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Thread vs Process

  • Thread: a lightweight process that can run concurrently with other threads within the same process.
  • Process: a program in execution, which can contain multiple threads.

Thread Control Block (TCB)

Shared Info:

  • Memory
  • IO and Files

Per-Thread info:

  • state
  • PC, registers

  • execution stack

Thread State

stateDiagram-v2
    Ready --> Running: schedule thread
    Running --> Ready: timer interrupt, yield
    Running --> Waiting: block for resource<br>(I/O, page fault, etc.)
    Waiting --> Ready: resource free,<br>I/O completion interrupt
    [*] --> Ready: create thread
    Running --> [*]: thread exit

Thread Queue and Scheduling

  • ready queue: threads that are ready to run, waiting for CPU time
  • waiting queue: threads that are waiting for some event to occur (eg: I/O completion)
  • current thread: the thread that is currently running on the CPU
  • scheduler: the part of the OS that decides which thread to run next
  • dispatcher: the part of the OS that switches between threads (context switch)
  • context switch: the process of saving the state of the current thread and loading the state of the next thread to run -
  • preemptive scheduling: the scheduler can interrupt a running thread to give CPU time to another thread (eg: timer interrupt)
  • non-preemptive scheduling: the scheduler cannot interrupt a running thread, the thread must yield the CPU voluntarily
    • yield() - voluntarily give up the CPU, context switch to another thread. @returns when another thread called yield()
    • sleep() - voluntarily give up the CPU for a specified amount of time