Threads

• Multiple execution units with a process
• 栈和pc不能共享

Definition

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

• Each thread has its own

  • thread ID
  • program counter
  • register set
  • Stack
• It shares the following with other threads within the same process
• code section
• data section
• the heap (dynamically allocated memory): 通过指针访问，不在cache里比较慢

• open files and signals

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

• Advantages of Threads

• Economy

  • Creating a thread is cheap

  如果已经有了一个线程，那么我们创建新的线程只需要给它分配一个栈。Code，data，heap 都已经在内存里分配好了。

  • Context switching between threads is cheap

  Cache is hot, no need to cache flush.

• Resource Sharing Threads naturally share memory

  不需要 IPC。

  • Having concurrent activities in the same address space is very powerful

• Responsiveness

如在 web server 中，一个线程在等待 I/O，当有请求来时就再分配一个线程去处理。（进程也可以，但是代价更大）

• Scalability

  • multi-core machine

• Drawbacks of Threads

• weak isolation between threads

  如果有一个线程出错，那么整个进程都会出错。 lead to process-based concurrency

• thread间没有隔离，保护措施不够

• Typical challenges of multi-threaded programming

• Deal with data dependency and synchronization

• Dividing activities among threads
• Balancing load among threads
• Split data among threads
• Testing and debugging

User Threads vs. Kernel Threads

如果内核不知道你这个 user thread，完全在 user space 执行，就是 user space-based thread; 如果内核知道你这个 user thread，就是 kernel-based thread。

• Many-to-One Model

• 优点：multi-threading is efficient and low-overhead.

• 缺点：内核只有一个线程，无法发挥 multi-core 的优势；一旦一个线程被阻塞，其他线程也会被阻塞。

• One-to-One Model

把线程的管理变得很简单，管理很简洁，现在 Linux，Windows 都是这种模型。硬件便宜了

缺点：overhead大

• Many-to-Many Model

m to n 线程，折中。缺点是太复杂。

• Two-Level Model

可以选择 many to many 或者 one to one。

1. implementation

• Java Threads / Kernel Threads: In modern JVMs, application threads are mapped to kernel threads

  

Threading Issues

1. Semantics of fork() and exec()

如果一个线程调用了 fork()，可能发生两种情况：只复制调用线程，或者复制所有线程。

Some OSes provide both options. In Linux the first option above is used

因为大部分时候 fork 之后会接 exec，抹掉所有的数据，因此直接复制调用线程就可以了。

2. Signals

• multiple options: signal给调用线程/所有线程/certain线程/指定一个线程接收所有signal

• Linux实现：提供多个接口自己选 dealing with threads and signals is tricky but well understood with many tutorials on the matter and man pages

3. Safe Thread Cancellation

• 可能出现问题：取消了一个正在工作的线程
• two approach
• Asynchronous cancellation：立即终止
  • problem: 可能会导致inconsistent state或synchronization problem，Absolutely terrible bugs lurking in the shadows
• Deferred cancellation: A thread periodically checks whether it should terminate 线程会自己进行周期性检查，如果取消掉不会影响系统的稳定性，就把自己取消掉。
  • problem: the code is cumbersome due to multiple cancellation points
• Invoking thread cancellation requests cancellation, but actual cancellation depends on thread state

Linux Threads

In Linux, a thread is also called a light-weight process (LWP)

The clone() syscall is used to create a thread or a process

TCB 用来存储线程的信息，Linux 并不区分 PCB 和 TCB，都是用 task_struct 来表示。

• single-threaded process vs multi-threaded process.
• 共享内容：通过指针指对应结构体

• PID 如果和 LWP 相同，说明这个进程只有这一个线程。如果不相同，说明进程有多个线程，此时进程的 PID 是主线程的 LWP。

• A process is
• either a single thread + an address space, PID is thread ID

• or multiple threads + an address space, PID is the leading thread ID

• Threads with Process – What is shared

• mm_struct is shared, task_struct, pid, stack and comm are not shared.

• One task in Linux
• Same task_struct (PCB) means same thread
  • One user thread maps to one kernel thread
  • But actually, they are the same thread
• Can be executed in user space: User code, user space stack
• Can be executed in kernel space: Kernel code, kernel space stack

例如我们使用了一个系统调用，线程切换到内核模式，相当于是用户线程对应的内核线程在执行，此时就使用内核空间的栈。

• example

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