1. OS
1

2. Operating System
 Just a program
 Provides a stable, consistent way for applications
to deal with the hardware
2

3. What constitutes an OS? 3
 Kernel
 System Programs
 Application Programs

4. Storage Device Hierarchy 5

5. Cache
 Writing policies
 Write back
 Initially, writing is done only to the cache
 Mark them as dirty for later writing to the
backing store
 Write through
 Write is done synchronously both to the cache
and to the backing store
6
 Replacement Policy?

6. System Calls
 Instructions that allow access to privileged or
sensitive resources on the CPU
 System calls provide a level of portability
 Parameters to system calls can be passed
through registers, tables or stack
 Is “printf” a system call?
7

7. System Calls
 File copy program
 Acquire input file name
 Acquire output file name
 If file cannot be created abort
 Open input file
 If file doesn't exist abort
 Read from input file
 Write to output file
 Repeat until read fails
 Close output file
 Terminate normally by returning closing status to OS
8

8. Operating System Operation
 Interrupt driven
 Dual Mode Operation
 “Kernel” mode (or “supervisor” or “protected”)
 “User” mode: Normal programs executed
9

9. Operating System Operation
 Timer
 Interrupts the
computer after a
specified period
 May be treated as a
fatal error or may give
the program more
time
10
Penny, Penny, Penny

10. Process
 A program in execution
 A single „thread‟ of execution
 Uniprogramming: one thread at a time
 Multiprogramming: more than one thread at a time
11

11. Process States
 New : Process is being created
 Ready : Process waiting to run
 Running : Instructions are being executed
 Waiting : Process waiting for some event to occur
 Terminated : The process has finished execution
12

12. Process Control Block(PCB)
 Only one PCB active at a time
13

13. Process Creation
vi catemacs ls
init
CSH CSH
pid=1
Pid=7778
Pid=1400
 Parent followed by a child process
14

14. Process Creation – Fork() 15

15. A Question (Microsoft) 16

16. Inter Process Communication 17
 Shared-Memory Systems
 A process creates a shared-memory segment
 Other processes attach it to their address space
 Message-Passing Systems
 send(P, message)
 receive(id, message)

17. IPC - Pipes 18
 Pipes are OS level communication links between processes
 Pipes are treated as file descriptors in most OS

18. Multithreading 19
 Each thread has its own stack – current execution state
 Threads encapsulate concurrency

19. Multithreading 20
 Advantages
 Responsiveness
 Resource Sharing
 Economy
 Scalability
 Pthreads
 POSIX standard defining an API for thread creation
and synchronization

20. A Question (Google) 21

21. Scheduling 22
Scheduling – Deciding which
threads are given access to
resources from moment to
moment
The CPU should not be idle
At least on process should
use CPU

22. Scheduling 23
Scheduling – Deciding which threads are given access to resources
from moment to moment

23. Scheduling 24
 Goals/Criteria
 Minimize Response Time
 Maximize Throughput
 Fairness
 First-Come, First-Served (FCFS) Scheduling
 Run until done
 Shorts jobs get behind long ones
P1 P2 P3
24 27 300

24. Preemption 25
 Capability to preempt a
process in execution
 Execution prioritized
 Higher priority processes
preempt the lower ones

25. Round Robin (RR) 26
 Each process gets a small unit of CPU time(time quantum)
 After quantum expires, process is preempted and added to the
end of ready queue
 N processes in ready queue and time quantum is q
 No process waits more than (N-1)q time units
 Performance
 q large -> FCFS
 q must be large with respect to context switch, otherwise
too much overhead

26. Round Robin (RR) 27
P1 P2 P3 P4 P1 P3 P4 P1 P3 P3
0 20 28 48 68 88 108 112 125 145 153
Process Burst Time
P1 53
P2 8
P3 68
P4 24
Waiting time for
 P1 = ?, P2 = ?, P3 = ?, P4 = ?
Average Waiting Time = ?
Average Completion Time = ?

27. Round Robin (RR) 28
P1 P2 P3 P4 P1 P3 P4 P1 P3 P3
0 20 28 48 68 88 108 112 125 145 153
Process Burst Time
P1 53
P2 8
P3 68
P4 24
Waiting time for
 P1 = (68-20) + (112-88) = 72, P2 = 20, P3 = 85, P4 = 88
Average Waiting Time = 66.5
Average Completion Time = 104.5

28. Round Robin (RR) 29
P1 P2 P3 P4 P1 P3 P4 P1 P3 P3
0 20 28 48 68 88 108 112 125 145 153
Process Burst Time
P1 53
P2 8
P3 68
P4 24
Pros and Cons:
 Better for short jobs
 Context-switching adds up for long jobs

29. What if we know future? 30
 Shortest Job First (SJF)
 Run whatever job has the least amount of
computation to do
 Shortest Remaining Time First (SRTF)
 Preemptive version of SJF: if job arrives and
has a shorter time to completion than the
remaining time on the current job,
immediately preempt CPU
Basically
 Idea is to get short jobs out of the system
 Big effect on short jobs, only small effect on long ones
 Result is better average response time

30. Synchronization 31
 Most of the time, threads are working on separate data, so
scheduling doesn‟t matter
 But, what happens when they work on a shared variable
Atomic Operations
 An operation that always runs to completion or not at all
 Indivisible
 Fundamental Building Block

31. Synchronization 32
 Synchronization
 Using atomic operations to ensure cooperation between
threads
 Mutual Exclusion
 Only one thread does a particular thing at a time
 Critical Section
 Piece of code that only one thread can execute at once
 Lock
 Prevents someone from doing something

32. Synchronization 33
Load/Store Disable Ints Test&Set Comp&Swap
Locks Semaphores Monitors Send/Receive
Shared Programs
Hardware
Higher-
level
API
Programs
Everything is pretty painful if only atomic primitives are load and store

33. Semaphores 34
 A kind of generalized lock
 Definition: A semaphore has a non-negative integer value and
supports the following two operations
 P() : An atomic operation that waits for semaphore to
become positive, then decrements it by 1
 Also called the wait() operation
 V() : An atomic operation that increments the semaphore
by 1, waking up a waiting P, if any
 Also called the signal() operation

34. Semaphores Implementation 35

35. Semaphore vs Mutex 36
 Nope, the purpose of mutex and semaphore are different
 Mutex is locking mechanism used to synchronize access to
resource. Ownership associated with mutex. Only the owner
can release the lock.
 Semaphore is Signaling mechanism (“I am done, you can carry
on” kind of signal)
 More at http://www.geeksforgeeks.org/mutex-vs-semaphore/
 Are binary semaphore and mutex same?

36. Readers-Writers 37
 Problem : Several readers and writers accessing the same file
 Need to control access to buffer
 If several readers are reading, no problem
 If writer is writing and reader is reading, some problem
 Variations
 First-Writers-Then-Readers
 First-Readers-Then-Writers

37. Readers-Writers 38

38. Deadlocks 39
P0
Wait(Q)
Wait(S)
Some code here
Signal(Q)
Signal(S)
P1
Wait(S)
Wait(Q)
Some code here
Signal(S)
Signal(Q)
A deadlock is a situation in which two or more
competing actions are each waiting for the other to
finish, and thus neither ever does.

39. Deadlock Requirements 40
 Mutual Exclusion
 Only one thread at a time can use a resource
 Hold and Wait
 Thread holding at least one resource is waiting to acquire
additional resources held by other threads
 No preemption
 Resources are released only voluntarily by the thread holding the
resource, after thread is finished with it
 Circular Wait
 There exists a set {T1, …, Tn} of waiting threads
 T1 is waiting for a resource that is held by T2
 T2 is waiting for a resource that is held by T3
 …
 Tn is waiting for a resource that is held by T1

40. Some Techniques 41
 Deadlock Detection
 Resource allocation graph etc.
 Prevention
 Circular Wait
 Avoidance
 Banker‟s algorithm