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Threads

How can we make a process run faster

  • Multiple execution units with a process

Thread 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)
    • open files and signals
  • Concurrency: A multi-threaded process can do multiple things at once
Example

线程执行的代码很有可能不一样。

  • 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。

  • 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

    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，现在 Linux，Windows 都是这种模型。

  • 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。

Thread Libraries

  • In C/C++: pthreads and Win32 threads
  • In C/C++: OpenMP
  • In Java: Java Threads

Pthreads

Specification, not implementation

只要符合接口的定义，就可以叫 pthreads。

Example

OpenMP

Identifies parallel regions – blocks of code that can run in parallel

#pragma omp parallel

Example 
#include <omp.h>
#include <stdio.h>
int main(int argc, char *argv[])
{
    /* sequential code */
    #pragma omp parallel
    {
        printf("I am a parallel region.");
    }
    /* sequential code */
    return 0;
}

#pragma omp parallel for
for (i = 0; i < N; i++)
{
    c[i] = a[i] + b[i];
}

使用之后编译器会为我们切分出若干个并行块，创造出对应的线程，最后使用 join 把线程合并。

Java Threads

  • Java threads may be created by:
    • Extending Thread class
    • Implementing the Runnable interface
Example 
class MyThread extends Thread {
    public void run() {
        . . .
    }
}
    MyThread t = new MyThread();
    public interface Runnable {
    public abstract void run();
}

Threading Issues

线程的加入让进程的操作变得更复杂。

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。

Signals

信号是给一个线程，给所有线程，给一个固定的线程还是用一个特定的线程来处理信号？

Most UNIX versions: a thread can say which signals it accepts and which signals it doesn’t accept.

On Linux: dealing with threads and signals is tricky but well understood with many tutorials on the matter and man pages

Safe Thread Cancellation

把一个线程的工作取消掉，如何保证取消后不影响系统的稳定性。

  • Asynchronous cancellation

    立即终止。

  • Deferred cancellation

    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。

Invoking thread cancellation requests cancellation, but actual cancellation depends on thread state.

带来很多问题：比如一个线程正在写变量，值还没有同步到内存或者 cache，这个 bug 很难被复现。

In Java, the Thread.stop() method is deprecated, and so cancellation has to be deferred.

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.

Note

clone 有一个参数 CLONE_VM，如果不设置那么类似于 fork，每个线程都有自己的内存空间；如果设置了那么线程跑在同一地址空间上。

注意线程能访问到其他线程的栈，也能读写，只是正常使用情况下是用的自己栈。？？？

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。

Example

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

可以看到 mm_struct （与内存管理相关的信息，如页表，vm_struct）是共享的，task_struct, pid, stack, comm 是不共享的。

  • One task in Linux
    • Same task_struct (PCB) means same thread（一个 PCB 指的是一个线程）
      • Also viewed as 1:1 mapping
      • 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。

Takeaway

Takeaway

  • Thread is the basic execution unit
    • Has its own registers, pc, stack
  • Thread vs Process
    • What is shared and what is not
  • Pros and cons of thread

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