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Os

Operating Systems#

Table of Contents

Process Scheduling#

Preemptive Scheduling#

  • is a kind of process scheduling
  • which have facility to interrupt an on going process to proceed with other high priority process (and the resource, mainly CPU, is allocated to the new process)
  • in such case, the running process moves to the ready state & the process in ready state moves to the running state

Non-Preemptive Scheduling#

Process Synchronization#

The shared resources can be used by all the processes but the processes should make sure that at a particular time, only one (or, a finite number of) process should be using that shared resource. This is called process synchronization.

Methods used for Process Synchronization#

  • Semaphore
  • Mutex (Mutual Exclusive object)

Semaphore AND Mutex#

  • Are kernel resources and provides synchronization service
  • Also known as synchronization primitives
  • both serves the same purpose but the mechanism, implementation, and use-cases are different

Semaphore#

  • is a Signalling Mechanism
Types#
  • Binary Semaphore
  • Counting Semaphore

Mutex#

  • an object
  • is a Locking Mechanism
  • Ownership involved: The thread/process which have mutex and accessing data can only release mutex
Types#
  • Recursive mutex
Pros#
Cons#

Mutex VS Semaphore#

  • Mutex uses a locking mechanism i.e. if a process wants to use a resource then it locks the resource, uses it and then release it. But on the other hand, semaphore uses a signalling mechanism where wait() and signal() methods are used to show if a process is releasing a resource or taking a resource.
  • A mutex is an object but semaphore is an integer variable.
  • In semaphore, we have wait() and signal() functions. But in mutex, there is no such function.
  • A mutex object allows multiple process threads to access a single shared resource but only one at a time. On the other hand, semaphore allows multiple process threads to access the finite instance of the resource until available.
  • In mutex, the lock can be acquired and released by the same process at a time. But the value of the semaphore variable can be modified by any process that needs some resource but only one process can change the value at a time.

Why 2 different synchronization premitives?#

Producer-Consumer Problem#
Critical Section#

In conncurrent programming * A group of instructions/statements or region of code * that region to be executed atomically (i.e. all or nothing)

A simple solution to critical section 

1
2
3
acquireLock()
executeCriticalSection()
releaseLock()

Preemption#

Inter Process Communication#

Concurrency vs Parallelism#

IO bound, Memory bound, Cache bound, CPU bound operations#

speed of IO bound \lt Memory bound \lt Cache bound \lt CPU bound operations

Memory Management#

Virtual Memory#

  • Virtual memory is a memory manage scheme
    • as per which the secondary memory could act as a part of the primary memory
  • implemented using demand paging or demand segmentation
  • other insights
    • a program does not refers to actual physical (primary) or machine (secondary) memory address
    • rather it refers to the logical addresses
    • logical addresses are translated into physical addresses during execution of the programs
    • thus a process could be stored & executed in non-contiguos chunks

Paging#

Demand Paging#

Page Fault#

Swapping#

  • Swapping a process out: marking its all pages as free/unloaded from memory
  • Swapping a process in: bringing in all the pages of a process from secondary memory to primary memory

Thrasing#

  • When a process is busy swapping pages in and out then it is called thrasing