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CSCI/CMPE 4334 Operating Systems CSCI/CMPE 4334 Operating Systems Review: Exam 2Review: Exam 2

Review

• Chapters 7 ~ 10 in your textbook

• Lecture slides

• In-class exercises (on the course website)

• Review slides

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Review

• 5 questions (100 points) + 1 bonus question (20 points)

• Question types– Q/A

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Time & Place & Event

• 2:35pm ~ 3:50am, April 22, Tuesday

• ENGR 1.290

• Closed-book exam

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Process Manager

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ProgramProgram ProcessProcess

Abstract Computing Environment

FileManager

MemoryManager

DeviceManager

ProtectionProtection

DeadlockDeadlockSynchronizationSynchronization

ProcessDescription

ProcessDescription

CPUCPU Other H/WOther H/W

SchedulerScheduler ResourceManager

ResourceManagerResource

Manager

ResourceManagerResource

Manager

ResourceManager

MemoryMemoryDevicesDevices

Process Mgr

Chapter 7: Scheduling

• Thread scheduling– Ready, running, and blocked states – Context switching

• Mechanism to call scheduler– Voluntary call

• yield function– Involuntary call

• Interval timer

• Scheduling methods (see lecture slides and exercises)– First-come, first served (FCFS)– Shorter jobs first (SJF) or Shortest job next (SJN)– Higher priority jobs first– Job with the closest deadline first

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Thread Scheduler Organization

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ReadyList

ReadyList SchedulerScheduler CPUCPU

ResourceManager

ResourceManager

ResourcesResources

Preemption or voluntary yield

Allocate Request

DoneNewThread job

jobjob

jobjob

“Ready”“Running”

“Blocked”

Context Switching

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CPU

New Thread Descriptor

Old ThreadDescriptor

Process Model and Metrics P will be a set of processes, p0, p1, ..., pn-1

S(pi) is the state of pi {running, ready, blocked} τ(pi), the service time

The amount of time pi needs to be in the running state before it is completed

W (pi), the waiting timeThe time pi spends in the ready state before its first

transition to the running state TTRnd(pi), turnaround time

The amount of time between the moment pi first enters the ready state and the moment the process exits the running state for the last time

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Everyday scheduling methods

• First-come, first served (FCFS)

• Shorter jobs first (SJF)– or Shortest job next (SJN)

• Higher priority jobs first

• Job with the closest deadline first

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Invoking the Scheduler• Need a mechanism to call the scheduler• Voluntary call

– Process blocks itself– Calls the scheduler– Non-preemptive scheduling

• Involuntary call– External force (interrupt) blocks the process– Calls the scheduler– Preemptive scheduling

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Chapter 8: Basic Synchronization

• Critical sections– ensure that when one process is executing in its

critical section, no other process is allowed to execute in its critical section, called mutual exclusion

• Requirements for Critical-Section Solutions– Mutual Exclusion– Progress– Bounded Waiting

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Chapter 8: Basic Synchronization (cont'd)

• Semaphore and its P() and V() functions– Definition– Usage

• Problems– Shared Account Balance Problem– Bounded Buffer Problem– Readers-Writers Problem– Sleepy Barber Problem– Dining-Philosophers Problem

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Structure of process Pi

repeat entry section critical section exit section remainder sectionuntil false;

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shared double balance;

Code for p1 Code for p2

. . . . . .balance = balance + amount; balance = balance - amount; . . . . . .

balance+=amount balance-=amount

balance

Mutual Exclusion. If process Pi is executing in its critical section,

then no other processes can be executing in their critical sections.

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.

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.

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Mutual Exclusion. If process Pi is executing in its critical section,

then no other processes can be executing in their critical sections.

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.

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.

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Disable interrupts Software solution – locks Transactions FORK(), JOIN(), and QUIT() [Chapter

2] Terminate processes with QUIT() to

synchronize Create processes whenever critical section

is complete … something new …

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A semaphore, s, is a nonnegative integer variable that can only be changed or tested by these two indivisible (atomic) functions:

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V(s): [s = s + 1]P(s): [while(s == 0) {wait}; s = s - 1]

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Proc_0() { proc_1() { while(TRUE) { while(TRUE { <compute section>; <compute section>; P(mutex); P(mutex); <critical section>; <critical section>; V(mutex); V(mutex); } }} }

semaphore mutex = 1;fork(proc_0, 0);fork(proc_1, 0);

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Proc_0() { proc_1() { . . . . . ./* Enter the CS */ /* Enter the CS */ P(mutex); P(mutex); balance += amount; balance -= amount; V(mutex); V(mutex); . . . . . .} }

semaphore mutex = 1;

fork(proc_0, 0);fork(proc_1, 0);

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ProducerProducer ConsumerConsumer

Empty Pool

Full Pool

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Readers

Writers

Barber can cut one person’s hair at a time Other customers wait in a waiting room

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Waiting Room

Entrance to WaitingRoom (sliding door)

Entrance to Barber’sRoom (sliding door)

Shop Exit

Shared data semaphore chopstick[5];

(=1 initially)

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while(TRUE) { think(); eat();}

Chapter 9: High-Level Synchronization

• Simultaneous semaphore Psimultaneous(S1, ...., Sn)

• Event– wait()

– Signal()

• Monitor– What is condition? Its usage?

– What are 3 functions of the condition?

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Chapter 9: High-Level Synchronization (cont'd)

– Examples• Shared Balance

• Readers & Writers

• Synchronizing Traffic

• Dining Philosophers

• Interprocess Communication (IPC)

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As we have seen, relatively simple problems, such as the dining philosophers problem, can be very difficult to solve

Look for abstractions to simplify solutions AND synchronization Events Monitors … there are others ...

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The orders of P operations on semaphores are critical Otherwise deadlocks are possible

Simultaneous semaphores Psimultaneous(S1, ...., Sn) The process gets all the semaphores or none

of them

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Dining Philosophers Problem

philosopher(int i) { while(TRUE) { // Think // Eat Psimultaneous(fork[i], fork [(i+1) mod 5]); eat(); Vsimultaneous(fork[i], fork [(i+1) mod 5]); }}semaphore fork[5] = (1,1,1,1,1);fork(philosopher, 1, 0);fork(philosopher, 1, 1);fork(philosopher, 1, 2);fork(philosopher, 1, 3);fork(philosopher, 1, 4);

Exact definition is specific to each OS A process can wait on an event until

another process signals the event Have event descriptor (“event control

block”) Active approach

Multiple processes can wait on an event Exactly one process is unblocked when a

signal occurs A signal with no waiting process is ignored

May have a queue function that returns number of processes waiting on the event

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Construct ensures that only one process can be active at a time in the monitor – no need to code the synchronization constraint explicitly

High-level synchronization construct that allows the safe sharing of an abstract data type among concurrent processes.

class monitor {variable declarations

semaphore mutex = 1;public P1 :(…)

{ P(mutex); <processing for P1> V(mutex);

}; ........

}

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To allow a process to wait within the monitor, a condition variable must be declared, as

condition x, y; Condition variable can only be used with the

operations wait and signal. The operation

x.wait;means that the process invoking this operation is suspended until another process invokes

x.signal; The x.signal operation resumes exactly one

suspended process. If no process is suspended, then the signal operation has no effect.

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Essentially an event (as defined previously)

Occurs only inside a monitor Operations to manipulate condition

variable wait: Suspend invoking process until

another executes a signal signal: Resume one process if any are

suspended, otherwise do nothing queue: Return TRUE if there is at least one

process suspended on the condition variable

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Refined IPC Mechanism

• OS manages the mailbox space• More secure message system

Info to beshared

Info to beshared Info copyInfo copy

Address Space for p0 Address Space for p1

MessageMessageMessageMessageMessageMessage

Mailbox for p1

send function receive function

OS Interfacesend(… p1, …); receive(…);

Chapter 10: Deadlock

• 4 necessary conditions of deadlock– Mutual exclusion– Hold and wait– No preemption– Circular wait

• How to deal with deadlock in OS– Prevention– Avoidance– Recovery

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Chapter 10: Deadlock (cont'd)

• Prevention– Ensure that at least one of the necessary conditions is false at all times– How to?

• Hold and wait• Circular Wait

• Avoidance– Safe state– Banker’s algorithm (see more examples in lecture slides)

• Safety algorithm to detect a safe sequence of process execution• Resource-request algorithm to allocate more resources for a process

• Detection and recovery– Detection algorithm to detect deadlock

• Deadlock detection from graph– Reusable Resource Graphs (RRGs)– (CRGs) Consumable Resource Graphs

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Deadlock can arise if four conditions hold simultaneously Mutual exclusion Hold and wait: No preemption Circular wait

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Three ways Prevention

place restrictions on resource requests to make deadlock impossible

Avoidance plan ahead to avoid deadlock.

Recovery Check for deadlock (periodically or

sporadically) and recover from it Manual intervention (the ad hoc approach)

Reboot the machine if it seems too slow

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Need to be sure a process does not hold one resource while requesting another

Approach 1: Force a process to request all resources it needs at one time

Approach 2: If a process needs to acquire a new resource, it must first release all resources it holds, then reacquire all it needs

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Occurs when a set of n processes that hold units of a set of n different resources Impose a total ordering of all resource types,

and require that each process requests resources in an increasing order of enumeration

Semaphore example semaphores A and B, initialized to 1

P0 P1

wait (A); wait(A)wait (B); wait(B)

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Allow a process to time-out on a blocked request -- withdrawing the request if it fails r = request resource w = withdraw request d = release or deallocate resource

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Si

ru

dv ru

Sj

Sk

wuNo guarantee!

Define a model of system states, then choose a strategy that will guarantee that the system will not go to a deadlock state

Requires extra information, e.g., the maximum claim for each process

Allows resource manager to see the worst case that could happen, then to allow transitions based on that knowledge

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Best known of avoidance strategies Modeled after lending policies used by banks Each new process entering system declares

the maximum use of resources it may need. When a process requests a resource it may

have to wait (until system in a safe state). When a process gets all its resources it must

return them in a finite amount of time.

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Available Vector of length m. If available [j] = k, there are k

instances of resource type Rj available. Max

n x m matrix. If Max [i,j] = k, then process Pi may request at most k instances of resource type Rj.

Allocation n x m matrix. If Allocation[i,j] = k then Pi is currently

allocated k instances of Rj. Need

n x m matrix. If Need[i,j] = k, then Pi may need k more instances of Rj to complete its task. Need [i,j] = Max[i,j] – Allocation [i,j]

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Let n = number of processes, and m = number of resources types.

1. Let Work and Finish be vectors of length m and n, respectively. Initialize:

Work := AvailableFinish [i] = false for i - 1, 2, 3, …, n.

2. Find an i such that both: (a) Finish [i] = false(b) Needi WorkIf no such i exists, go to step 4.

3. Work := Work + Allocationi

Finish[i] := truego to step 2.

4. If Finish [i] = true for all i, then the system is in a safe state.

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Requesti = request vector for process Pi. If Requesti

[j] = k then process Pi wants k instances of resource type Rj.

1. If Requesti Needi go to step 2. Otherwise, raise error condition, since process has exceeded its maximum claim.

2. If Requesti Available, go to step 3. Otherwise Pi must wait, since resources are not available.

3. Pretend to allocate requested resources to Pi by modifying the state as follows:

Available := Available – Requesti;

Allocationi := Allocationi + Requesti;

Needi := Needi – Requesti;;

If safe the resources are allocated to Pi.

If unsafe Pi must wait, and the old resource-allocation state is restored

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1. Let Work and Finish be vectors of length m and n, respectively Initialize:

(a) Work := Available(b) For i = 1,2, …, n, if Allocationi 0, then

Finish[i] := false;otherwise, Finish[i] := true.

2. Find an index i such that both:(a) Finish[i] = false(b) Requesti Work

If no such i exists, go to step 4.

3. (a) Work := Work + Allocationi; (b) Finish[i] := true; go to step 2.

4. If Finish[i] = false, for some i, 1 i n, then the system is in deadlock state. Moreover, if Finish[i] = false, then Pi is deadlocked.

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Algorithm requires an order of m x n2 operations to detect whether the system is in deadlocked state.

Good Luck!

Q/A

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