Threads

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Why thread?

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. (线程是执行单元。)
  • 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.

• Advantages of Threads
  • economy:
    • creating a thread is cheap(much cheaper than creating a process since code, data and heap are already in memory)
    • context-switching between threads is cheap
      • 换进程 switch_mm + cpu_switch_to 换进程不需要换 memory
      • no cache flush
  • Resource sharing:
    • threads naturally share memory
      • with processes you hae to use possibly complicated IPC
    • Having concurrent activities in the same address space is very powerful.
  • Responsiveness
    • A program that has concurrent activities is more responsive.
  • Scalability
• Drawbacks of Threads
  • weak isolation between threads: if one thread fails, then he process fails.
  • may be more memory-constrained than processes.
  • do not benefit from memory protection.
• 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

• Many-to-One Model
  • advantage: multi-threading is efficient and low-overhead (no syscalls to the kernel)
  • 如果一个线程卡着，其它就都卡着了(一个 thread syscall 了，其它 thread 就不能 syscall.)
  • cannot take advantage of a multi-core architecture.
• One-to-One Model
  • overhead. 开销大
  • Linux and Windows 用这个
• Many-to-Many Model
• Two level Model

Threading Issues

Semantics of fork() and exec()

如果一个 thread 调用了 fork() ，那么我们要复制调用的线程还是复制所有的线程呢？

• Some OSes provide both options
• In Linux the first option above is used
  • If one calls exec() after fork(), all threads are wiped out anyway.

Signals

• Multiple options:
  • deleiver the signal to the thread to which the signal applies
  • deliver the signal to every thread in the process
  • deliver the signal to certain threads in the process
  • assign a specific thread to receive all signals
• 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，但是不影响我们的系统(有可能取消的时候它在处理 shared data 之类的)

• Asynchronous cancellation
• Deferred cancellation (thread 觉得自己能够安全地被关掉时，就把自己取消掉。)

Default type is deferred. (Cancellation only occurs when thread reaches cancellation point)

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_strcut 来表示。
• 如果 PID 和 LWP 相同，说明这个进程只有一个线程，否则说明线程有多个进程，此时进程的 PID 是主进程的 LWP.
• A process is
  • either a signal thread + an address space
  • or multiple threads + an address space.
• mm_struct (与内存管理相关的信息如页表，heap)是共享的，task_struct, pid, stack 是不共享的。(stack 4K 对齐)

• 用 thread_group 链表把 thread 串起来。

• One task in Linux

  • Same task_struct(PCB) means same thread
    • Also viewed as 1:1 mapping
    • One user thread maps to one kernel thread
    • But actually, they are the same thread.(kenerl space 线程可以理解为 user space 的小号)
  • Can be executed in user space(user code, user space stack)
  • Can be executed in kernel space(kenerl code 和 user code 不一样，其实就是执行 syscall, kernel space stack, kernel space 的栈固定 16K,user space 的栈大小不固定)
• 调完 syscall 之后要求 kernel stack 是空。

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最后更新: 2024年10月22日 15:51:32
创建日期: 2024年10月22日 15:51:32