第三讲 线程

中国科学技术大学计算机系 陈香兰

xlanchen@ustc.edu.cnFall 2013

内容提要

为什么要引入线程？什么是线程？BenefitsUser and Kernel ThreadsMultithreading ModelsSolaris 2 Threads

Reading ：汤子瀛，二版， 2.4Operating Systems Concepts ， 7th ， chapter4

为什么要引入线程？

Review 进程考虑一个 Web 服务器

A big process that can do everything itself? For every client, FORK a new process?

创建一个进程是有代价的 Fork a new process to do I/O?

进程间通信是有代价的 If Multi-processors, ?

思路： 引入“分工合作”的概念 在进程内部进一步引入“并发”

可行性：A task = computation/IO units + resourcesComputation/IO units

Are (almost) independent in sense of computation/IOShould share the process’ address space and other resources

So, separate computation/IO and resources

什么是线程？

线程：有时又称为轻量级进程是 CPU 调度的基本单位Thread ID + PC + Register Set + stack是进程中的一个运行实体

同属一个进程的线程之间，共享该进程的 code section, data section, and other resources

Process, a heavyweight processTraditionally, has only one thread & can do only one

task at a timeNow, may contains multiple threads

资源的拥有者还是进程！

Single- & multithreaded process

stack

code

registers

data files

stack

Thread

Single-threaded

code

registers

data files

registers

Thread

Multi-threaded

stack

registers

stack

引入线程概念后的应用举例

网页浏览器显示图片，文字，放音乐，接收数据，…

文档编辑器显示、输入、拼写检查

线程与进程的比较

调度并发性拥有资源系统开销

引入线程的好处

Responsiveness Interactive app.

Resource sharing 1:N

Economy, when Create a new …,

E.g., in Solaris, 30 times slower

Context switch, 5 times slower

Utilization of multiprocessor architectures Threads of the same process may be running in parallel

引入线程，概念上的一些转变

进程基本属性的变化独立性、调度和资源分配的单位

切换的变化在同一进程内，线程的切换不会引起进程的切换；

地址空间上的变化：在同一进程内，各线程共享同一地址空间；

一进程中的线程在另一进程中是不可见的；通信手段上的变化：

同一进程内的线程间的通信主要是基于全局变量进行的；而以同步手段为辅。

线程的实现方式

User Threads Kernel threads

Level User Kernel (OS)

Implementation Thread Lib. Kernel (OS)

Speed of operations Fast Slower

Effect of block OP. Block all No effect

Parallelism no yes

E.X. POSIX Pthreads, Mach c-threads, Solaris threads

WinNT, Solaris, Digital UNIX

User threads VS. Kernel threads

多线程的实现模型

User threads + kernel threads

Three types

N:1(N>1)

1:1

N:M(N>=M)

N:1, 1:1 & N:M

Many-to-One One-to-One Many-to-Many

Manager User level User + kernel User + kernel

Speed of OP Fast Slowest slower

Effect of Block OP Block all Block itself Block itself

Concurrency Only in its process Whole Whole (limited)

Parallel NO YES YES (limited)

E.g. WinNT, OS/2 Solaris, IRIX, Digital UNIX

k Kernel thread

User thread

k Kernel thread

User thread

k

k

k

k Kernel thread

User thread

k k

Solaris 2 Threads

User thread + LWP + kernel thread N(1):1(0):1

Bound VS. unbound user-level thread

LWP pool for a App.

SMPtask3task2task1

CPU

kernel

Kernel thread

User-level thread

LWP

回顾

Benefits User and Kernel Threads Multithreading Models

作业

阅读参考书，请给出线程的定义。讲明出处。阅读参考书，说明线程概念有几种实现方式？什么是用

户态线程，什么是内核态线程？它们之间的对应关系有几种？说明出处。

与进程相比，引入线程主要有哪些好处？

无上机作业