Lecture 6 - Thread

• A thread is a basic unit of execution within a process

线程是进程内的基本执行单元，进程是资源分配和隔离[allocation and protection]的基本单元

• A Process contains at least one thread, and can contain multiple threads

Revisit IMPORTANT!!

Advantages of Threads

Economy:

• Creating a thread is cheap: Much cheaper than creating a process -- Code, data and heap are already in memory
• Context-switching between threads is cheap: Much cheaper than between processes -- No cache flush

Resource Sharing:

• Threads naturally share memory

1. With processes you have to use possibly complicated IPC (e.g., Shared Memory Segments)
2. IPC is not needed

NOTE: Threads DO NOT share the following:

1. Registers
2. Stack
3. Program Counter
4. thread ID

It shares the following with other threads within the same process

1. code section
2. data section
3. the heap (dynamically allocated memory)
4. open files and signals

• Concurrency: A multi-threaded process can do multiple things at once

• Having concurrent activities in the same address space is very powerful -- But fraught with danger

Responsiveness

• A program that has concurrent activities is more responsive

1. While one thread blocks waiting for some event, another can do something
2. e.g. Spawn a thread to answer a client request in a client-server

• This is true of processes as well, but with threads we have better sharing and economy

Drawbacks of Threads

• 隔离差

Models of Threading

• Linux : One-to-One

Many-to-One Model

One-to-One Model

耗资源

• Removes both drawbacks of the Many-to-One Model
• Creating a new threads requires work by the kernel

1. Not as fast as in the Many-to-One Model

• Example: Linux Windows Solaris 9 and later

Many-to-Many Model

Two-Level Model

Threading Issues

1. fork() and exec()

2. fork() fork which thread or all threads?

   Linux will fork the caller thread only

3. exec() exec which thread or all threads?

   Linux : all threads are “wiped out” anyway

Safe Thread Cancelation

Asynchronous cancellation

• One thread terminates another immediately

Deferred cancellation [Default]

• A thread periodically checks whether it should terminate

Thread Scheduling

process-contention scope (PCS)

• 每个进程分到时间片一样，然后进程内部再对线程进行调度。

system-contention scope (SCS)

• 所有线程进行调度。

Linux Thread

• In Linux, a thread is also called a light-weight process (LWP)
• The clone() syscall is used to create a thread or a process

Thread Control Block -- Task Struct [TCB & PCB]

• When the FIRST thread is created, its Thread ID is the same as the Process ID -- Leading Thread
• struct list_head thread_group
• All threads in the same process are linked together

• User Space Stack 大小不固定

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最后更新: 2025年1月6日 21:18:56
创建日期: 2024年12月27日 21:05:43