ECM 1413 Processes

Question 1

• A program:

Answer 1

• is a set of instructions for performing a specific task (passive)
• is stored on disk

Question 2

• A process:

Answer 2

• An instance of a program in execution (active)
• requires CPU resources, memory and I/O
• Multiple process instances of one program

Question 3

Why do we use processes?

Answer 3

Modularity
Simplifies OS development

Speedup through parallelism
Subdivision of tasks so they can be ran in parallel

Security and stability through isolation

Question 4

nodes of the process lifecycle

Answer 4

born
ready
waiting
running
died

Question 5

Born/new

Answer 5

process created but not admitted

Question 6

Ready

Answer 6

process in main memory, ready to execute

Question 7

Running

Answer 7

The process has been allocated to a cpu pool and is currently running
The processor might the running of a process on the grounds of an interrupt or timeout and it is back to the ready state

Question 8

What is a Pressure Control Block (PCB)

Answer 8

Data structure that stores information, details, metadata, about a block. It is important in context switching
Usually every process has parent (except the first). As such, each process stores the ID of the parent process
PCB is the Kernel’s representation of the process

Question 9

Context Switching. Context:

Answer 9

the current state of the process (the files it has access to, the condition of the variables in the process)

- A context switch is pure overhead
- Time required varies with factors like the number of registers that must be copied

Question 10

To stop running process 1 and start process 2:

Answer 10

1) Change the state of process 1 from running to ready
2) Save the context of process (since it’s back in ready, we will need to use it later again)
3) Load the context of process 2
4) Change the process scheduling state of process 2

This whole process is referred to as a context switch or process switch

Question 11

Overhead:

Answer 11

any combination of excess or indirect computation time, memory, bandwidth, or other resources that are required to perform a specific task.

Question 12

With too few context switching:

Answer 12

• There will be the starvation of some processes. No fairness between the processes as some will have to wait too long

Question 13

With too many context switching:

Answer 13

• Takes too many resources and causes slowdown on overhead

Question 14

Layout of process in memory

Answer 14

Stack
Heap
data
Text

Question 15

Text

Answer 15

The actual executable code; fixed-size and read-only

Question 16

Data

Answer 16

Global and static variables; Fixed-size - determined at compile time

Question 17

Heap

Answer 17

Memory that is dynamically allocated during runtime; Size can shrink/grow

Question 18

Stack

Answer 18

Temporary storage (function parameters, return addresses and local variables) ; Size shrinks/grows

Question 19

Stack as a data structure

Answer 19

A LIFO (Last In First Out) data structure
Operations:
- Push: adding to the stack
- Pop: removing from the stack
Upon Function Call: Activation record (parameters, local variables, return address) pushed to stack
Upon Function Return: Activation record popped from stack

Question 20

The Heap as a data structure

Answer 20

Dynamically allocated (grows or shrinks based on the needs of the process), the only similarity to a stack
Blocks of memory are allocated and removed in an arbitrary (any) order
Used when you need:
- To store a large block of memory for a longer period of time
- Variables that can change size dynamically
If allocated during runtime, it goes onto the heap

Question 21

Process spawning:

Answer 21

The method by which a process (parent) creates a new process (child)

Question 22

In Linux there are four system calls for process spawning

Answer 22

fork
exec
wait
exit

Question 23

What is forking

Answer 23

Fork creates a child process with a unique process ID. The child is a copy of the parent process
Fork returns
* 0 to the child process
* the process ID of the child process to the parent process

Question 24

Wait

Answer 24

parent process is put in waiting state until child process finishes

Question 25

Exit

Answer 25

when done, the child process issues the exit system call, and the parent process can resume.