Operating Systems

Review

Questions

What are two functions of an OS? From top to down, providing abstractions to

applications From down to top, resource manager,

multiplexing( sharing) resource

Questions

What causes OS to change? What is multi-programming? What is time-sharing? Or, why aren’t we still running MS-DOS?

What is a process? What is a thread? What is address space? What is a file?

Questions

How does a shell work? Or … arrange the commands in order:wait()pid = fork()exec()gets()while(1) {}

Review

Process Scheduling Explain how SJF,FCFS,RR etc. works True or False:

FCFS is optimal in terms of average turnaround time Most processes are CPU bound The shorter the time quantum, the better

6

P1

20

P2

32

P3 P4 P5

40 56 60

FCFS

P1

4

P2

12

P3 P4P5

24 40 60

SJF

P1 P2

4

P3 P4 P5 P1

8 12 16 20 24

P2 P3

28

P4 P1 P2 P4

32 36 40 44 48

P1 P4

52

P1

56 60

RR

Response time Estimate by time from job submission to time to first CPU dispatch Assume all jobs submitted at same time, in order given

Turnaround time Time interval from submission of a process until completion of the process Assume all jobs submitted at same time

7

Response Time Calculations

Job FCFS SJF RR

P1 0 40 0

P2 20 12 4

P3 32 4 8

P4 40 24 12

P5 56 0 16

Average 29.6 16 8

8

Turnaround Time Calculations

Job FCFS SJF RR

P1 20 60 60

P2 32 24 44

P3 40 12 32

P4 56 40 56

P5 60 4 20

Average 41.6 28 42.4

Review

What is a “race condition”? What are 4 properties necessary for a correct

“critical region” solution? mutual exclusion, no assumption, bounded

waiting, let others use when unusing What is Peterson’s Solution? Why is semaphore better than Peterson’s

solution and TSL?

Review

What is deadlock?

What is the four conditions for deadlock to happen?

Review of Banker’s Algorithm

Allocation Max Available

A B C A B C A B C

P0 0 1 0 7 5 3 3 3 2

P1 2 0 0 3 2 2

P2 3 0 2 9 0 2

P3 2 1 1 2 2 2

P4 0 0 2 4 3 3

P = {P0, P1, …, P4}; R={A(10), B(5), C(7)}

Snapshot at time T0:

Can request for (1,0,2) by P1 be granted?Can request for (3,3,0) by P4 be granted? Can request for (0,2,0) by P0 be granted?

Review

What is the Memory Management Unit?

How does it work during a virtual—physical address translation?

Translate virtual address to physical address1.202.41003.8300

Review

A paging scheme uses a Translation Loo-aside Buffer (TLB) A TLB access takes 10 ns and a main memory access takes 50 ns. What is the effective access time (in ns) if the TLB hit ratio is 90% and there is no page-fault?

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

Multilevel Page Tables

Figure 3-13. (a) A 32-bit address with two page table fields. (b) Two-level page tables.

What is the PT1 and PT2 value for a virtual address 0x00403004?

? Suppose that a machine has 38-bit virtual

address ad 32-bit physical addresses. What is the main advantage of a multilevel page table

over a single-level one? With a two-level page table, 16-KB pages, and 4-byte

entries, how many bits should be allocated for the top-level page table field and how many for the next-level page table filed? To answer this question, consider the internal fragmentation resulting from page table size. For example, when a page table is less than 1 page in size, you have internal fragmentation that is unused and wasted memory.

True or False

With paging, a process’ logical address spaces is contiguous

With paging, a process’ physical address spaces is contiguous

Review

Does paging have fragmentation? No? Then why not? Yes? Then what kind?

What are the overheads associated with paging?

Review True or False:

a) The logical address space cannot be bigger than the physical address space

b) Processes have big address spaces because they always need them

Demand paging:a) Is unrelated to prepagingb) Brings logical pages into physical memory

when requested by a processc) Increases memory requirements for a

system

Questions

What is static relocation? What is dynamic relocation? What is static linking? What is dynamic linking?

Review What is a Page Replacement Algorithm?

What is “Belady’s Anomaly”? How does the Optimal algorithm work? How does Enhanced Second Chance work?

What is thrashing? How do we fix it?

?

A computer has four page frames. The time of loading, time of last access, and the R and M bits for each page are as shown below (the times are in clock ticks):

Page Loaded Last Ref. R M 0 126 285 1 0 1 120 265 0 0 2 140 270 0 1 3 110 280 1 1 (a) Which page will NRU replace? (b) Which page will

FIFO replace? (c) Which page will LRU replace? (d) Which page will second chance replace?

Cause of thrashing

Degree of multiprogramming

CP

U u

tili

zati

on

thrashing increases

?

Array A[1024, 1024] of integer, each row is stored in one page

Program 1for j := 1 to 1024 do

for i := 1 to 1024 doA[i,j] := 0;

1024 × 1024 page faults Program 2

for i := 1 to 1024 dofor j := 1 to 1024 do

A[i,j] := 0; 1024 page faults

First-In-First-Out (FIFO)

1

2

3

3 Frames / Process

1,2,3,4,1,2,5,1,2,3,4,5

4

1

2

5

3

4

9 Page Faults

How many page faults would we have if we had4 Frames/Process?

?

How long does it take to load a 64KB program from a disk whose average seek time and average rotation time are 10 msec, and whose tracks hold 32KB. For 2KB block size? For 4KB block size?

(10+10+2/32*20)*32=(20+1.25)*32=680 (10+10+4/32*20)*16=(20+2.5)*16 =360

Figure 4-34. A UNIX i-node.

The UNIX V7 File System (2)

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

I-Node

How many data blocks are there?

If you added 3 more data blocks to the file, what would happen?D

isk

bloc

ks

i-node

null

null

null

null

null

62

77

Review

Directories: In what way is a directory different than a file? In what way is a directory similar to a file?

Aliases: Describe a hard-link Describe a soft-link

Free space management: If the size of a block is B bytes, then what is the

biggest size of a group in ext2?

Figure 4-35. The steps in looking up /usr/ast/mbox.

The UNIX V7 File System (3)

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639