Processes and Threads

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Processes and Threads

• Chapter 2

2.1 Processes 2.2 Threads 2.3 Interprocess
communication 2.4 Classical IPC problems 2.5
Scheduling
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Agenda

• 2.1 Processes
• 2.2 Threads
• 2.3 Interprocess communication
• 2.4 Classical IPC problems
• 2.5 Scheduling

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Process

• The most central concept in any OS
• An abstraction of a running program
• Modern Computers
• Can do more than one thing at the same time
• Can run user programs
• Can read disk and work with user terminal
• In a multiprogramming system
• More than one user program can be scheduled
• Each may run for tens of msecs.

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ProcessesThe Process Model

• Multiprogramming of four programs
• Conceptual model of 4 independent, sequential
  processes
• Only one program active at any instant

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Process Creation

• Principal events that cause process creation
• System initialization
• Execution of a process creation system
• User request to create a new process
• Initiation of a batch job

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Process Termination

• Conditions which terminate processes
• Normal exit (voluntary)
• Error exit (voluntary)
• Fatal error (involuntary)
• Killed by another process (involuntary)

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Process Hierarchies

• Parent creates a child process, child processes
  can create its own process
• Forms a hierarchy
• UNIX calls this a "process group"
• Windows has no concept of process hierarchy
• all processes are created equal

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Process States (1)

• Possible process states
• running
• blocked
• ready
• Transitions between states shown

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Process States (2)

• Lowest layer of process-structured OS
• handles interrupts, scheduling
• Above that layer are sequential processes

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Implementation of Processes (1)

• Fields of a process table entry

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Implementation of Processes (2)

• Skeleton of what lowest level of OS does when an
  interrupt occurs

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Agenda

• 2.1 Processes
• 2.2 Threads
• 2.3 Interprocess communication
• 2.4 Classical IPC problems
• 2.5 Scheduling

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Threads

• In traditional OS, each process has
• An address space
• A single tread of control
• In modern OS, each process may have
• Multiple threads of control
• Same address spare
• Running in quasi-parallel
• Thread or a Lightweight Process has
• a program counter, registers, stack

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ThreadsThe Thread Model (1)

• (a) Three processes each with one thread
• (b) One process with three threads

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The Thread Model (2)

• Items shared by all threads in a process
• Items private to each thread

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The Thread Model (3)

• Each thread has its own stack

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Thread Usage (1)

• A word processor with three threads

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Thread Usage (2)

• A multithreaded Web server

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Thread Usage (3)

• Rough outline of code for previous slide
• (a) Dispatcher thread
• (b) Worker thread

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Thread Usage (4)

• Three ways to construct a server

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Implementing Threads in User Space

• A user-level threads package

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Implementing Threads in the Kernel

• A threads package managed by the kernel

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Hybrid Implementations

• Multiplexing user-level threads onto kernel-
  level threads

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Scheduler Activations

• Goal mimic functionality of kernel threads
• gain performance of user space threads
• Avoids unnecessary user/kernel transitions
• Kernel assigns virtual processors to each process
• lets runtime system allocate threads to
  processors
• Problem
• Fundamental reliance on kernel (lower layer)
• Calling procedures in user space (higher layer)
• Called upcall

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Pop-Up Threads

• Creation of a new thread when message arrives
• (a) before message arrives
• (b) after message arrives

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Making Single-Threaded Code Multithreaded (1)

• Conflicts between threads over the use of a
  global variable

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Making Single-Threaded Code Multithreaded (2)

• Threads can have private global variables

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Agenda

• 2.1 Processes
• 2.2 Threads
• 2.3 Interprocess communication
• 2.4 Classical IPC problems
• 2.5 Scheduling

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Interprocess Communication

• Processes may need to communicate
• E.g., output of one goes to input of the other
  one
• Issues
• How to pass information
• different address spaces
• Making critical activities
• Two process try to grab the last 1MB of memory
• Proper sequencing
• One process is producing data for the other one

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Interprocess CommunicationRace Conditions

• Two processes want to access shared memory at
  same time

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Critical Regions (1)

• Critical Regions or Critical Sections
• The part of the program where shared memory is
  accessed.
• Four conditions to provide mutual exclusion
• No two processes simultaneously in critical
  region
• No assumptions made about speeds or numbers of
  CPUs
• No process running outside its critical region
  may block another process
• No process must wait forever to enter its
  critical region

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Critical Regions (2)

• Mutual exclusion using critical regions

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Mutual Exclusion with Busy Waiting (0)

• Mutual Exclusion
• Only one process can be in the critical section.
• Disabling Interrupts (HW Solution)
• Dangerous May lead to the end of the system
• Lock Variables (SW Solution)
• Fatal Flow Similar to Spooler Directory
• Strict Alternation (SW Solution)
• Busy Waiting, Violating Condition 3
• Petersons Solution (SW Solution)
• Busy Waiting
• TSL Instruction (HW/SW Solution)
• Busy Waiting, but Faster

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Mutual Exclusion with Busy Waiting (1)Strict
Alternation (SW Solution)

• Proposed solution to critical region problem
• (a) Process 0. (b) Process 1.

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Mutual Exclusion with Busy Waiting (2)Petersons
Solution (SW Solution)
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Mutual Exclusion with Busy Waiting (3)Using TSL
Instruction (HW/SW Solution)

• Entering and leaving a critical region using the
• TSL instruction

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Mutual Exclusion with Sleep and Wakeup
(0)Producer-Consumer Problem

• Priority Inversion Problem
• busy waiting with priority!
• Solution with Fatal Race Condition
• Waking up a consumer that is not asleep yet!
• Semaphore
• An integer that counts the number of wake-up
  calls.
• Mutexes
• Binary semaphores, good for mutual exclusion.
• Monitors
• Easier to program (Synchronized in Java).
• Message Passing
• N messages used for communication coordination.
• Barriers
• Synchronization of N processes/threads.

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Mutual Exclusion with Sleep and Wakeup (1)Fatal
Race Condition
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Mutual Exclusion with Sleep and Wakeup (2) Using
Semaphores
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Mutual Exclusion with Sleep and Wakeup (3) Using
Mutexes

• Implementation of mutex_lock and mutex_unlock

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Mutual Exclusion with Sleep and Wakeup (4) Using
Monitors (1)

• Example of a monitor

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Mutual Exclusion with Sleep and Wakeup (4) Using
Monitors (2)

• Outline of producer-consumer problem with
  monitors
• only one monitor procedure active at one time
• buffer has N slots

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Mutual Exclusion with Sleep and Wakeup (4) Using
Monitors (3)

• Solution to producer-consumer problem in Java
  (part 1)

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Mutual Exclusion with Sleep and Wakeup (4) Using
Monitors (4)

• Solution to producer-consumer problem in Java
  (part 2)

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Mutual Exclusion with Sleep and Wakeup (5)
Message Passing

• The producer-consumer problem with N messages

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Barriers

• Use of a barrier
• processes approaching a barrier
• all processes but one blocked at barrier
• last process arrives, all are let through

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Agenda

• 2.1 Processes
• 2.2 Threads
• 2.3 Interprocess communication
• 2.4 Classical IPC problems
• 2.5 Scheduling

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Dining Philosophers (1)

• Philosophers eat/think
• Eating needs 2 forks
• Pick one fork at a time
• How to prevent deadlock

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Dining Philosophers (2)

• A nonsolution to the dining philosophers problem

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Dining Philosophers (3)

• Solution to dining philosophers problem (part 1)

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Dining Philosophers (4)

• Solution to dining philosophers problem (part 2)

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The Readers and Writers Problem

• A solution to the readers and writers problem

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The Sleeping Barber Problem (1)
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The Sleeping Barber Problem (2)
Solution to sleeping barber problem.
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Agenda

• 2.1 Processes
• 2.2 Threads
• 2.3 Interprocess communication
• 2.4 Classical IPC problems
• 2.5 Scheduling

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Scheduling

• Scheduler
• The part of the OS that make the choice of which
  process to run next.
• Scheduling Algorithm
• The algorithm used for scheduling

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SchedulingIntroduction to Scheduling (1)

• Bursts of CPU usage alternate with periods of I/O
  wait
• a CPU-bound process spends most of its time on
  computing
• an I/O bound process spends most of its time
  waiting for I/O

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Introduction to Scheduling (2)System Algorithm
Goals
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Scheduling Algorithm Goals

• Throughput
• The number of jobs per hour that the system
  completes.
• Turnaround time
• The statically average time from the moment that
  a batch job is submitted until the moment it is
  completed.

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Scheduling in Batch Systems (1)

• First-Come First-Served
• Shortest Job First (non-preemptive)
• An example of shortest job first scheduling
• Shortest Remaining Time First (preemptive)
• Three-Level Scheduling

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Scheduling in Batch Systems (2)

• Three level scheduling

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Scheduling in Interactive Systems (1)

• Round Robin Scheduling
• list of runnable processes
• list of runnable processes after B uses up its
  quantum
• Priority Scheduling
• Multiple Queues
• Shortest Process Next
• Guaranteed Scheduling
• Lottery Scheduling
• Fair-Share Scheduling

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Scheduling in Interactive Systems (2)

• A scheduling algorithm with four priority classes

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Scheduling in Real-Time Systems

• Schedulable real-time system
• Given
• m periodic events
• event i occurs within period Pi and requires Ci
  seconds
• Then the load can only be handled if

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Policy versus Mechanism

• Separate what is allowed to be done with how it
  is done
• a process knows which of its children threads are
  important and need priority
• Scheduling algorithm parameterized
• mechanism in the kernel
• Parameters filled in by user processes
• policy set by user process

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Thread Scheduling (1)

• Possible scheduling of user-level threads
• 50-msec process quantum
• threads run 5 msec/CPU burst

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Thread Scheduling (2)

• Possible scheduling of kernel-level threads
• 50-msec process quantum
• threads run 5 msec/CPU burst