Zybook Ch2: Processes, Threads, and Resources

Question 1

What is a process?

Answer 1

an instance of a program being executed by an OS

Ex: the OS creates a new process when a user opens a new application

Question 2

What is the OS organized as?

Answer 2

The OS is organized as a collection of processes

Question 3

How does the OS keep track of each process?

Answer 3

Process Control Blocks (PCB)

Question 4

What is a process control block (PCB)?

Answer 4

A data structure that holds information for a process, including the current instruction address, the execution stack, the set of resources used by the process, and the program being executed. The PCB is the concrete representation of a process.

Question 5

What are the 3 basic states that are implemented by most OSs for a process?

Answer 5

Running state (running)

Ready state (ready)

Blocked state (blocked)

Question 6

What is the running state?

Answer 6

When the process has all necessary resources and the CPU is actively executing the program’s instructions.

Question 7

What is the ready state?

Answer 7

When the process has all necessary resources to run but the CPU is currently unavailable.

Question 8

What is the blocked state?

Answer 8

When the process is waiting on a currently unavailable resource.

Question 9

What is the new state?

Answer 9

State a newly created process is placed into before the process is allowed to compete for the CPU

Question 10

What is the terminated state?

Answer 10

The state the process is placed into when execution can no longer continue but the PCB has not been deleted.

Question 11

What is the suspended state?

Answer 11

The state the process is placed into even though the CPU and all resources are available?

Question 12

What is a context switch?

Answer 12

The transfer of control from one process to another?

Question 13

What is the CPU state?

Answer 13

All of the intermediate values held in any CPU registers and hardware flags at the time of interruption.

Question 14

What is a physical CPU?

Answer 14

The real hardware instance of a CPU

Question 15

What is a virtual CPU?

Answer 15

A CPU the process thinks is available only to itself. Occurs when multiple processes run on one physical CPU using time sharing

Question 16

What is a virtual CPU in relation to the physical CPU?

Answer 16

it can be an abstraction of the physical CPU or it can be a software that emulates the behavior of a different CPU

Question 17

What are the benefits of virtual CPUs?

Answer 17

Multi-user support, multi-cpu transparency, portability

Question 18

What is multi-user support?

Answer 18

Multiple users, each represented by one or more separate processes, can share the same machine without being aware of each other.

Question 19

What is multi-CPU transparency?

Answer 19

An application written to utilize multiple CPUs will run correctly, although perhaps more slowly, if only one CPU is available.

Question 20

What is portability for virtual CPUs?

Answer 20

An application compiled for one type of CPU can run on a different CPU without being modified or even recompiled.

Question 21

What are the benefits of using multiple cooperating processes?

Answer 21

1) The interfaces between the processes are simple and easy to understand.
2) Each process can be designed and studied in isolation.
3) The implementation reduces idle time by overlapping the execution of multiple processes.
4) Different processes can utilize separate CPUs, if available, thus speeding up the execution.

Question 22

What are the fields for a generic PCB?

Answer 22

CPU_state, process_state, memory, scheduling_information, accounting_information, open_files, other_resources, parent, children

Question 23

What is the CPU_state field?

Answer 23

When p is stopped, the current state of the CPU, consisting of various hardware registers and flags, is saved in this field. The save information is copied back to the CPU when p resumes execution.

Question 24

What is the process_state field?

Answer 24

Stores p’s current state. Ex: Running, ready, or blocked.

Question 25

What is the PCB memory field?

Answer 25

Describes the area of memory assigned to p. In the simplest case the field would point to a contiguous area of main memory. In systems using virtual memory (to be introduced in a later chapter) the field could point to a hierarchy of memory pages or segments.

Question 26

What is the scheduling_information field?

Answer 26

Contains information used by the scheduler to decide when p should run. The information typically records p’s CPU time, the real time in the system, the priority, and any possible deadlines.

Question 27

What is the accounting_information field?

Answer 27

Keeps track of information necessary for accounting and billing purposes. Ex: The amount of CPU time or memory used.

Question 28

What is the open_files field?

Answer 28

Keeps track of the files currently open by the process.

Question 29

What is the other_resources field?

Answer 29

Keeps track of any resources, such as printers, that p has requested and successfully acquired.

Question 30

What is the parent field?

Answer 30

Every process is created by some other running process. The parent process of a process p is the process that created p. The parent field records the identity of p’s parent.

Question 31

What is the children field?

Answer 31

A child process c of process p is a process created by p. Process p is c’s parent. The identity of every child process c of p is recorded in the children field.

Question 32

How is each PCB implemented and why?

Answer 32

Each PCB is implemented as a heterogeneous data structure because they are composed of different data types?

Question 33

What are the two ways to organize all PCBs?

Answer 33

1) An array of structures. The PCBs are marked as free or allocated, which eliminates the need for any dynamic memory management. The main drawback is a lot of wasted memory space to maintain a sufficient number of PCB slots.
2) An array of pointers to dynamically allocated PCBs. The pointer array wastes little space and can be made much larger than the array of structures. The drawback is the overhead of dynamic memory management to allocate each new PCB and to free the memory when the process terminates.

Question 34

Why are linked lists avoided in OS?

Answer 34

They require dynamic memory management, which is costly.

Question 35

How are PCBs organized without linked lists?

Answer 35

The links are distributed over the child PCBs such that each points to the immediate younger sibling and immediate older sibling. The original 2 fields, parent and children, in the PCB of a process p are replaced by 4 new fields:

• parent: points to p’s single parent as before
• first_child: points to p’s first child
• younger_sibling: points to the sibling of p created immediately following p
• older_sibling: points to the sibling of p created immediately prior to p

Question 36

What is the waiting list?

Answer 36

It is associated with every resource and contains all processes blocked on that resource because the resource is not available.

Question 37

What is the ready list (RL)?

Answer 37

A list containing all processes that are in the ready state and thus are able to run on the CPU. The RL also includes the currently running process.

Question 38

What is a process creation hierarchy?

Answer 38

A graphical representation of the dynamically changing parent-child relationships among all processes. The process creation hierarchy changes each time a process is created or destroyed.

Question 39

What is the create process function?

Answer 39

create() is a function that allocates a new PCB, fills the PCB entries with initial values, and links the PCB to other data structures in the system

Question 40

What are the 8 steps of the create process function (create())?

Answer 40

1_ A new PCB is allocated. The PCB is uniquely identified by a pointer or an array index p.

2) The fields cpu_state, memory, scheduling_information, and accounting_information are filled using the initial values supplied to the function as parameters.
3) Assuming the state transition diagram consists of only the three states, running, ready, and blocked, the process state field is set to ready.
4) The parent field of p is set to point to self, which is the calling process and thus the parent of p.
5) In turn, p is inserted into the calling process’s list of children as a new child.
6) The remaining fields are set to NULL, since at creation p has no children, no open files, and no other resources.
7) p is inserted into the RL.
8) The scheduler function is called to select the process to run. Depending on the priorities of the two processes, the scheduler could choose to start the new child process p or to continue the execution of p’s parent process.

Question 41

How are processes destroyed?

Answer 41

destroy(p). The destroy process function destroys a process by freeing the PCB data structure and removing any references to the PCB from the system.

Question 42

What are the steps of destory(), the destory process function?

Answer 42

1) After calling destroy(c) on all child processes, remove p from either the RL (when p is ready) or from the waiting list of a resource (when p is blocked).
2) Remove p from the list of children of the calling process.
3) Release all memory and other resources, close all open files, and deallocate the PCB.
4) Call the scheduler to select the next process to run. The call must be made outside of the destroy(p) function to guarantee that the scheduler executes only once, after the entire hierarchy of processes has been destroyed, rather than as part of every recursive call to destroy(c).