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

(Chapter-3) Process-Concept

Chapter 3: Process-Concept Process Scheduling Inter-process Communication Dr. Abeer Mahmoud

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

( 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) Dr. Abeer Mahmoud

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 Dr. Abeer Mahmoud

The Process (cont..) which contains global variables. 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. is memory that is dynamically allocated during process run time. which contains global variables. which is program code . Dr. Abeer Mahmoud

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. Dr. Abeer Mahmoud

Diagram of Process State Dr. Abeer Mahmoud

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 Values of program counter & CPU registers must be saved when an interrupt occurs to allow the process to be continued correctly afterward 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 . Dr. Abeer Mahmoud

Process Control Block (PCB) (cont..) Dr. Abeer Mahmoud

CPU Switch From Process to Process Dr. Abeer Mahmoud

PROCESS SCHEDULING Dr. Abeer Mahmoud

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. Dr. Abeer Mahmoud

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 Dr. Abeer Mahmoud

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. Dr. Abeer Mahmoud

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. Dr. Abeer Mahmoud

Representation of Process Scheduling Queuing diagram Dr. Abeer Mahmoud

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 Dr. Abeer Mahmoud

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 Dr. Abeer Mahmoud

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. Dr. Abeer Mahmoud

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) Dr. Abeer Mahmoud

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 Dr. Abeer Mahmoud

Addition of Medium Term Scheduling Dr. Abeer Mahmoud

INTERPROCESS COMMUNICATION Dr. Abeer Mahmoud

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 Dr. Abeer Mahmoud

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). Dr. Abeer Mahmoud

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. Dr. Abeer Mahmoud

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

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 . Dr. Abeer Mahmoud

Communications Models Message passing model Shared memory model Dr. Abeer Mahmoud

Thank you End of Chapter 3 Dr. Abeer Mahmoud