Lecture: threads & switching

preparation

• read OSPP §4, Concurrency and Threads

processes and threads

• locks recap
  • mutual exclusion
  • boils down to xchg on x86
  • safety? progress? fairness?
  • performance? scalability?
• threads and processes
  • review CSE 333 on processes/threads
  • problem: # 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
  • generalization: namespaces & containers
• 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

• thread = stack + virtual CPU registers
  • registers
    • SP: point to a separate stack
    • PC: where to resume execution
    • how/where are other registers saved
  • scheduling state: running, runnable, killed, etc.
• cooperative scheduling: threads give up control by calling yield
  • useful?
  • downsides? how would you improve it?
• example: uthread implementation
  • what’s needed for switching
  • save
    • do we need to save the PC
    • save current thread’s registers on the stack
    • save the SP - to where
  • restore
    • where to get the SP value
    • restore saved registers
    • what address to return to
• scheduler activations