Examples in OS synchronization for UG

 Process Synchronization is a way to
coordinate processes that use shared data.
 It occurs in an operating system among
cooperating processes.
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Dr.M.Karthika/AP/IT/MTNC

 The main purpose of synchronization is the
sharing of resources without interference
using mutual exclusion.
 The other purpose is the coordination of the
process interactions in an operating system.
Semaphores and monitors are the most
powerful and most commonly used
mechanisms to solve synchronization
problems.
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 It is the task phenomenon of coordinating the
execution of processes in such a way that no two
processes can have access to the same shared
data and resources.
 It is a procedure that is involved in order to
preserve the appropriate order of execution of
cooperative processes.
 In order to synchronize the processes, there are
various synchronization mechanisms.
 Process Synchronization is mainly needed in a
multi-process system when multiple processes
are running together, and more than one
processes try to gain access to the same shared
resource or any data at the same time.
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 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 to occur
while a shared variable or resource is being modified.
 In this manner, we could be sure that the current sequence of
instructions would be allowed to execute in order without pre-
emption. Unfortunately, this solution is not feasible in a
multiprocessor environment.
 Disabling interrupt on a multiprocessor environment can be
time-consuming as the message is passed to all the processors.
 This message transmission lag delays the entry of threads into
the critical section, and the system efficiency decreases.
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 As the synchronization hardware solution is
not easy to implement for everyone, a strict
software approach called Mutex Locks was
introduced.
 In this approach, in the entry section of code, a
LOCK is acquired over the critical resources
modified and used inside the 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.
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 Solaris 2
 Windows 2000
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 Solaris implements variety of locks to support
multitasking, multithreading and
multiprocessing.
 It uses adaptive mutexes, conditional variables,
semaphores, read-write locks, turnstiles to
control access to critical sections.
 On A multiprocessor system it starts as a
standard semaphore spin-lock.
 If the lock is held by a thread which is running
on another CPU then the thread spins.
 If the lock is held by a thread which is currently
in run state,the thread blocks, going to sleep
until it is awakened by the signal of releasing the
lock.
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 The spin-waiting method is exceedingly
inefficient if code segment is longer.
So conditional variables, semaphores are
used for them.
 Solaris provides Read-Write lock to protect
the data are frequently accessed by long
section of code usually in read-only manner.
 It uses turnstiles to order the list of threads
waiting to acquire either an adaptive mutex
or read-writer lock.
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 Turnstile is a queue structure containing
threads blocked on a lock.
 They are per lock holding thread, not per
object.
 Turnstiles are organized according to
priority-inheritance which gives the running
thread the highest of the priorities of the
threads in its turnstiles to prevent priority
inversion.
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 Locking mechanisms are used by kernel is
also used by user-level threads, so that the
locks are available both inside and outside of
the kernel. The difference is only that
priority-inheritance in only used in kernel,
user-level thread does not provide this
functionality.
 To optimize Solaris performance, developers
refine the locking methods as locks are used
frequently and typically for crucial kernel
functions, tuning their implementations and
use to gain great performance.
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 Windows operating system is a multithreaded
kernel that provide support for real time
application and multiprocessors.
 On uniprocessor system, Windows provides
interrupt masks to protect access to global
resources.
 It protects access to global resource using
spinlock.
 The kernel uses spinlocks only to protect short
code segment like Solaris.
 The kernel ensures that while holding a spinlock,
a thread will never be preempted.
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 Windows provide dispatcher object for thread
synchronization according to several different
mechanisms including mutexes, semaphores,
events and timers.
 The system protects shared data by requiring
a thread to gain ownership of a mutex for
accessing the data and when it is finished,
releases the ownership.
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 Events acts as a conditional variable to notify
a waiting thread when desired condition
occurs.
 Timers are used to notify one or more thread
when time expired.
 Dispatcher objects may be either signaled
state or a non-signaled state.
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 Signaled state indicates that an object is
available and a thread will not block when
acquiring the object.
 Non-signaled state indicates that an object is
not available and a thread will block when
trying to acquire the object.
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THANK YOU
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Examples in OS synchronization for UG

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