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Lecture 2 Processes
Lecture 1 Introduction

Multithreading models

  • Many to One Model
    • Maps many user threads to one kernel thread
    • Allow the application to create any number of threads that can execute concurrently.
    • All thread activity is restricted to the user space
    • However only one thread can access the kernel at a time, so multiple threads are unable to run in parallel on multiprocessors

Multithreading models

  • Many to Many Model
    • Maps many user-level threads to many kernel-level threads
    • Aka the two-level model, minimizes programming effort while reducing the cost and weight of each thread
    • A program can have as many threads as are appropriate without making the process too heavy.
    • This implementation provides a standard interface, a simpler programming model, and optimal performance for each process

Process Scheduling

  • refers to a set of policies and mechanisms supported by operating system that controls the order in which jobs are completed.
  • A scheduler is an operating system program (module) that select the next job to be admitted for execution.
  • In this section we will describe:
    • Scheduling objectives
    • Types of schedulers
    • Various scheduling algorithms

Scheduling Objectives

  • Maximise throughput – service the largest possible number of processes per unit time
  • Minimise overhead – scheduling should minimise the waster resources
  • Balance resource use – should keep the resources of the system busy
  • Maximise interactive users – maximise the number of interactive users receiving acceptable response times
  • Enforce priorities – the scheduling mechanism should favour the higher-priority processes
  • Avoid indefinite postponement – all processes should be treated the same and no process can suffer indefinite postponement

Types of Schedulers

  • Long – term Scheduler
    • Also called job scheduler
    • Selects processes from the queue and loads them into memory for execution
    • It is the change from of state from new to ready
    • Its main objectives is to provide a balanced mix of jobs, such as I/O bound and processor bound
  • Short-term Scheduler
    • Also called the CPU scheduler/dispatcher
    • It is the change of state from ready to running for the process
    • Selects from among the processes that are ready to execute and allocates the CPU to one of them
    • Faster than long term scheduler

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