The document discusses process synchronization and coordination between independent processes. It covers concepts like race conditions, critical sections, solutions like Peterson's algorithm using shared variables, synchronization primitives like semaphores, and classical synchronization problems like the bounded buffer, producer-consumer, readers-writers, and dining philosophers problems. Implementation techniques like busy waiting, signaling with wait/signal operations, and avoidance of starvation and deadlock are described. Examples of solutions to these classic problems using semaphores and other synchronization methods are outlined.

1. Process Synchronization
Organized By: V.A.
V.A.
CSED,TU

2. Disclaimer
This is NOT A COPYRIGHT MATERIAL
Content has been taken mainly from the following books:
Operating Systems Concepts By Silberschatz & Galvin ,
Operating systems By D M Dhamdhere,
System Programming By John J Donovan
etc…
VA.
CSED,TU

3. Process & Synchronization
 Process – Program in Execution.
 Synchronization – Coordination.
 Independent process cannot affect or be affected by the execution of another
process
 Cooperating process can affect or be affected by the execution of another
process
 Advantages of process cooperation
 Information sharing
 Computation speed-up
 Modularity
 Convenience
VA.
CSED,TU

4. Buffer
VA.
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5. Bounded-Buffer – Shared-Memory Solution
 Shared Data
#define BUFFER_SIZE 10
typedef struct {
...
} item;
item buffer [BUFFER_SIZE];
int in = 0;
int out = 0;
 Can only use BUFFER_SIZE-1 elements
VA.
CSED,TU

6. Producer – Consumer
VA.
CSED,TU

7. Bounded-Buffer – Producer
while (true) {
/* Produce an item */
while (((in = (in + 1) % BUFFER SIZE count) == out)
; /* do nothing -- no free buffers */
buffer[in] = item;
in = (in + 1) % BUFFER SIZE;
}
VA.
CSED,TU

8. Bounded Buffer – Consumer
while (true) {
while (in == out)
; // do nothing -- nothing to consume
// remove an item from the buffer
item = buffer[out];
out = (out + 1) % BUFFER SIZE;
return item;
}
VA.
CSED,TU

9. Setting Final value of Counter
VA.
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10. Buffer Types
 Paradigm for cooperating processes, producer process produces information that
is consumed by a consumer process
 Unbounded-buffer places no practical limit on the size of the buffer
 Bounded-buffer assumes that there is a fixed buffer size
VA.
CSED,TU

11. RC & CS
 Race Condition – Where several processes access and manipulate the same data
concurrently and the outcome of the execution depends on the particular order
in which access takes place.
 Critical Section – Segment of code in which Process may be changing common
variables, updating a table, writing a file and so on.
VA.
CSED,TU

12. Peterson’s Solution
VA.
CSED,TU

13. Peterson’s Solution
Var flag : array [0…1] of Boolean;
Turn : 0..1;
Begin
Flag[0] = false;
Flag[1] = false;
Parbegin
Repeat Repeat
Flag[0] = true Flag[1] = true
Turn = 1 Turn = 0
While flag[1] && turn==1 While flag[0] && turn==0
Do {nothing}; Do {nothing};
{Critical Section} {Critical Section}
Flag[0] = false; Flag[1] = false;
{Remainder} {Remainder}
Forver; Forver;
Parend
end
VA.
CSED,TU

14. Peterson’s Solution
VA.
CSED,TU

15. Synchronization Hardware
 Many Systems provide hardware support for critical section code
 Uni-Processors – Could disable Interrupts
 Currently running code would execute without preemption
 Generally too inefficient on multiprocessor systems
 Modern machines provide special atomic hardware instructions
 Atomic :- Uninterruptible
 Either Test memory word and Set value
 Or Swap contents of two memory words
VA.
CSED,TU

16. TestAndSet Instruction
Definition:
boolean TestAndSet (boolean *target)
{
boolean rv = *target;
*target = TRUE;
return rv:
}
VA.
CSED,TU

17. 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);
VA.
CSED,TU

18. Swap Instruction
Definition:
void Swap (boolean *a, boolean *b)
{
boolean temp = *a;
*a = *b;
*b = temp:
}
VA.
CSED,TU

19. 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);
VA.
CSED,TU

20. 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++;
}
VA.
CSED,TU

21. 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 S; // initialized to 1
 wait (S);
Critical Section
signal (S);
VA.
CSED,TU

22. 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 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.
VA.
CSED,TU

23. 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.
VA.
CSED,TU

24. Semaphore Implementation with no Busy waiting
(Cont.)
 Implementation of WAIT:
wait (S){
value--;
if (value < 0) {
add this process to waiting queue
block(); }
}
 Implementation of SIGNAL:
Signal (S){
value++;
if (value <= 0) {
remove a process P from the waiting queue
wakeup(P); }
}
VA.
CSED,TU

25. 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.
VA.
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26. Producer Consumer
VA.
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27. Solution to PC must satisfy 3 conditions
VA.
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28. Solution to PC with Busy Wait
VA.
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29. Solution to PC with Signaling
VA.
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30. Indivisible Operations for PC
VA.
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31. Readers & Writers in Banking System
VA.
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32. Correctness Condition for R-W Problem
VA.
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33. Solution outline for R-W without Writer priority
VA.
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34. Dining Philosophers
VA.
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35. Outline of a Philosopher Process Pi
VA.
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36. An Improved Outline of a Philosopher
Process
VA.
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37. Reference List
Operating Systems Concepts By Silberschatz & Galvin,
Operating systems By D M Dhamdhere,
System Programming By John J Donovan,
www.os-book.com
www.cs.jhu.edu/~yairamir/cs418/os2/sld001.htm
http://gaia.ecs.csus.edu/~zhangd/oscal/pscheduling.html
http://www.edugrid.ac.in/iiitmk/os/os_module03.htm
http://williamstallings.com/OS/Animations.html
etc…
VA.
CSED,TU

38. Thnx…
VA.
CSED,TU