Processes 4

Overview

• Multi-level feedback queues
• Scheduling in Window 7 + illustration Scheduling in
• Linux (implementation in labs) Scheduling related processes/threads

Multi-level Feedback Queues

• Different scheduling algorithms can be used for the individual queues (e.g., round robin, SJF, FCFS)

• Feedback queues allow priorities to change dynamically[动态的], i.e., jobs can move between queues:

  • Move to lower priority queue if too much CPU time is used (prioritise I/O and interactive processes)
  • Move to higher priority queue to prevent starvation and avoid inversion[反转] of control

• Defining characteristics of feedback queues include:

  • The number of queues
  • The scheduling algorithms used for the individual queues
  • Migration policy[迁移政策] between queues
  • Initial access to the queues
    Feedback queues are highly configurable[可配置] and offer significant flexibility[灵活性，适应性]

Processes 3

Overview

1 Threads vs. processes
2 Different thread implementations
3 POSIX Threads (PThreads)

Threads

Threads from an OS Perspective[观点]

• A process consists of two fundamental units
  • Resources: all related resources are grouped together
    • A logical address space containing the process image (program, data, heap, stack)
    • Files, I/O devices, I/O channels, . . .
  • Execution trace[执行追踪], i.e., an entity that gets executed
• A process can share its resources between multiple execution traces, i.e., multiple threads running in the same resource environment
  

Processes 2

Overview

• Introduction to process scheduling
• Types of process schedulers
• Evaluation criteria for scheduling algorithms
• Typical process scheduling algorithms

Process Scheduling

Context

• The OS is responsible for managing and scheduling processes
  • Decide when to admit processes to the system (new → ready)
  • Decide which process to run next (ready → run)
  • Decide when and which processes to interrupt (running → ready)
• It relies on the scheduler (dispatcher) to decide which process to run next, which uses a scheduling algorithm to do so
• The type of algorithm used by the scheduler is influenced by the type of operating system (e.g., real time vs. batch)

Processes 1

Overview

• Introduction to processes and their implementation
• Process states and state transitions
• System calls for process management

Processes and Implementation

Definition: “a process is a running instance of a program”
进程是程序的运行实例

• A process is registered with the OS using its “control structures”: i.e. an entry in the OS’s process table to a process control blocks (PCB)
• The process control block contains all information necessary to manage the process and is necessary for context switching in multi-programmed systems
• A process’ memory image contains:
  • The program code (could be shared between multiple processes running the same code)
  • A data segment, stack and heap
• Every process has its own logical address space, in which the stack and heap are placed at opposite[相反的] sides to allow them to grow

Introduction 2

Overview

• CPU design
• Address spaces, interruts
• OS structures/implementation

CPU design

• A CPU basic cycle consist of fetch[取], decode, execute
• Every CPU has his own instruction set
• CPU has a set of registers
• Registers are used to store data and for special functions (e.g. program counter, program status word – mode bit)
• The compiler/programmer decides what to keep in the registers
• Context switching[上下文切换] must save and restore the CPU’s internal state, including its registers

Introduction 1

Overview

• “Defining“ operating systems
• What is multi-programming
• Kernel-user mode

Defining Operating Systems

• In the early days, programmers had to deal directly with hardware

  • Real computer hardware is urgly
  • Hardware is extremely difficult to program

• An operating system is a layer[层] of indirection[间接] on top of the hardware:

  • It provide abstractions for application programs
  • it provide a cleaner and easier interface to the hardware