Process.md

Process

Process Practice

Basic Concepts

Definition of Process

• Is a single running activity of a program with independent function working on some data set
• Is a independent unit for system to manage resources and scheduling (maybe "Thread")
• Is a abstraction of CPU
• Every process has independent address space

Alias: Task, Job

• Wiki - Process (computing)

Process Status

3 Basic Status

• Running
  • Occupy the CPU resouces
• Ready
  • Ready to run, just waiting for the CPU
• Waiting/Blocked (Sleeping)
  • Waiting a event to be completed
    • I/O
  • Mostly wake by interrupt

2 More Status

• New/Created
  • Finish some necessary task
    • allocate PID
    • allocate PCB
  • Haven't allow to run (or it will be Running state)
• Terminated (Zombie)
  • Some resouce reclaim or waiting for others (parent) to terminate

5 Status Model

1 More Status

• Suspend
  • SWAP out the process from memory to secondary memory (disk)

7 Status Model

Linux Process Status

• TASK_RUNNING
• TASK_UNINTERRUPTABLE
• TASK_INTERRUPTABLE
• TASK_STOPPED
• TASK_ZOMBIE

Process Control Block (PCB)

• It's the only sign that system know a process existing

Alias: Task Controlling Block, Entry of the Process Table, Task Struct, Switchframe

• PCB structure (wiki)

  • Process identification data
  • Process state data
  • Process control data

• PCB structure (Modern OS)

  • Process management
    • Process ID
    • Program counter (PC)
    • Priority
    • Process state
  • Memory management
    • Kernel stack
    • Page table
  • File management
    • User ID
    • Group ID

• Wiki - Process control block

PCB Table: A set for all the process's PCB

• This can be an array (XV6)
• Or can be a linked list

Real-World Example

• In Linux: task_struct (kernel process)
  • Thread in Linux Notes
• In XV6: proc
• In Windows: EPROCESS, KPROCESS, PEB

Process Address Space

• Kernel Space
• User Space
  • Stack
    • Stack Pointer (SP) pointing to its bottom
  • [Empty space]
  • Heap
    • mmap
    • malloc
  • Data (global data)
    • shared memory
    • shared libraries (e.g. DLLs)
  • Code

For 32-bit OS, the process address space is 2³² = 4GB

For 64-bit OS, in the theory the process address space should be 2⁶⁴ but it's way too big. So in practice the system will give it 2⁴⁸ or 2³⁹. (The decision of number is by multi-level paging)

Virtual Address Space (VAS)

• Wiki - Virtual address space

Context Switch

Cost for Context Switching: saving and loading registers and memory maps, updating various tables and lists, etc

Example of Linux Kernel: context switching involves switching registers, stack pointer, and program counter, but is independent of address space switching, though in a process switch an address space switch also happens.

• Wiki - Context switch

CPU Scheduling

Process Queue

Process Control and System Call

System Call

Are primitive, atomic operations

• Create Process
  • Example
    • UNIX: fork/exec
    • Windows: CreateProcess
• Terminate Process
  • Example
    • UNIX: exit
    • Windows: ExitProcess
• Block Process
  • Example
    • UNIX: wait
    • Windows: WaitForSingleObject

Example of UNIX Process System Calls

• fork()

  • Copy one page a time from parent process to child process's address space
    • Copy-on-write (COW) fork in Linux
  • Return 0 to child and return child process's pid to parent

• exec()

  • Use new code to override the original address space

• wait()

• exit()

• Wiki - System call

Others

Process classification

• System Process
• User Process

• Frontground Process
• Background Process

• CPU-bound Process
  • Wiki - CPU-bound
• I/O-bound Process
  • Wiki - I/O bound

Process hierarchy structure

• UNIX Process family tree
  • init as root
• Windows
  • same position

Resource

• Wiki - Process management (computing)
• Shichao's Notes Chapter 3. Process Management
• In-Memory Layout of a Program (Process)

Book

Operating System Concepts 9ed.

• Notes
  • Processes