ITFN 2601 Introduction to Operating Systems

1
ITFN 2601Introduction to Operating Systems

• Lecture 3
• Processes, Threads
• IPC

2
Agenda

• Processes
• Threads
• IPC Basics
• Semaphores Mutex
• Monitors Barriers
• Message Passing

3
Why Processes?

• Separation of Tasks
• Pseudo-Parallel Execution
• One Job, One Process

4
Process Fundamentals

• Creation
• Implicit (by operating system daemons)
• Explicit (programmatically, by you)
• Termination
• Voluntary (normal or error exit)
• Involuntary (fatal error or killed by another)

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Process Internals

• States
• Running
• Ready
• Blocking
• Process Control Block
• State
• Registers Memory
• PC, SP, Files

Running
Blocked
Ready
6
Threads?

• Lightweight Processes
• Less Individual Information
• Decomposition of Task
• User vs. Kernel

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User vs. Kernel Threads

• Software Threads
• No System Calls for Swapping
• Process-Blocking System Calls
• Dual-Conflicting Swaps
• Process Level Thread
• Kernel Level Process

• OS Threads
• Kernel Calls to Swap Threads
• Thread-Blocking System Calls
• Dual-Coordinated Swaps
• Kernel Level Thread
• Kernel Level Process

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Hybrid Implementation

• Scheduler Activation
• Thread-aware Kernel for User-threads
• Violates Fundamental Flow-of-Control
• Upcalls
• Pop-Up Threading
• Implicit Thread Creation
• Receives Message and processes them

9
Process Control

• Each Process must maintain its own PCB
• Information that Multiple Processes may use is
  duplicated

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Thread Control

• Each Process has a PCB
• Each Thread has a TCB
• Shared information is Shared, not Duplicated

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IPC Basics

• Race Conditions
• Critical Region
• One at a Time
• Speed Independent
• Non-Critical Regions Immaterial
• Finite Waiting
• Solution?
• Mutual Exclusion

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Basic Mutual Exclusion

• Disable Interrupts (BAD!)
• Only works for one Processor
• User must remember to re-enable interrupts
• Lock Variable (GETTING CLOSER)
• How do you lock the lock?
• Lockstep Alternation (ALMOST)
• Violates 3rd rule of Critical Regions

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Basic Mutual Exclusion (cont.)

• Interested Flags
• Test Set
• Atomic Action
• Read Value to see locked
• Automatically set to locked, just in case
• Sleep Wakeup

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Semaphores

• Tracks number of Sleeps Wakeups
• Down Up
• Historically P V Dutch (Thanks, Dijkstra!)
• Special Cases
• Down from a 0
• Process or Thread Sleeps until a new Wakeup
• Up from 0
• Random Sleeper for this Wakeup is Triggered

15
Binary Semaphores

• Mutex
• Like a Standard Lock
• Atomicity Provides Self-Locking
• Idle Wait
• If Locked, Thread Yields
• Simpler Semaphores
• Fundamental Mutual Exclusion Tool

16
Monitors Barriers

• Monitors
• Compiler Based Mutual Exclusion
• Condition Variables
• Wait
• Signal
• Barriers
• Forced Resynchronization of Processes

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Barriers
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Networked Critical Region?

• How do you enforce Critical Regions across a
  network?
• Message Passing
• Each signal is explicitly given a destination
• Authentication?
• Acknowledgement of Receipt?

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Summary

• Processes Threads allow for pseudo-simultaneous
  execution of tasks
• Coordination between processes how?
• Locking Sleeping
• TSL Command
• Counting Wakeups
• Monitors
• Networked Coordination?
• Message Passing
• Barriers