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Threads. CSCE 351: Operating System Kernels Witawas Srisa-an Chapter 4-5. Threads The Thread Model (1). (a) Three processes each with one thread (b) One process with three threads. The Thread Model (2). Items shared by all threads in a process Items private to each thread.

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threads

Threads

CSCE 351: Operating System Kernels

Witawas Srisa-an

Chapter 4-5

threads the thread model 1
ThreadsThe Thread Model (1)

(a) Three processes each with one thread

(b) One process with three threads

the thread model 2
The Thread Model (2)
  • Items shared by all threads in a process
  • Items private to each thread
the thread model 3
The Thread Model (3)

Each thread has its own stack

thread usage 1
Thread Usage (1)

A word processor with three threads

thread usage 2
Thread Usage (2)

A multithreaded Web server

thread usage 3
Thread Usage (3)
  • Rough outline of code for previous slide

(a) Dispatcher thread

(b) Worker thread

thread usage 4
Thread Usage (4)

Three ways to construct a server

implementing threads in user space
Implementing Threads in User Space

A user-level threads package

implementing threads in the kernel
Implementing Threads in the Kernel

A threads package managed by the kernel

hybrid implementations
Hybrid Implementations

Multiplexing user-level threads onto kernel- level threads

scheduler activations
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)

pop up threads
Pop-Up Threads
  • Creation of a new thread when message arrives

(a) before message arrives

(b) after message arrives

making single threaded code multithreaded 1
Making Single-Threaded Code Multithreaded (1)

Conflicts between threads over the use of a global variable

making single threaded code multithreaded 2
Making Single-Threaded Code Multithreaded (2)

Threads can have private global variables

interprocess communication race conditions
Interprocess CommunicationRace Conditions

Two processes want to access shared memory at same time

critical regions 1
Critical Regions (1)
  • 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
critical regions 2
Critical Regions (2)

Mutual exclusion using critical regions

mutual exclusion with busy waiting 1
Mutual Exclusion with Busy Waiting (1)

Proposed solution to critical region problem

(a) Process 0. (b) Process 1.

No process running outside its critical region may block another process

mutual exclusion with busy waiting 3
Mutual Exclusion with Busy Waiting (3)

Entering and leaving a critical region using the

TSL instruction

sleep and wakeup
Sleep and Wakeup

Producer-consumer problem with fatal race condition

semaphores
Semaphores
  • A variable type
    • 0 or any positive values (counting)
    • 0 or 1 (binary)
  • Support two operations
    • down (p)
      • if value > 0 then decrement
      • if value = 0 then suspend process without completing the down
      • indivisible atomic action
    • up (v)
      • increment the semaphore value and if there are processes sleeping on the semaphore, wake one of them up
semaphore
Semaphore

arena

queue

semaphores1
Semaphores

The producer-consumer problem using semaphores

mutexes
Mutexes

Implementation of mutex_lock and mutex_unlock

example program
Example Program
  • suspend.c and wake.c
monitors
Monitors
  • Language construct
    • higher level synchronization primitive
  • Only one process can be active in a monitor at any instant
  • Use condition variables to block processes
    • wait operation
    • signal operation
monitors 1
Monitors (1)

Example of a monitor

monitors 2
Monitors (2)
  • Outline of producer-consumer problem with monitors
    • only one process can be active in a monitor at one time
    • buffer has N slots
the readers and writers problem
The Readers and Writers Problem

A solution to the readers and writers problem

the sleeping barber problem 2
The Sleeping Barber Problem (2)

Solution to sleeping barber problem.

scheduling introduction to scheduling 1
SchedulingIntroduction to Scheduling (1)
  • Bursts of CPU usage alternate with periods of I/O wait
    • a CPU-bound process
    • an I/O bound process
introduction to scheduling 2
Introduction to Scheduling (2)

Scheduling Algorithm Goals

scheduling in batch systems 1
Scheduling in Batch Systems (1)

An example of shortest job first scheduling

(8 + 12 + 16 + 20) / 4

= 14 units

(4 + 8 + 12 + 20) / 4

= 11 units

scheduling in batch systems 2
Scheduling in Batch Systems (2)

Three level scheduling

scheduling in interactive systems 1
Scheduling in Interactive Systems (1)
  • Round Robin Scheduling
    • list of runnable processes
    • list of runnable processes after B uses up its quantum
scheduling in interactive systems 2
Scheduling in Interactive Systems (2)

A scheduling algorithm with four priority classes

scheduling in real time systems 1
Scheduling in Real-Time Systems (1)

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
scheduling in real time systems 2
Scheduling in Real-Time Systems (2)
  • Example
    • four periodic events: 100, 200, 400, 600 ms
    • required CPU times: 25, 40, 100, 120 ms

Is this system schedulable?

thread scheduling 1
Thread Scheduling (1)

Possible scheduling of user-level threads

  • 50-msec process quantum
  • threads run 5 msec/CPU burst
thread scheduling 2
Thread Scheduling (2)

Possible scheduling of kernel-level threads

  • 50-msec process quantum
  • threads run 5 msec/CPU burst
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