Lecture 12: Processes and threads

Preparation

• Read OSPP §4, Concurrency and Threads.
• Review the Minimal Intel Architecture Boot Loader from the first lecture. Focus on Processor Discovery and Initialization this time.

Processes and threads

• # of tasks > # of CPUs
• programming abstraction: virtual CPUs
  • process API: fork/execve/exit/kill/wait/getpid
  • thread API: pthread etc.
• definitions I
  • process: address space, hosting one or more threads
  • thread: basic execution and scheduling unit
• definitions II
  • threads and processes are both tasks
  • threads: tasks with shared address space
  • example: Linux clone syscall
• questions
  • T/F - writing to memory location x in process A will change the value at the same memory location x in process B
    • how about threads?
  • T/F - thread A creates a file descriptor; thread B can access it
    • how about processes?

A thread implementation

• virtual CPU state
  • registers
    • SP: implies a separate stack
    • IP: for resume
  • scheduling state: running, runnable, killed, etc.
• exercise
  • new: git clone -b uthread https://github.com/xiw/xv6.git
  • if you have cloned xv6: git pull; git checkout -b uthread
  • uthread.c
  • uthread_switch.S
• cooperative scheduling: threads give up control by calling yield
  • useful?
  • downsides? how would you improve it?