1. Chapter 3: Process Management

2. Important Terminologies
Task
When a program is requested for execution it is called a task.
Before pressing enter key.
Job
When the OS checks the validity of the task by searching it in internal page table, if present then it is said to be a valid job for further processing.
Process
The job is then loaded into computer memory, after memory allocation, the program is set into execution.

3. Example When we write on the console, C:\>dir
A task has been given to OS
OS checks its validity by searching it into internal page table, if found, then it is valid, become a job
When it become a job, OS must load it into memory.
Now this program under execution along with its environment is called a Process.

4. Process Management
A process is a program in execution. It is a unit of work within the system. Program is a passive entity, process is an active entity.
Process needs resources to accomplish its task
CPU, memory, I/O, files
Initialization data
Process termination requires reclaim of any reusable resources
Single-threaded process has one program counter specifying location of next instruction to execute
Process executes instructions sequentially, one at a time, until completion
Multi-threaded process has one program counter per thread
Typically system has many processes, some user, some operating system running concurrently on one or more CPUs
Concurrency by multiplexing the CPUs among the processes / threads

5. Process Management Activities
The operating system is responsible for the following activities in connection with process management:
Creating and deleting both user and system processes
Suspending and resuming processes
Providing mechanisms for process synchronization
Providing mechanisms for process communication
Providing mechanisms for deadlock handling

6. Process State As a process executes, it changes state
Ready: The process is waiting to be assigned to a processor
Running: Instructions are being executed

7. Interrupts Resume Answer the Phone
The occurrence of an event is usually signaled by an interrupt from either the hardware or the software.
Hardware may trigger an interrupt at any time by sending a signal to the CPU usually by way of the system bus.
Software may trigger an interrupt by executing a special operation called a system call.
Answer the Phone .
Resume

8. Example Process Burst Time Start/Arrival Time P1 40 sec 0 sec P2
CPU time slice = 10 sec

9. P1 & P2 States
Time Instant P1 P2
Running
Ready 10 20 30 40 50 60 70 -

10. Class Exercise

11. Example Process Burst Time Start/Arrival Time P1 30 sec 0 sec P2
CPU time slice = 10 sec

12. P1 & P2 States
Time Instant P1 P2
Running
Ready 10 20 30 40 50 - 60 70

13. Operations on Processes
System must provide mechanisms for process creation, termination, and so on as detailed next

14. 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 options
Parent and children share all resources
Children share subset of parent’s resources
Parent and child share no resources
Execution options
Parent and children execute concurrently
Parent waits until children terminate

15. A Tree of Processes in Linux

16. 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 de-allocated 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 systems do not allow child to continue if its parent terminates
All children terminated - cascading termination

17. Process State Modified
As a process executes, it changes state
New: The process is being created
Running: Instructions are being executed
Ready: The process is waiting to be assigned to a processor
Terminated: The process has finished execution

18. Process State Modified
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

19. Diagram of Process State

20. Example Process Burst Time Start/Arrival Time P1 40 sec 0 sec P2
CPU time slice = 10 sec
P1 does an I/O for 10 sec at 20
P2 does an I/O for 20 sec at 40

21. P1 & P2 States Time Instant P1 P2 Running Ready 10 20 Blocked for I/O
Running
Ready 10 20
Blocked for I/O 30 40 50 60
Exit 70 -

22. Class Exercise

23. Example Process Burst Time Start/Arrival Time P1 40 sec 0 sec P2
CPU time slice = 10 sec
P1 does an I/O for (10 x 2) sec at 20 and 50
P2 does an I/O for (10 x 2) sec at 40 and 60

24. P1 & P2 States Time Instant P1 P2 Running Ready 10 20 Blocked for I/O
Running
Ready 10 20
Blocked for I/O 30 40 50 60 70
Exit

25. To make sure… You have Understood !
Process
Burst Time
Start/Arrival Time P1
30 sec
0 sec P2
40 sec
CPU time slice = 10 sec
P1 does an I/O for 10 sec at 20
P2 does an I/O for 30 sec at 30

26. P1 & P2 States Time Instant P1 P2 Running Ready 10 20 Blocked for I/O
Running
Ready 10 20
Blocked for I/O 30 40 50
Exit 60 - 70

27. Process Control Block (PCB)
Information associated with each process
(also called task control block)
Process state – running, waiting, etc.
Program counter – location of instruction to next execute
CPU registers – contents of all process-centric registers
CPU scheduling information – priorities, scheduling queue pointers
Memory-management information – memory allocated to the process
Accounting information – CPU used, clock time elapsed since start, time limits
I/O status information – I/O devices allocated to process, list of open files

28. Threads So far, process has a single thread of execution
Consider having multiple program counters per process
Multiple locations can execute at once
Multiple threads of control -> threads
Must then have storage for thread details, multiple program counters in PCB.

29. Single and Multithreaded Processes

30. Threads
Example: Consider a multi-threaded application. If one thread is blocked by some I/O request, the other can continue its execution.
Switching among threads is faster.
Only PC and registers
Threads are also known as ‘lightweight’ process.
Only one thread can be executed at a time
A thread can be in one of the several states, like Rn, Rd etc.
Different stacks and registers
But same memory

31. Synchronization Multithreaded applications Multiprocessor systems
Concurrent processing
Various problems are associated with it
Critical section

32. Critical section Critical Section Starts
Cout<<“Result before increment”<<i;
i++;
Cout<<“Result after increment”<<i;
Cout<<“Result before multiplication”<<i;
i=i*3;
Cout<<“Result after multiplication”<<i;
Critical Section Ends

33. Mutual Exclusion While (entry == True) entry = False
Cout<<“Result before increment”<<i;
i++;
Cout<<“Result after increment”<<i;
Cout<<“Result before multiplication”<<i;
i=i*3;
Cout<<“Result after multiplication”<<i;
entry = True

34. Semaphores
A semaphore is a variable or abstract data type that provides a simple but useful abstraction for controlling access, by multiple processes, to a common resource in a parallel programming or a multi user environment.
A library has 10 study rooms, to be used by one student at a time.
Students must request a room from the front counter. When a student has finished using a room, (s)he must return to the counter.
The clerk does not keep track of which room is occupied or who is using it.
When a student requests a room, the clerk decreases this number. When a student releases a room, the clerk increases this number.

35. Semaphores The front desk represents a semaphore,
the rooms are the resources
the students represent processes.
The value of the semaphore in this scenario is initially 10.
When a student requests a room he or she is granted access and the value of the semaphore is changed to 9.
After the next student comes, it drops to 8, then 7 and so on. 
A semaphore is a variable or abstract data type that provides a simple but useful abstraction for controlling access, by multiple processes, to a common resource in a parallel programming or a multi user environment.