Chapter 3: Processes

Question 1

What is a process?

Answer 1

The unit of work in a modern time-sharing system

Question 2

List and define the 5 states

Answer 2

New - The process is being created.
Running - Instructions are being executed.
Waiting - The process is waiting for some event to occur (such as an I/O completion or reception of a signal).
Ready - The process is waiting to be assigned to a processor.
Terminated - The process has finished execution.

Question 3

What state(s) can a process transition to from running?

Answer 3

waiting
blocked/ready
blocked/suspended
terminated

Question 4

When a process is swapped out or suspended in secondary memory, what state does it have to transition to in order to be scheduled?

Answer 4

Ready/suspended

Question 5

What three state(s) can process transition to ready?

Answer 5

suspended
waiting
running

Question 6

In the UNIX state diagram, there is a user running state and a kernel running state. Why is this?

Answer 6

for dual mode, processes to run simultaneously.

Question 7

What scheduler determines which process will be run by the CPU?

Answer 7

Short term scheduler

Question 8

Describe long and short term scheduling

Answer 8

Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue. Is invoked very infrequently and can be slow. (seconds, minutes) Controls the degree of multiprogramming.

Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU.
Is invoked very frequently (milliseconds). Has to be fast.

Question 9

What’s the difference between I/O and CPU bound processes?

Answer 9

I/O spends more time doing I/O than computations. Has many short CPU bursts.

CPU spends more time doing computations. Has few very long CPU bursts.

Question 10

What happens to processes that are running and then request an I/O?

Answer 10

The processes are placed in an I/O queue.

Question 11

Why are processes put in a swapped/ready state?

Answer 11

Because theres too much contention so it must wait.

Question 12

What are some possible reasons that cause a process to give up the CPU or become blocked and eventually be moved to the ready queue?

Answer 12

Interrupts

Question 13

What’s the importance of degree of multiprogramming?

Answer 13

It has make a careful selection and select a good process mix. I/O and CPU bound processes .

If all processes are I/O bound, the ready queue will almost always be empty, and the short-term scheduler will have little to do.

If all processes are CPU bound, the I/O waiting queue will almost always be empty, devices will go unused, and again the system will be unbalanced.

Question 14

What operating systems don’t have long-term schedulers?

Answer 14

Unix and microsoft windows

Question 15

What’s the idea behind medium-term scheduler?

Answer 15

To remove processes from memory and reduce the degree of multiprogramming and to possible add processes back into memory later using the swapping method. The key advantage of doing this is to improve the process mix or to free up memory.

Question 16

Describe Context Switching

Answer 16

When CPU switches to another process, the system must save the state of the old process and load the saved state for the new process.

Context-switch time is overhead; the system does no useful work while switching.

Time dependent on hardware support.

Question 17

What resources does a child need to complete a task?

Answer 17

CPU time, memory, files, I/O devices

Question 18

Give a characteristic of Independent and Cooperating processes

Answer 18

Independent process cannot affect or be affected by the execution of another process.

Cooperating process can affect or be affected by the execution of another process.

Question 19

What are the advantages of Cooperating processes?

Answer 19

Information sharing
Computation speed-up
Modularity
Convenience

Question 20

List and Define the two Interprocess Communication models

Answer 20

Message system – processes communicate with each other without resorting to shared variables.

Shared Memory- a region of memory is shared by co-operating processes.

Question 21

How’s information exchanged through shared memory communication?

Answer 21

Through reading and writing to shared region.

Question 22

How’s information exchanged through message system communication?

Answer 22

Through messages.

Question 23

What are some characteristics of shared memory and message system communication?

Answer 23

Shared memory communication: Faster than message system communication.

Message system: Easier to implement, useful for exchanging small amounts of data, and Implemented using system calls and thus requires intervention of kernel.

Question 24

What’s the difference between an unbounded buffer and a bounded buffer?

Answer 24

Unbounded: places no practical limit on the size of the buffer.

Bounded: assumes that there is a fixed buffer size.

Question 25

Give a characteristic and name some properties of communication links in Direct Communication

Answer 25

Characteristic: Processes must name each other explicitly.

Properties of communication links:
Links are established automatically.

A link is associated with exactly one pair of communicating processes.

Between each pair there exists exactly one link

The link may be unidirectional, but is usually bi-directional.

Symmetric vs. Asymmetric

Question 26

Give a characteristic and name some properties of communication links in Indirect Communication

Answer 26

Characteristics: Messages are directed and received from mailboxes (also referred to as ports)

Each mailbox has a unique id.

Processes can communicate only if they share a mailbox.

Properties of communication link:
Link established only if processes share a common mailbox.

A link may be associated with many processes.

Each pair of processes may share several communication links.

Link may be unidirectional or bi-directional.

Question 27

List the operations of Indirect Communication

Answer 27

reate a new mailbox
send and receive messages through mailbox
destroy a mailbox

Question 28

What are some reasons a parent may terminate the execution of a child?

Answer 28

Child has exceeded allocated resources.
Task assigned to child is no longer required.

All children terminated - cascading termination

Question 29

What happens when a process is terminated?

Answer 29

Process executes last statement and asks the operating system to delete it (exit)

Output data from child to parent (via wait)

Process’ resources are deallocated by operating system

Question 30

Describe process creation

Answer 30

Parent process create children processes, which, in turn create other processes, forming a tree of processes.

Question 31

Give characteristics of resource sharing, execution, and address in process creation.

Answer 31

Resource sharing:
Parent and children share all resources
Children share subset of parent’s resources
Parent and child share no resources

Execution:
Parent and children execute concurrently
Parent waits until children terminate

Address Space: Child duplicate of parent
Child has a program loaded into it

Question 32

Give some unix examples of process creation

Answer 32

fork system call creates new process

exec system call used after a fork to replace the process’ memory space with a new program

Question 33

What are the two characteristics of execution that happen when a process creates a process?

Answer 33

The parent continues to execute concurrently with it’s children.

The parent waits until some or all of its children have terminated.

Question 34

How are message passing systems implemented?

Answer 34

They are implementing with the use of system calls

Question 35

Why is shared memory faster than message passing?

Answer 35

because message-passing systems are typically implemented using system calls and require the more time-consuming task of kernel intervention

Question 36

What are some issues of shared memory?

Answer 36

cache coherency

Question 37

What’s the difference between symmetry and asymmetry in direct communication?

Answer 37

Symmetry: both the sender process and the receiver process must name the other to communicate.

Asymmetry:only the sender names the recipient; the recipient is not required to name the sender.

Question 38

What’s the disadvantage of symmetry and asymmetry schemes?

Answer 38

limited modularity of the resulting process definitions.

Changing the identifier of a process may necessitate examining all other process definitions.

Question 39

What an advantage of the multiprocess approach?

Answer 39

Websites run in isolation from one another. If one website crashes, only its renderer process is affected; all other processes remain unharmed.

Question 40

Define Sandbox in multiprocess

Answer 40

Access to disk and network I/O is restricted, minimizing the effects of any security exploits.

Question 41

What is the purpose of remote procedure call?

Answer 41

calls information that is to be received.

Question 42

What does the semantics of at most once mean in terms of remote procedure calls?

Answer 42

Only receives the first set of information set and none of the rest. This is to avoid duplicates and possible loss/corruption of data.