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[UCSD CSE120]分时系统-timesharing

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本文是我在上UCSD的 CSE 120: Principles of Operating Systems (Winter 2020) 整理的笔记，第二课主要介绍了分时系统里时间分配的概念，以及内核 (kernel) 和用户 (user) 层面上实现thread的区别以及优缺点。

  1. Definition

    • Multiple processes share single CPU resources
    • Conceptually, each process makes progress over time
    • Practically, each perioadcally get quantum of CPU time
      • quantum : a basic time unit for CPU to allocate for a cycle
    • Illusion of parallel progress by rapidly switching CPU

  2. Implementation

    • Kernel keeps track of progress of each process
    • Divided the states (progress) of each process:
      • Running: actually running (making progress), using CPU
      • Ready: able to make progress, but not using CPU
      • Blocked: unable to make progress (waiting other resources like memory, I/O etc), cannot use CPU
    • Kernel selects a ready process and let it use CPU

  3. Process State Diagram

    • Dispatch: allocated the CPU to a process
    • Preempt: take away CPU from process
    • Sleep: process gives up CPU to wait for event
    • Wakeup: event occurred, make process ready

  4. Kernel

    • A seperate memory space that store kernel code that support user processes to run
      • systems calls: fork(), exit(), read(), write(), yield(),…
      • management: context switching, scheduling,…
    • Keep track of state of each process
      • each process has a unique ID
    • Store other info needed
      • areas of memory being used
      • contents of CPU contexts
      • other…
    • runs as an extension of current process
      • when system call (process give up control to kernel voluntarily)
      • hardware interrupt (preemption)
        • timer

    • Has text, data and multiple stack (each for each process/thread)

      • even if two process share the same code, use seperate memory(text, data, stack) to store state of each process

  5. Threads

    • It’s a single sequential path of execution
    • Abstraction: independent of memory(may have different implementation like user-level and kernel-level)
    • A thread is a part of a process
      • Lives in the memory of a process (share global variable)
      • Multiple threads may exist in a process
    • To the user: unit of parallelism
    • To the kernel: unit of schedulability

  6. User-level threads vs Kernel-level threads

    • user-level thread:

      • Implement stacks for different threads in user space (actually share the stack in kernel)
      • Pros:
        • Threads call and management in user level
        • Efficient: no need to trapped into kernel (which is heavy)
        • No need for kenerl support of threads

      • Cons:

        • no true parallelism (kernel see no threads but process)

        • mulitple CPU cannot let multiple threads in one process run at the same time

    • kernel-level thread:

      • Implement stacks for different threads in kernel space
      • Pros:
        • can achieve true prallelism

      • Cons:

        • overhead: thread switch requires kernel call

    • Distinguish:

      • Where is the thread abstraction supported: kernel code or user code
      • Where is the thread running: user space or kernel space