Processes are programs in execution that include a program counter, stack, and data section. Each process is represented in the operating system by a process control block that contains the process state, CPU registers, scheduling information, memory management information, and I/O status. When switching between processes, the CPU saves the state of the old process and loads the saved state of the new process via a context switch. Process scheduling aims to maximize CPU usage by quickly switching processes onto the CPU for time sharing using scheduling queues.

1. Processes

2. Process Concept
• Process – a program in execution;
• A process includes:
– program counter
– stack
– data section

3. Process in Memory

4. Process State
• As a process executes, it changes state
– new: The process is being created
– running: Instructions are being executed
– waiting: The process is waiting for some event to
occur
– ready: The process is waiting to be assigned to a
processor
– terminated: The process has finished execution

5. Diagram of Process State

6. Process Control Block (PCB)
Each process is represented in operating system by process
control block
Information associated with each process
• Process state
• Program counter: Next Instruction to be executed
• CPU registers
• CPU scheduling information: includes process priority, and
any other scheduling parameters.
• Memory-management information: : includes information
of the base and limit registers, the page tables etc.
• Accounting information: includes amount of CPU, time
limits etc.
• I/O status information: includes list of I/O devices allocated
to the process, a list of open files and so on.

7. Process Control Block (PCB)

8. Context Switch
• When CPU switches to another process, the system must save the
state of the old process and load the saved state for the new process
via a context switch
• Context of a process represented in the PCB
• Context-switch time is overhead; the system does no useful work
while switching
– The more complex the OS and the PCB  the longer the context
switch
• Time dependent on hardware support
– Some hardware provides multiple sets of registers per CPU 
multiple contexts loaded at once

9. CPU Switch From Process to
Process

10. Process Scheduling
• Maximize CPU use, quickly switch processes onto CPU for time
sharing
• Process scheduler selects among available processes for next
execution on CPU
• Maintains scheduling queues of processes
– Job queue – set of all processes in the system
– Ready queue – set of all processes residing in main memory,
ready and waiting to execute
– Device queues – set of processes waiting for an I/O device
– Processes migrate among the various queues

11. Representation of Process Scheduling
n Queueing diagram represents queues, resources, flows

12. Schedulers
• Short-term scheduler (or CPU scheduler) – selects which process should be
executed next and allocates CPU
– Sometimes the only scheduler in a system
– Short-term scheduler is invoked frequently (milliseconds)  (must be fast)
• Long-term scheduler (or job scheduler) – selects which processes should be
brought into the ready queue
– Long-term scheduler is invoked infrequently (seconds, minutes)  (may be
slow)
– The long-term scheduler controls the degree of multiprogramming
• Processes can be described as either:
– I/O-bound process – spends more time doing I/O than computations, many
short CPU bursts
– CPU-bound process – spends more time doing computations; few very long
CPU bursts
• Long-term scheduler strives for good process mix

13. Addition of Medium Term
Scheduling
n Medium-term scheduler can be added if degree of multiple
programming needs to decrease
l Remove process from memory, store on disk, bring back in
from disk to continue execution: swapping

14. Operations on Processes
• System must provide mechanisms for:
– process creation,
– process termination,

15. Process creation
 Creation
ways a process can be created:
• Running an application.
• Created by OS to provide some service (e.g. background
printing).
• Spawned by existing process (parent/child).

16. Process Creation
• Parent process create children processes, which, in turn
create other processes, forming a tree of processes
• Generally, process identified and managed via a process
identifier (pid)
• Resource sharing
– Parent and children share all resources
– Children share subset of parent’s resources
– Parent and child share no resources
• Execution
– Parent and children execute concurrently
– Parent waits until children terminate

17. A Tree of Processes in Linux
init
pid = 1
sshd
pid = 3028
login
pid = 8415
kthreadd
pid = 2
sshd
pid = 3610
pdflush
pid = 200
khelper
pid = 6
tcsch
pid = 4005
emacs
pid = 9204
bash
pid = 8416
ps
pid = 9298

18. Process Creation (Cont)
• Address space
– Child duplicate of parent
– Child has a program loaded into it
• UNIX examples
– fork system call creates new process
– exec system call used after a fork to replace the process’
memory space with a new program

19. C Program Forking Separate Process

20. Process termination
 Termination
• 7 ways of dying:
• Normal completion.
• Time out (e.g. user terminal idle).
• OS intervention (e.g. deadlock).
• Parent dies/is killed.
• Parent kills child.
• Error condition (e.g. memory violation/ zero divide).
• Memory unavailable.

21. Process Termination
• Process executes last statement and asks the operating
system to delete it (exit)
– Output data from child to parent (via wait)
– Process’ resources are deallocated by operating system
• Parent may terminate execution of children processes
(abort)
– Child has exceeded allocated resources
– Task assigned to child is no longer required
– If parent is exiting
• Some operating system do not allow child to continue
if its parent terminates
– All children terminated - cascading termination