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2. Processes

07/10/22

Processes​

A process is a running instance of a program. The program is passive and 'sits' on the disk. A process may be active and may hold resources

A process has control structures - they store all necessary information related to the management of the program

Context Switching​

A process control box contains:

  • Process identification (PiD, UID, Parent PiD)
  • Process control information (process state, scheduling information)
  • Process state information (user registers, pc etc)

The process control block is necessary for context switching in multi programmed systems    

Process control blocks are kernel data structures:

  • They are stored in the process table
  • They are accessible in kernel mode only
  • Direct "user-mode access" could compromise their integrity
  • They are accessed through system calls

Process Implementation​

  • Process table holds a process control block for each process
  • Info about status of resources is maintained in tables
    • Process tables - process control blocks
    • Memory tables - memory allocation, protection, virtual memory etc
    • I/O tables - Availability, status, transfer information
    • File tables - Location, status
  • Tables are in kernel space and cross referenced.

Process States and Transitions​

  • New process has just been created and is waiting to be admitted
  • Ready process is waiting for the CPU to become available
  • Running process 'owns' the CPU
  • Blocked process cannot continue (waiting for IO)
  • Terminated process is no longer executable
  • Suspended process is swapped out The interrupts lie on the basis of the transitions

Context Switching - Multi-programming​

Mutli-programming goes back to the 'MULTICS' age This is achieved by alternating processes and context switching

  • Single processor systems result in concurrent execution
  • True parallelism requires multiple processors

Save CPU State -> The process control block is updated Restore CPU State -> the process control block read

A trade-off exists between the length of time-slice and context switching.

  • Slow time slices - good response times. low utilisation
  • Long time slices - poor response times, better utilisation

Process​

  • A process memory image contains the program code and a data segment (stack or heap)
  • Every process has its own logical address space
  • Some OS use address space layout randomisation.

System Calls​

True system calls are 'wrapped' in the OS libraries following a well defined interface

  • Unit: fork() generates an exact copy of the parent
  • Windows: NTCreateProcess()
  • Linux: Clone()

System calls are necessary to notify the OS and the process has terminated.

  • Resources must be de-allocated
  • Output must be flushed
  • Process admin may have to be carried out

Fork​

  • fork() creates an exact copy of the current process. The first instruction carried out by the child in the first one after the fork call
  • fork() returns the process identifier of the child process to the parent process
  • fork() returns 0 to the child process