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Lecture 3: Processes

This lecture covers the process concept, process scheduling, operations on processes, cooperating processes, interprocess communication, and communication in client-server systems.

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Lecture 3: Processes

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  1. Lecture 3 : Processes

  2. Topics covered • Process Concept • Process Scheduling • Operations on Processes • Cooperating Processes • Interprocess Communication • Communication in Client-Server Systems

  3. Process Concept • Early computer systems allowed only one program to be executed at a time . This program had complete control of the system and had access to all the system's resources. In contrast, current-day computer systems allow multiple programs to be loaded into memory and executed concurrently. This evolution required firmer control and more compartmentalization of the various programs; and these needs resulted in the notion of :- • A process which is a program in execution. • A process is the unit of work in a modern time-sharing system.

  4. Process Concept • the operating system consists of a collection of processes:- 1- operating system processes executing system code 2- user processes executing user code. • Potentially/ all these processes can execute concurrently/ with the CPU (or CPUs) multiplexed among them. By switching the CPU between processes, the operating system can make the computer more productive

  5. Process Component • A process is includes :- 1- The current activity, as represented by the value of the program counter and the contents of the processor's registers. 2- The process stack, which contains temporary data (such as function parameters, return addresses, and local variables). 3 - A data section, which contains global variables. 4- A process may also include a heap, which is memory that is dynamically allocated during process run .

  6. Process in Memory

  7. What the Process • program by itself is not a process; a program is a passive entity, such as a file containing a list of instructions stored on disk (often called an executable file), whereas a process is an active entity, . Two common techniques for loading executable files are double-clicking an icon representing the executable file and entering the name of the executable file on the command line

  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 • ready: The process is waiting to be assigned to a processor • terminated: The process has finished execution

  9. Diagram of Process State

  10. Process Control Block (PCB) Information associated with each process • Process state • Program counter :- The counter indicates the address of the next instruction to be executed for this process • CPU registers:- The registers vary in number and type, depending on the computer architecture. They include accumulators, index registers, stack pointers, and general-purpose registers. Along with the program counter, this state information must be saved when an interrupt occurs, to allow the process to be continued correctly afterward .

  11. Process Control Block (PCB) • CPU scheduling information :- This information includes a process priority, pointers to scheduling queues, and any other scheduling parameters . • Memory-management information :- This information may include such information as the value of the base and limit registers, the page tables, depending on the memory system used by the operating system . • Accounting information :- This information includes the amount of CPU and real time used. • I/O status information :- This information includes the list of I/O devices allocated to the process, a list of open files .

  12. Process Control Block (PCB)

  13. Operation of processes • The processes in most systems can execute concurrently, and they may be created and deleted dynamically. Thus, these systems must provide a mechanism for process creation and termination. In this section, we explore the mechanisms involved in creating processes and illustrate process creation on Windows systems

  14. Operation of processes • process may create several new processes, via a create-process system call, during the course of execution. The creating process is called a parent process, and the new processes are called the children of that process. Each of these new processes may in turn create other processes, forming a tree of processes. Most operating systems (including UNIX and the Windows family of operating systems) identify processes according to a unique process identifier (or pid), which is typically an integer number

  15. CPU Scheduling • A process migrates among the various scheduling queues throughout its lifetime. The operating system must select, for scheduling purposes, processes from these queues in some fashion . The selection process is carried out by the appropriate scheduler • in a batch system, more processes are submitted than can be executed immediately. These processes are spooled to a mass-storage device (typically a disk), where they are kept for later execution. • The types of processes scheduler:- • - The long-term scheduler, or job scheduler, selects processes from this pool and loads them into memory for execution. • - The short-term scheduler, or CPU scheduler, selects from among the processes that are ready to execute and allocates the CPU to one of them

  16. CPU Scheduling • CPU-scheduling decisions may take place under the following four circumstances: 1- When a process switches from the running state to the waiting state (for example, as the result of an I/0 request or an invocation of wait for the termination of one of the child processes 2- When a process switches from the running state to the ready state (for example, when an interrupt occurs)

  17. CPU Scheduling 3- When a process switches from the waiting state to the ready state (for example, at completion of I/0) 4- When a process terminates

  18. P1 P2 P3 0 24 27 30 Scheduling Algorithms • First-Come, First-Served Scheduling:- ProcessBurst Time P1 24 P2 3 P3 3 • Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is: • Waiting time for P1 = 0; P2 = 24; P3 = 27 • Average waiting time: (0 + 24 + 27)/3 = 17

  19. Scheduling Algorithms • Shortest-Job-First (SJF) Scheduling:- • Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time • Two schemes: • no preemptive – once CPU given to the process it cannot be preempted until completes its CPU burst • preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF) • SJF is optimal – gives minimum average waiting time for a given set of processes

  20. Shortest-Job-First (SJF) Scheduling:- ProcessBurst Time P1 5 P2 17 P3 68 P4 24 • The Gantt chart is:

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