COMP2310 - Week 3 - Processes and Threads

CPUs and Flows

One CPU per Control Flow

• Single CPU per control flow

  • Individual stack, code

• Process management (scheduling) not required as each process runs on its own CPU

• Coordinated shared memory access is required

• I.e. important real-time systems such as brakes

CPU for all Control Flows

• Single CPU where each control flow has its own emulated CPU

• Process management required

• Coordinated memory access required + memory protection

Symmetric Multi-Processing - SMP

• All CPUs share same physical address space + resource access

• Any process/thread can be executed on any one avaliable CPU

• Use cases

  • Moving processes across CPUs for heat dissipation and load sharing
  • Big-Little architecture, a powerful CPU with many other less powerfull CPUS

Processes or Threads?

• Threads -> A group of processes which can share some resources (Process hierachy)

  • Threads tend to share memory
  • Processes tend not to (specific exceptions apply)

• Threads have less priority/less expensive than 'first-class-citizen-processes'

• OS dependent

• Scheduling

  • User-level -> No kernel support
  • Kernel-level -> Handled as processes with some restrictions

Processes

An instance of a program being executed in a multitasking OS

A process comprises of an address space + control flow(s)

• Kernel must have full knowledge about all processes -> Allows kernel to protect and schedule access to appropriate resources
  • State
  • Requirements
    • Access
  • Held resources

Threads

Smallest sequence of programmed instructions that can be managed independently by a scheduler

• Multiple control flows per process are known as a thread

• Uses

  • If inside OS
    • Kernel scheduling
    • Thread performs I/O
  • If outside OS
    • User-level scheduling
    • Parent process performs I/O, each thread cannot directly perform I/O as kernel doesn't know about it

Process Control Blocks (PCBs)

• Some information is essential
  • Process ID -> Unique identifier
  • Process State
    • Created
    • Ready
    • Executing
    • Blocked
    • Suspended
  • Scheduling attributes
    • Priorities
    • Deadlines
    • Consumed CPU-time
  • CPU State
    • Saved/restored information on context switches (i.e. interrupts)
      • Program counter, stack pointer, ..
  • Memory attributes / privileges
    • Memory base
    • Limits
    • Shared/protected areas
  • Allocated resources / privileges
    • Open or requested files/devices

Process States

1. Created

Task ready to run, but is waiting for admission by a dispatcher

2. Ready

Ready to run and waiting on a avaliable CPU

3. Running

Holding a CPU and executing

4. Blocked

Task not ready to run, i.e. waiting for a resource such as I/O

5. Suspended

Rare. A task is swapped out of main memory (non-time-critical-processes) which are waiting for main memory space or other resources

Dispatching and suspending can now be considered independent modules

Unix Processes

pid = fork();

if (fork() == 0) {
    || Child process
    exec("path_to_exe",args);
    exit(0); // Terminate
} else { // If positive value
    // Parent process
    ...
    pid = wait();
    // Waiting for termination of one child process
}

1. Duplicates current process, returns 0 to newly created process ('child' process)
2. Returns process id of child process to creating process ('parent' process)
3. Returns -1 as indication of failure