Topic #3: Threads and Processes

Question 1

It is an abstraction of a running program, an activity of some kind

Answer 1

Process

Question 2

It is a series of instructions for a computer to perform that may be stored on a disk

Answer 2

Program

Question 3

It is the rapid switching back and forth of pseudo parallel sequential processes

Answer 3

Multiprogramming

Question 4

Events which cause Processes Creation

Answer 4

• System Initialization
• Execution of a process creation system call by a running process
• a user request to create a new process
• initiation of a batch job

Question 5

Events which cause Termination

Answer 5

Normal Exit (Voluntary)
Error Exit (Voluntary)
Fatal Error (Involuntary)
Killed by another process (Involuntary)

Question 6

Three States a process may be in

Answer 6

Running
Ready
Blocked

Question 7

A state that a process can be in that refer to actually using the CPU at that instant

Answer 7

Running

Question 8

A state that a process can be in that refers to a runnable or a temporarily stopped process to let others run

Answer 8

Ready

Question 9

A state that a process can be in wherein it is unable to run until some external event happpens

Answer 9

Blocked

Question 10

It is where information that must be saved when the process is switched from running to ready or blocked state so that it can be restarted later

Answer 10

Process table

Question 11

Other name for Processs Table

Answer 11

Process Control Blocks

Question 12

How many entries are there per process

Answer 12

One entry per process

Question 13

It contains the address of the interrupt service procedure

Answer 13

Interrupt Vector

Question 14

It is a signal emitted by a device attached to a computerr or a from a program that requires the OS to stop and figure out what to do next

Answer 14

Interrupt

Question 15

These are sometimes referred to as lightweight processes

Answer 15

Threads

Question 16

It is the situation that allows multiple threads to run within the same process

Answer 16

Multithreading

Question 17

Advantages of Threads

Answer 17

• threads are more responsive as it executes an output and can be immediately returned
• context switch time between threads is lower
• multiple threads can be scheduled on multiple processor
• resources can be shared among all threads in a process
• communication between threads is eeasier as they share an address space
• multiple threads = multiple jobs = multiple completion = increased throughput

Question 18

It is the thread package defined by the IEEE to make it possible to write protable threaded programs

Answer 18

Pthreads

Question 19

It is a thread call that creates a new thread

Answer 19

Pthread_create

Question 20

It is a thread call that terminates the calling thread

Answer 20

Pthread_exit

Question 21

It is a thread call that waits for a specific thread to exit

Answer 21

Pthread_join

Question 22

It is a thread call that releases the CPU to let another thread run

Answer 22

Pthread_yield

Question 23

It is a thread call that creates and initializes a thread’s attribute structure

Answer 23

Pthread_attr_init

Question 24

It is a thread call that removes a thread’s attribute structure

Answer 24

Pthread_attr_destroy

Question 25

All Pthreads have certain properties

Answer 25

- an identifier - a set of registers - a set of attributes

Question 26

Main places to implement threads

Answer 26

- user space - the kernel

Question 27

Implementing Threads in User Space

Answer 27

- kernel knows nothing about them; it is managing single-threaded proccesses - each process needs its own thread table - implemented by a library

Question 28

Advantages of Implementing Threads in User Space

Answer 28

- Can be implemented in an OS that does not support threads - customized scheduling algorithm for each process - it scales better

Question 29

Disadvantages of Implementing Threads in User Space

Answer 29

- problem of howw blocking system calls are implemented - problem of page faults - one thread can only use the CPU - programmers want theads where threads are blocked often

Question 30

Implementing Threads in a Kernel

Answer 30

- No run-time system is needed - no thread table for each process; one thread table for the entire system - information about the threads is now ketp in the kernel - no need for nonblocking system calls

Question 31

Pros and Cons of Nonblocking system calls

Answer 31

- a system call is susbtantial and requires more overhead but has issues with signals

Question 32

Hybrid Implementation of Threads

Answer 32

- kernel is aware of only kernel-level threads and schedules those - some of the kernel threads may have user-level threads multiplexed on top - user level threads are created, destroyed, scheduled like other user-level threads even if OS cant multihread - each kernel-level thread takes turns using user-level threads in the kernel

Question 33

IPC

Answer 33

Inter-process Communication

Question 34

Mechanisms for IPC

Answer 34

- Atomic Read/Write - Locks - Semaphores - Monitors - Message Passing

Question 35

Need to communicate with other processes

Answer 35

- to pass info form process to process - to enable proper sequencing when dependencies are present - to ensure that no two process are the in the critical region at the same time

Question 36

Race Conditions

Answer 36

Two or more processes are reading and writing shared data

Question 37

Conditions in order to avoid race conditions

Answer 37

- Mutual Exclusion: No two processes may be simultaneously in the critical regions - No Assumption: no assumptions may be made about speeds or the number of CPUs - Progress: no process running outside its critical region may be blocked - No Starvation: no progress should have to Wait forever to enter the critical region

Question 38

it is a condition required for the avoidance of a racce condition wherein no two proccesses may be simultaneously be inside their critical regions

Answer 38

Mutual Exclusion

Question 39

it is a condition required for the avoidance of a racce condition wherein no assumptions may be made about the speeds or the number of CPUs

Answer 39

No assumptions

Question 40

it is a condition required for the avoidance of a racce condition wherein no process running outside its critical region may block other processes

Answer 40

Progress

Question 41

it is a condition required for the avoidance of a racce condition wherein no process should have to wait forever to enter its critical region

Answer 41

No Starvation

Question 42

It is the part of the program where shared variables are accessed

Answer 42

Critical Region

Question 43

Disabling Interrupts

Answer 43

Each process disables all interrupts just after entering the critical region and re-enable them before leaving

Question 44

It is a single shared variable where the process sets it to 1 and enters the critical region, if it is 1 the process has to wait for it to become 0

Answer 44

Lock Variables

Question 45

Classsical IPC Problem

Answer 45

- Producer-Consumer Problem - Dining Philosopher Problem - Reader Writers Problem

Question 46

Explain the Dining Philosophers Problem

Answer 46

It models processes competing for exclusive access to limited resources

Question 46

Explain the Producer-Consumer Problem

Answer 46

- It models acess to a bounded buffer - Producers wont try to add data into the buffer if its full and consumers wont try to remove data if buffer is empty

Question 47

Explain the Readers-Writers Problem

Answer 47

- Models access to a database - two readers can read at once: a writer should not wait longer than needed

Question 48

What are the units of execution

Answer 48

Processes

Question 49

How are the units of execution represented

Answer 49

Process Control Blocks

Question 50

How is work scheduled in the CPU

Answer 50

- Process States - Process Queues - Context Switches

Question 51

How does a process move from one state to another

Answer 51

- Scheduling - I/O - Creation - Termination

Question 52

How are processes created

Answer 52

- CreateProccess (Windows) - Fork/exec (Unix)

Question 53

Comparea and Contrast Kernel-Level Threads ot User-level threads

Answer 53

Kernel-level threads are better but requires significant overhead; User-level threads are better but are not well integrated in the OS