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CSCI/CMPE 4334 Operating Systems CSCI/CMPE 4334 Operating Systems Review: Final ExamReview: Final Exam

Review

• Chapters 1 ~ 12 in your textbook

• Lecture slides

• In-class exercises 1~10 (on the course website)

• Review slides• Review for 2 mid-term exams

• Review for final exam

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Review

• 5 questions (100 points) + 1 bonus question (20 points)– Process scheduling, concurrent processing

– Memory allocation, memory management policy

– I/O device management

• Question types– Q/A

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Time & Place & Event

• 1:15pm ~ 3pm, May 12 (Tuesday)

• ENGR 1.272

• Closed-book exam

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Basic OS Organization

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Processor(s) Main Memory Devices

Process, Thread &Resource Manager

MemoryManager

DeviceManager

FileManager

Chapter 5: Device Management

• Abstract all devices (and files) to a few interfaces

• Device management strategies

• Buffering & Spooling

• Access time of disk device (see lecture slides and exercises)– FCFS, SSTF, Scan/Look, Circular Scan/Look

– See examples in Exercise (2)

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Track (Cylinder)

Sect

or(a) Multi-surface Disk (b) Disk Surface (b) Cylinders

Cylinder (set of tracks)

Several techniques used to try to optimize the time required to service an I/O request

FCFS – first come, first served Requests are serviced in order received No optimization

SSTF – shortest seek time first Find track “closest” to current track Could prevent distant requests from being

serviced

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Scan/Look Starts at track 0, moving towards last track,

servicing requests for tracks as it passes them

Reverses direction when it reaches last track Look is variant; ceases scan in a direction

once last track request in that direction has been handled

Circular Scan/Look Similar to scan/look but always moves in

same direction; doesn’t reverse Relies on existence of special homing

command in device hardware

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Chapter 6: Implementing Processes,Threads, and Resources

• Modern process– Concurrent processing

– Fully utilize the CPU and I/O devices

• Address space

• Context switching

• State diagram

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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

• 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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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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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

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Structure of process Pi

repeat entry section critical section exit section remainder sectionuntil false;

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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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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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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);

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 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 11: Memory Management

• Functions of memory manager• Abstraction• Allocation• Address translation• Virtual memory

• Memory allocation (see lecture slides and exercises)• Fixed partition method

• First-fit• Next-fit• Best-fit• Worst-fit

• Variable partition method24

Chapter 11: Memory Management (cont'd)

• Dynamic address space binding• Static binding

• Dynamic binding

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Primary & Secondary Memory

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CPU

Primary Memory(Executable Memory)

e.g. RAM

Secondary Memorye.g. Disk or Tape

• CPU can load/store• Ctl Unit executes code from this memory•Transient storage

• Access using I/O operations• Persistent storage

Load

Information can be loaded statically or dynamically

Functions of Memory Manager

• Allocate primary memory space to processes

• Map the process address space into the allocated portion of the primary memory

• Minimize access times using a cost-effective amount of primary memory

• May use static or dynamic techniques

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Memory Manager• Only a small number of interface functions

provided – usually calls to:– Request/release primary memory space– Load programs– Share blocks of memory

• Provides following– Memory abstraction– Allocation/deallocation of memory– Memory isolation– Memory sharing

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Memory Abstraction• Process address space

– Allows process to use an abstract set of addresses to reference physical primary memory

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Mapped to objectother than memory

Process Address Space Hardware Primary Memory

Memory layout / organization how to divide the memory into blocks for

allocation? Fixed partition method: divide the memory once before

any bytes are allocated. Variable partition method: divide it up as you are

allocating the memory. Memory allocation

select which piece of memory to allocate to a request

Memory organization and memory allocation are close related

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OperatingSystem

Region 3

Region 2

Region 1

Region 0 N0

N1

N2

N3

pi

pi needs ni units

ni

How to satisfy a request of size n from a list of free blocks First-fit: Allocate the first hole that is big enough Next-fit: Choose the next block that is large

enough Best-fit: Allocate the smallest hole that is big

enough; must search entire list, unless ordered by size. Produces the smallest leftover hole.

Worst-fit: Allocate the largest hole; must also search entire list. Produces the largest leftover hole.

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CPU

0x02010

0x10000+

MARload R1, 0x02010

0x12010

•Program loaded at 0x10000 Relocation Register = 0x10000•Program loaded at 0x04000 Relocation Register = 0x04000

Relocation Register

Relative Address

We never have to change the load module addresses!

Performed automagically by processor

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Image for pj

Image for pi

Swap pi outSwap pi out

Swap pj in

A characteristic of programs that is very important to the strategy used by virtual memory systems is spatial reference locality Refers to the implicit partitioning of code and

data segments due to the functioning of the program (portion for initializing data, another for reading input, others for computation, etc.)

Can be used to select which parts of the process should be loaded into primary memory

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Chapter 12: Virtual Memory

• Address translation between virtual memory and physical memory

• Terms• Page

• Page frame

• Paging algorithms• Static allocation

• Dynamic allocation

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Chapter 12: Virtual Memory (cont'd)

• Fetch policy • when a page should be loaded• On demand paging

• Replacement policy (see lecture slides and exercises)• which page is unloaded• Policies

• Random • Balady’s optimal algorithm*• LRU*• LFU• FIFO• LRU approximation

• Placement policy • where page should be loaded

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Chapter 12: Virtual Memory (cont'd)

• Thrashing• Load control

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Memory Image for pi

Secondary Memory

Primary Memory

Since binding changes with time, use a dynamic virtual address map, t

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VirtualAddressSpace

t

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PrimaryMemory

Virtual AddressSpace

Supv spaceUser space

Paging Disk(Secondary Memory)

2. Lookup (Page i, Addr k)

3. Translate (Page i, Addr k) to (Page Frame j, Addr k)

4. Reference (Page Frame j, Addr k)

1. Reference to Address k in Page i (User space)

Virtual memory system transfers “blocks” of the address space to/from primary memory

Fixed size blocks: System-defined pages are moved back and forth between primary and secondary memory

Variable size blocks: Programmer-defined segments – corresponding to logical fragments – are the unit of movement

Paging is the commercially dominant form of virtual memory today

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A page is a fixed size, 2h, block of virtual addresses

A page frame is a fixed size, 2h, block of physical memory (the same size as a page)

When a virtual address, x, in page i is referenced by the CPU If page i is loaded at page frame j, the

virtual address is relocated to page frame j If page is not loaded, the OS interrupts the

process and loads the page into a page frame

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Page # Line #Virtual Address

g bits h bits

tMissing Page

Frame # Line #j bits h bits

Physical Address

MAR

CPU Memory

“page table”

Two basic types of paging algorithms Static allocation Dynamic allocation

Three basic policies in defining any paging algorithm Fetch policy – when a page should be loaded Replacement policy –which page is unloaded Placement policy – where page should be

loaded

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Determines when a page should be brought into primary memory

Usually don’t have prior knowledge about what pages will be needed

Majority of paging mechanisms use a demand fetch policy Page is loaded only when process references it

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When there is no empty page frame in memory, we need to find one to replace Write it out to the swap area if it has been

changed since it was read in from the swap area Dirty bit or modified bit

pages that have been changed are referred to as “dirty”

these pages must be written out to disk because the disk version is out of date this is called “cleaning” the page

Which page to remove from memory to make room for a new page We need a page replacement algorithm

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There are too many processes in memory and no process has enough memory to run. As a result, the page fault is very high and the system spends all of its time handling page fault interrupts.

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Good Luck!

Q/A

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