1. Operating Systems (CS 340 D)
Princess Nora University
Faculty of Computer & Information Systems
Computer science Department
Operating Systems (CS 340 D)

2. (Chapter-3) Process-Concept

3. Chapter 3: Process-Concept
Process Scheduling
Inter-process Communication

4. OBJECTIVES:
To introduce the notion of a process
To describe the various features of processes, including scheduling, creation and termination, and communication

5. ( a program that is loaded into memory and in execution)
Process Concept
System consists of a collection of processes:
Operating system processes executing system code
User processes executing user code
The terms job and process are used almost interchangeably, and both refer to the same meaning: =
( a program that is loaded into memory and in execution)

6. Process Concept (cont..)
Program : is a collection of instruction in a code manner , which is sometimes known as the text section.
Program = passive entity
Process =Active entity
Process :is more general than program as it includes more details such as :
Text section
Stack
Data section
The current activity:
Heap

7. The Process (cont..)
contains temporary data (such as function parameters, return addresses, and local variables )
>>The current activity: the value of the program counter & contents of the processor’s registers.
(some times): is memory that is dynamically allocated during process run time.
which contains global variables.
which is program code .

8. 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 (e.g. An I/O completion).
Ready: the process is waiting to be assigned to a processor
Terminated: the process has finished execution
Only one process can be running on any processor at any instant. Many processes may be ready and waiting.

9. Diagram of Process State

10. Process Control Block (PCB)
Each process is represented in the operating system by a process control block (PCB)
PCB contains many information including :
Process state
Program counter
CPU registers
CPU scheduling information
e.g. a process priority
Memory-management information
Accounting information:
e.g . when the process was last run, how much CPU time it has used
I/O status information
It includes the list of I/O devices allocated to the process, a list of open files….etc .
Values of program counter
& CPU registers must be
saved when an interrupt occurs
to allow the process to be
continued correctly afterward

11. Process Control Block (PCB) (cont..)

12. CPU Switch From Process to Process

13. PROCESS SCHEDULING

14. Process Scheduling
Multiprogramming aims to have some process running at all times to maximize CPU utilization.
Time sharing aims to switch the CPU among processes so frequently user interact with each program while it is running.
Process scheduler selects an process (from a set of several available processes) for execution on the CPU.

15. Process Scheduling Queues
Job queue – set of all processes in the system
Ready queue – set of all processes residing in main memory, ready and waiting to execute
This queue is generally stored as a linked list.
Device queues – set of processes waiting for an I/O device
Each device has its own device queue

16. Process Scheduling Queues (cont.)
Processes migrate among the various queues:
A new process is initially put in the ready queue.
It waits there until it is selected for execution.
Once the process is allocated the CPU and is executing, one of several events could occur:
The process could issue an I/O request and then be placed in an I/O queue.
The process could create a new sub-process and wait for the sub-process’s termination
The process could be removed forcibly from the CPU as a result of an interrupt, and be put back in the ready queue.

17. Process Scheduling Queues (cont.)
A process continues this cycle until it terminates, at which time it is removed from all queues and has its PCB and resources deallocated.

18. Representation of Process Scheduling
Queuing diagram

19. Schedulers
The operating system must select processes scheduling for these queues.
The selection process is carried out by the appropriate scheduler.
Long-term scheduler (or job scheduler) – selects which processes should be brought from mass-storage into RAM (to the ready queue )
Short-term scheduler (or CPU scheduler) – selects which process (from ready queue) should be executed next and allocates CPU

20. Schedulers controls the degree of multiprogramming
Short-term scheduler is invoked very frequently (milliseconds)  (must be fast)
Long-term scheduler is invoked very infrequently (seconds, minutes)  (may be slow)
The long-term scheduler:
controls the degree of multiprogramming

21. Schedulers (Cont) Processes can be described as either:
I/O-bound process
CPU-bound process
– spends more time doing I/O than computations, many short CPU intervals
– spends more time doing computations; few very long CPU intervals.

22. Schedulers (Cont)
The long-term scheduler must select a good process mix of I/O- bound and CPU-bound processes to improve total performance.
If all processes are I/O bound
If all processes are CPU bound
the ready queue will almost always be empty,
the short-term scheduler will have little to do…
.(minimize CPU utilization)
the I/O waiting queue will almost always be empty,
devices will go unused….
(minimize devices utilization)

23. Medium Term Scheduling
Sometimes it can be advantageous to remove processes from RAM …….(swap out)
Later, the process can be reintroduced into RAM …….(swap in), and its execution can be continued where it left off.
This scheme is called swapping. ( done by medium-term scheduler)
Swapping may be necessary to improve the process mix
Or because a change in memory requirements has exceeded available memory

24. Addition of Medium Term Scheduling

25. INTERPROCESS COMMUNICATION

26. Interprocess Communication
Processes within a system may be
independent
cooperating
Cannot affect or be affected by the execution of another process.
Can affect or be affected by the execution of another process

27. Interprocess Communication
Reasons (/advantages) for cooperating processes:
Information sharing : (e.g. shared file)
Computation speedup:
To make a particular task run faster, we must break it into subtasks, each of which will be executing in parallel with the others.
This can be achieved only if the computer has multiple processing elements (e.g. CPUs).

28. Interprocess Communication
Reasons (/advantages) for cooperating processes:
3. Modularity:
We may want to construct the system in a modular fashion (i.e. dividing the system functions into separate processes)
4. Convenience
An individual user may work on many tasks at the same time.

29. Interprocess Communication (Cont.)
Cooperating processes need interprocess communication (IPC)
IPC: mechanism that will allow cooperating processes to exchange data and information.

30. Interprocess Communication (Cont.)
Two models of IPC: process an exchange information by
Shared memory:
Message passing
reading and writing data to the shared region.
Shared memory is faster than message passing ….
.Because message passing systems requires more system calls (interrupts) .
By messages exchanged between the cooperating processes.
Useful for exchanging smaller amounts of data, because no conflicts need be avoided.
Easier to implement than shared memory .

31. Communications Models
Message passing model
Shared memory model

32. Thank you End of Chapter 3