
} while (TRUE);
- 11. 5.11 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
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
Protect a critical section by first acquire() a lock then
release() the lock
Boolean variable indicating if lock is available or not
Calls to acquire() and release() must be atomic
Usually implemented via hardware atomic instructions
But this solution requires busy waiting
This lock therefore called a spinlock
- 12. 5.12 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
Semaphore
Semaphores are integer variables that are used to solve the critical section
problem by using two atomic operations, wait and signal that are used for process
synchronization.
The definitions of wait and signal are as follows −
Wait The wait operation decrements the value of its argument S, if it is positive. If
S is negative or zero, then no operation is performed.
wait(S)
{
while (S<=0);
S--;
}
Signal The signal operation increments the value of its argument S.
signal(S)
{
S++;
}
- 13. 5.13 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
Types of Semaphores
There are two main types of semaphores i.e. counting semaphores and
binary semaphores. Details about these are given as follows −
Counting Semaphores These are integer value semaphores and have
an unrestricted value domain. These semaphores are used to coordinate
the resource access, where the semaphore count is the number of
available resources. If the resources are added, semaphore count
automatically incremented and if the resources are removed, the count is
decremented.
Binary Semaphores The binary semaphores are like counting
semaphores but their value is restricted to 0 and 1. The wait operation
only works when the semaphore is 1 and the signal operation succeeds
when semaphore is 0. It is sometimes easier to implement binary
semaphores than counting semaphores.
- 14. 5.14 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
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
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
- 15. 5.15 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
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
Solved via priority-inheritance protocol
- 16. 5.16 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
Classical Problems of Synchronization
Semaphore can be used in other synchronization problems besides
Mutual Exclusion.
Below are some of the classical problem depicting flaws of process
synchronaization in systems where cooperating processes are
present.
We will discuss the following three problems:
Bounded Buffer (Producer-Consumer) Problem
Dining Philosophers Problem
The Readers Writers Problem
- 17. 5.17 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
Bounded-Buffer Problem
This problem is generalised in terms of the Producer
Consumer problem, where a finite buffer pool is used to
exchange messages between producer and consumer
processes.
Because the buffer pool has a maximum size, this problem
is often called the Bounded buffer problem.
Solution to this problem is, creating two counting
semaphores "full" and "empty" to keep track of the current
number of full and empty buffers respectively.
- 18. 5.18 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
Dining Philosophers Problem
The dining philosopher's problem involves the allocation of limited resources
to a group of processes in a deadlock-free and starvation-free manner.
- 19. 5.19 Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
The Readers Writers Problem
In this problem there are some processes(called readers) that only read the
shared data, and never change it, and there are other
processes(called writers) who may change the data in addition to reading,
or instead of reading it.