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Chapter 2.5 : Threads

• Process concept • Process scheduling • Interprocess communication • Deadlocks• Threads

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These lecture notes have been adapted from• How to program with threads

An introduction to multithreaded programming

By Bil Lewis and Daniel J. Bergand

• Tanenbaum slides

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Processes & Threads• Processes and threads are related concepts• A process is a kernel-level entity

– Process structure can only be accessed through system calls

• A thread (or a lightweight process) is a user-level entity– The thread structure is in user space – It is accessed directly with the thread library

calls, which are just normal user-level functions (threads do not use system calls)

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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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Process and Thread Data Structures

Kernel Space

User Space

PCB

Code Data Stack

TCB1 TCB2 TCB3

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Characteristics of Threads

• The TCB (thread control block) consist of

– program counter

– register set

– stack space

Thus the TCB is a reduced PCB

• A traditional process is equal to a task with one thread

• All threads in a process share the state of that process

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Characteristics of Threads (Cont.)

• They reside in the exact same memory space (user

memory), see the same code and data

• When one thread alters a process variable (say, the

working directory), all the others will see the

change when they next access it

• If one thread opens a file to read it, all the other

threads can also read from it.

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Characteristics of Threads (Cont.)

• Because no system calls are involved, threads are

fast

• There are no kernel structures affected by the

existence of threads in a program, so no kernel

resources are consumed -- threads are cheap

• The kernel doesn't even know that threads exist

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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

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Threads of a Task

Threads

Code segmentData segment

Program Counter

Threads

Task

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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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Single vs. Multiple Threads of Execution

Start

End

Edit Document

Print Document

Single Thread Multiple Threads

End

Edit Document

Start

Print Document

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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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Some Benefits of Writing Multithreaded Programs:

Performance gains from multiprocessing hardware (parallelism)

Increased application throughput Increased application responsiveness Enhanced process-to-process

communications

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Parallellism

• Different threads can run on different processors simultaneously with no special input from the user and no effort on the part of the programmer

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Throughput

• When a traditional, single-threaded program requests a service from the operating system, it must wait for that service to complete, often leaving the CPU idle

• Multithreading provides progress even though one or more threads wait for an event as long as other threads are active

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Responsiveness

• Blocking one part of a process need not block the whole process. Single-threaded applications that do something lengthy when a button is pressed typically display a "please wait" cursor and freeze while the operation is in progress

• If such applications were multithreaded, long operations could be done by independent threads, allowing the application to remain active and making the application more responsive to the

user

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Communications

• An application that uses multiple processes to accomplish its tasks can be replaced by an application that uses multiple threads to accomplish those same tasks

• Processes-to-process communication through traditional IPC (interprocess communications) facilities (e.g., pipes or sockets)

• The threaded application can use the inherently shared memory of the process