Synchronization is the coordination of events to operate a system in unison. For example, the conductor of an orchestra keeps the orchestra synchronized or in time. Systems that operate with all parts in synchrony are said to be synchronous or in sync—and those that are not are asynchronous.

1. 19/6/2020
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2. 29/6/2020
Synchronization
Submitted to
Submitted ByMs. Rif…
Senior Lecturer
Department of CSE
Daffodil International University
1. M…. K… : 143-15-***
2. T… H…: 143-15-***
3. Ja… Fe…: 143-15-***

3. Process Synchronization means sharing system
resources by processes.
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4. •Race Condition.
•Critical Section.
•Mutual Exclusion
•Bounded waiting.
•Progress.
•Deadlock.
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5. • Many systems provide hardware support for critical
section code.
• The critical section problem could be solved easily
in a single-processor environment if we could
disallow interrupts.
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6. • A strict software approach called Mutex Locks.
• In this approach, in the entry section of code, a
LOCK is acquired over the critical resources
modified
• And used inside critical section, and in the exit
section that LOCK is released.
•As the resource is locked while a process executes
its critical section hence no other process can
access it. 6

7. •In 1965, Dijkstra proposed this technique.
• By using the value of a simple integer variable to
synchronize the progress of interacting processes.
• ‘This integer variable is called semaphore.’
• It is basically a synchronizing tool.
• And is accessed only through two low standard atomic
operations, wait and signal.
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8. •Simple
•Works with many processes
•Can have many different critical sections with
different semaphores
•Each critical section has unique access semaphores
•Can permit multiple processes into the critical
section at once, if desirable.
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9. •Priority Inversion is a big limitation os semaphores.
•Their use is not enforced, but is by convention only.
•With improper use, a process may block
indefinitely. Such a situation is called Deadlock.
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10. A race condition is an undesirable situation that
occurs when a device or system attempts to perform
two or more operations at the same time, but
because of the nature of the device or system, the
operations must be done in the proper sequence to
be done correctly
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11. • shared int x =3;
• process_1 () {
• x = x +1;
• print x;
• }
• process_2 () {
• x = x +1;
• print x;
• }
• probably three machine instructions. A likely translation is:
• LOAD r1 x
• ADD r1+1 r1
• STORE r1 x 11

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14. •Non shareable mode of Shared resources
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15. •Out of a group of cooperating processes, only one
process can be in its critical section at a given point
of time.
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16. •int flag;
•void enter_region(int process) {
•while ( flag == 1 );
•flag = 1;
•}
•void leave_region(int process) {
•flag = 0;
•}
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17. •If no process is in its critical section
• And if one or more threads want to execute their
critical section.
• Then any one of these threads must be allowed to
get into its critical section.
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18. •After a process makes a request for getting into its
critical section
•There is a limit for how many other processes can
get into their critical section, before this process's
request is granted.
• So, after the limit is reached, system must grant the
process permission to get into its critical section.
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19. •Indefinite postponement of a process
• Because it requires some resource. but the
resource is never allocated to this process. It is
sometimes called livelock.
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20. Five philosophers sit at a round table and in the
centre lies a plate of spaghetti that is available to
everyone.
There are chopsticks on the table { one between
each Philosopher.
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21. • Think (because they are philosophers)
• Eat (as they are hungry) and
• Wait (two chopsticks are need to eat spaghetti but
only 5 are available)
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22.  The sections of code where more than one process
are accessing shared resources at a time.
Where shared resources are protected.
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23. •For solving this problem following conditions must
be Satisfied.
•i) Mutual Exclusion
•ii) Progress
•iii) Bounded Waiting
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25. Is a situation in which two computer programs
shares same resources are effectively preventing
each other from accessing the resource.
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26. In OS a deadlock occurs when
a process or thread enters a waiting state,
•Because a requested system resource is held by
another waiting process,
• Which in turn is waiting for another resource held
by another waiting process.
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28. Deadlock occurs if the following conditions hold
simultaneously:
•Mutual exclusion: only one process can use a
resource at a time.
•Hold and wait: Processes are allowed to request for
new resources without releasing the resources that
they are currently holding.
•No-preemption: If a process has a resource , cannot
be preempted.
•Circular-wait: Two or more processes from a circular
chain. 28

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31. •Prevention
•Avoidance
•Detection and recovery
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