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operating-system.md

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title Operating System
description Operating system basic


 
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Table of contents

1. OS Definition

Operating System is software or it can be seem as the intermediate between user and hardware that runs on a computing device and manages the hardware and software components that make up a functional computing system

2. OS Functions

Hardware management
Provide interface for users
Application installation
Connect hardware
Interaction between application and hardware
CPU scheduling
Process coordination and synchronization
Resource management
Access control and protect system
Integrity maintenance, error control and recovery

3. OS Types

Base on processing:

  • Uniprogramming OS
  • Multiprogramming OS
  • Time-sharing OS
  • Parallel/multiprocessor/tightly-coupled OS
  • Clustered/distributed/loosely-coupled OS
  • RTOS

4. OS Components

Process/thread management
Primary memory management
File management
I/O system management
Secondary memory management
Protect system
Command line interpreter system

5. Process

Process is the active application instant, application become process when executable file loaded into memory (called process image)
Process layout is as same as Memory Layout In C Program
Process initialization step: allocate identifier, allocate memory for process load, initialize Process Control Block (PCB), setup necessary relation (e.g. arrange PCB to queue)
PCB is one of the most important data structure in OS to manage and control the execution of processes, it typically contains the following components:

  • Process ID (PID): unique identifier assigned to each process by the operating system
  • Program counter (PC): address of the next instruction to be executed by the process
  • Process state: current state of the process, includes new, ready, running, waiting, terminated, suspended
  • CPU registers: values of the processor registers that are being used by the process
  • Memory management information: information about the process’s memory allocation, such as the base and limit registers
  • I/O status information: information about the process's I/O operations, such as open files and network connections
  • Accounting information: information about the resources used by the process, such as CPU time and memory usage
  • CPU scheduling information: information about the process’s priority and scheduling requirements

Thread is a segment of a process or a lightweight process, threads share memory, data, resources together
There are 3 thread mapping model

  • Multi user thread mapping to single kernel thread: any user thread blocked cause all user thread blocked, user threads can't run parallel because only one user thread can use kernel at one time
  • Single user thread mapping to single kernel thread: create a user thread also create a kernel thread, better concurrency because any user thread blocked don't affect to other threads, limited number of threads
  • Multi user thread mapping to multi kernel threads: solve the disavantages of the two below but hard to setting

OS Scheduling

Scheduling in OS is process scheduling with PCB: process scheduling is a crucial function in OS and PCB in OS plays a vital role in this function
Scheduler include several types:

  • Long-term scheduler used to schedule job (or process), decide which program accepted to load into system to run, control system's multiprogramming, maintain both CPU-bound and I/O-bound process
  • Short-term scheduler used to schedule CPU, decide which process in ready queue will take CPU to run next, trigger events: clock interrupt, I/O interrupt, OS call, signal, ...
  • Mid-term scheduler (in time-sharing system) used to adjust system's multiprogramming (swap in - move process from memory to disk and swap out - move process from disk to memory) Scheduler will change the CPU control permit for the choosen process, includes:
  • Context change
  • User mode change
  • Jump to the position in process to running proccess again Scheduling cause time consumption called dispatch latency, time that scheduler stop current process and start another process Scheduling criteria
  • User-oriented: response time - time since process receive request to first request is responsed (time-sharing, interactive system) minimum, turnaround time: time since process loaded into system to process terminated minimum, waiting time: sum of process waiting time in ready queue minumum
  • System-oriented: process utilization - schedule to make the CPU busiest maximum, fairness - all process treated peer, throughput - number of done process in a time maximum Scheduling features
  • Selection function: used to choose which process in ready queue run usually based on priority, resources request, process
  • Desicion mode: used to choose time to choose selection function for scheduling, there are two desicion mode: non-preemptive - when running process will run until end or blocked by I/O request and preemtive - running process can be interrupt to ready state, more cost but more better response time because no case that a process take CPU too long service time is the time process need CPU in a CPU-I/O period, processes have large service time are CPU-bound processes

Scheduling algorimth


 
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