vault backup: 2025-04-21 21:20:06
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201
Operating Systems/Processes and Threads.md
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201
Operating Systems/Processes and Threads.md
Normal file
@ -0,0 +1,201 @@
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---
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type: theoretical
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backlinks:
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- "[[Overview#Multiprogramming]]"
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- "[[Overview#Multitasking/Timesharing]]"
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---
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## Process
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A program in execution.
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Consists of:
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* The program code - **text section**
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* Current activity - **PC**, registers
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* **Stack** -> Function parameters, return addresses, local variables
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* Data section
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* **Heap** -> dynamically allocated (at run time) memory
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The difference between a process and a program is that the program is the executable file stored on disk, while the process **is running** (shocker).
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### Creation
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Four events could cause processes to be created
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1. System init - Daemons
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2. Executing a process by "running a program"
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3. A user process request to create a new process
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4. Initiation of a batch[^1] job
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### `fork()`
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A Linux [system call](Overview.md#System%20calls).
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```mermaid
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graph LR;
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A["`fork()`"] --> |parent| B["wait"]
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A --> |child|C["`exec()`"]
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C --> D["`exit()`"]
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D --> B
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B --> E["Resumes"]
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```
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### Hierarchy
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Linux creates a parent-child relationship between processes, Windows doesn't.
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Linux:
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```mermaid
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graph TD;
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init["init
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pid = 1"]
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login["login
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pid = 8415"]
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kthreadd["kthreadd
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pid = 2"]
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sshd["sshd
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pid=3028"]
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bash["bash
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pid=8416"]
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ps["ps
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pid=9298"]
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emacs["emacs
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pid=9204"]
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khelper["khelper
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pid=6"]
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pdflush["pdflush
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pid=200"]
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init --> login
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init --> kthreadd
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init --> sshd
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login --> bash
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bash --> ps
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bash --> emacs
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kthreadd --> khelper
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kthreadd --> pdflush
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```
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### Termination
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1. Normal
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Process should return a code to its parent. Child processes should wait until they know that the parent received it, becoming **zombie processes**. If the parent dies before the child, the child is called an orphan. Absolutely fucking crazy naming. Every linux process should have a parent process [source: unicef](https://unicef.org).
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2. Error - just a special return code
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3. Fatal error, involuntary - division by zero, invalid opcode; process is immediately terminated by the system
|
||||
4. Killed
|
||||
|
||||
|
||||
### States
|
||||
As a state machine
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||||
|
||||
1. Running
|
||||
2. Ready
|
||||
3. Blocked (blocking == waiting)
|
||||
|
||||
```mermaid
|
||||
graph TD;
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||||
A["Running"]
|
||||
B["Ready"]
|
||||
C["Blocked"]
|
||||
|
||||
A --> |1| C
|
||||
A --> |2| B
|
||||
B --> |3| A
|
||||
C --> |4| B
|
||||
```
|
||||
#### Ready State
|
||||
- In this state the process is not waiting for a resoucrce
|
||||
- Can be executed
|
||||
- Put in a queue (ready queue)
|
||||
|
||||
#### I/O queue
|
||||
- I/O device has its own
|
||||
- Multiple queues are created by OS
|
||||
## Timesharing: In-depth
|
||||
from [Multitasking/Timesharing](Overview.md#Multitasking/Timesharing)
|
||||
The output of running programs should not change when we stop and switch back to the same program later on.
|
||||
|
||||
|
||||
### Context switching
|
||||
Switching implies that we have to store the values of registers, flags, PC, etc. of the current process and load them into the next one. Then we continue.
|
||||
|
||||
|
||||
### The process control block - PCB
|
||||
The OS needs a place to store the status of each process. This is that data structure.
|
||||
|
||||
|
||||
### Process table
|
||||
A list of PCBs (one per process)
|
||||
|
||||
|
||||
|
||||
|
||||
* Timer (`ISR`[^2]) generates multiple interrupts per second
|
||||
* Store the status of the process in PCB
|
||||
|
||||
|
||||
|
||||
## Threads
|
||||
A thread is a basic unit of CPU utilization, consisting of a program counter, a stack, and a set of registers, ( and a thread ID. )
|
||||
|
||||
A light-weight process.
|
||||
|
||||
|
||||
|
||||
### Processes vs threads
|
||||
|
||||
|
||||
| Processes | Thread |
|
||||
| ----------------------------------------- | ------------------------------------------------ |
|
||||
| Heavyweight | Lighter |
|
||||
| Each process has its own memory | Threads use memory of the process they belong to |
|
||||
| Inter-Process Communication (IPC) is slow | Way faster inter-thread communication |
|
||||
| Context switching is more expensive | Less expensive |
|
||||
| Do not share memory | do share memory |
|
||||
|
||||
### Multithreading
|
||||
- Traditional processes have a single thread of control[^3]
|
||||
* If a process has multiple threads of control, it can perform more than one task
|
||||
|
||||
### Ways to Implement Threads
|
||||
* Kernel-Level Threads (KLT)
|
||||
* Managed by the OS kernel
|
||||
* Each thread is a separate scheduling entity
|
||||
* `pthread`, `thread`
|
||||
* User-Level Threads (ULT)
|
||||
* Managed by user-space libraries, OS is unaware
|
||||
* Faster context switching
|
||||
* Green threads
|
||||
|
||||
### User Threads and Kernel Threads
|
||||
- **User threads**
|
||||
- Implemented by a thread library at the user level
|
||||
- thread creation and scheduling are done in user space
|
||||
- **Kernel Threads**
|
||||
- Managed by OS
|
||||
|
||||
### Relationship modles
|
||||
*
|
||||
|
||||
[^1]: A batch job is a scheduled task or a set of commands that are executed without manual intervention - **cron**
|
||||
|
||||
[^2]: interrupt service routine - like in LC3
|
||||
|
||||
[^3]: sequence of programmed instructions that can be managed independently by a scheduler within a computer program
|
||||
|
||||
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@ -1,17 +0,0 @@
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||||
# Conflicts
|
||||
Please resolve them and commit them using the commands `Git: Commit all changes` followed by `Git: Push`
|
||||
(This file will automatically be deleted before commit)
|
||||
[[#Additional Instructions]] available below file list
|
||||
|
||||
- Not a file: .obsidian/workspace.json
|
||||
|
||||
# Additional Instructions
|
||||
I strongly recommend to use "Source mode" for viewing the conflicted files. For simple conflicts, in each file listed above replace every occurrence of the following text blocks with the desired text.
|
||||
|
||||
```diff
|
||||
<<<<<<< HEAD
|
||||
File changes in local repository
|
||||
=======
|
||||
File changes in remote repository
|
||||
>>>>>>> origin/main
|
||||
```
|
||||
0
unicef.org.md
Normal file
0
unicef.org.md
Normal file
Loading…
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Reference in New Issue
Block a user