Princess Nora University Operating Systems CS 340 Chapter 2 - Process Concept PDF

Princess Nora University Computer Sciences Department Operating Systems CS 340 1  Chapter-2 Process Concept (covered in chapter 3 of textbook) Introduction Chapter 2: Process-Concept 1. Process Concept 2. Process Scheduling 3. Inter-process Comm...

Princess Nora University Computer Sciences Department Operating Systems CS 340 1  Chapter-2 Process Concept (covered in chapter 3 of textbook) Introduction Chapter 2: Process-Concept 1. Process Concept 2. Process Scheduling 3. Inter-process Communication 3  OBJECTIVES:  To introduce the notion of a process  To describe the various features of processes, including scheduling, creation and termination, and communication  To explore interprocess communication using shared memory and message passing 4  PROCESS CONCEPT 5 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 : o Text section o Stack section o Data section o The current activity section o Heap section 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. is memory that is dynamically allocated during process run time. which contains global variables. which is program code. 8 Process State  Process state is defined by the current activity of that process.  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 : is a data structure contains information about process & used by OS.  PCB contains many information including :  Process state : new/ready/ running/waiting/terminated  Program counter & CPU registers: Values of program counter & CPU registers must be saved when an interrupt occurs to allow the process to be continued correctly afterward  CPU scheduling information: information about when the process will be executed in CPU.  Memory-management information: information about where the process allocated in RAM  Accounting information :e.g. process numbers ,when the process was last run, how much CPU time it has used  I/O status information.: the list of I/O devices allocated to the process, a list of open files, ….etc. 11 Process Control Block (PCB) (cont..) 12 CPU Switch From Process to Process Time 13  PROCESS SCHEDULING 14 Process Scheduling  Scheduler: it is an operating system module that responsible of selecting process(/es). Short-term scheduler (CPU scheduler/ process scheduler ) Long-term Schedulers scheduler (job scheduler ) Medium Term Scheduling 15 Process Scheduling Queues  Job queue – set of all processes in the system residing in mass storage.  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 The process could The process could be issue an I/O request create a new sub- removed forcibly from and then be placed in process and wait for the CPU as a result of an I/O queue. the sub-process’s an interrupt, and be termination 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 19 Queuing diagram 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  Short-term scheduler is invoked very frequently (milliseconds)  (must be fast)  Long-term scheduler is invoked very infrequently (seconds, minutes)  (may be slow) Number of processes in RAM  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 It spends more time doing I/O It spends more time doing than computations, computations; It has many short CPU burst It has few very long CPU burst. 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 the I/O waiting queue will always be empty, almost always be empty, the short-term scheduler will devices will go unused…. have little to do… (minimize devices utilization).(minimize CPU utilization)  System with the best performance will have a combination of CPU-bound and I/O bound processes. 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 (to improve total performance)  Or because a change in memory requirements has exceeded available memory (i.e. memory is almost full) 24 Context Switch  Context Switch Time: Is amount of time needed by OS to switch CPU from one process to another  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  Time dependent on hardware support 25 Operation on Processes  OS provide mechanisms for  process creation,  process termination 26 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)  Resources 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 27 Process Termination  Process executes last statement and then asks the operating system to delete it.  Returns status value (integer)from child to parent  Process’ resources are deallocated by operating system  Parent may terminate the execution of children processes. Some reasons for doing so:  Child has exceeded allocated resources  Task assigned to child is no longer required  The parent is exiting and the operating systems does not allow a child to continue if its parent terminates 28  INTERPROCESS COMMUNICATION 29 Interprocess Communication Processes within a system may be independent cooperating Cannot affect or be affected by Can affect or be affected by the the execution of another execution of another process process. 30 Interprocess Communication  Reasons (/advantages) for cooperating processes: 1. Information sharing : (e.g. shared file) 2. 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). 31 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. 32 Interprocess Communication (Cont.)  Cooperating processes need interprocess communication (IPC)  IPC: mechanism that will allow cooperating processes to exchange data and information. 33  Interprocess Communication (Cont.)  Two models of IPC: process can exchange information by Message passing Shared memory Method: Method: messages exchanged between the reading and writing data to the cooperating processes. shared region. Useful for exchanging smaller amounts of Useful for exchanging small & large data, because no conflicts need be amounts of data, avoided. More difficult to implement than Easier to implement than shared memory Message passing Requires more system calls (interrupts) Shared memory is faster than message >>> slower. passing ….because only one interrupt is generated 34 Communications Models Message passing model Shared memory model 35 Thank you End of Chapter 2 Some Helpful Notes  Program counter :It is a register in CPU holds the address of the next instruction to be executed for this process.  Some terminologies :  job pool= job queue= input queue  mass-storage =back-store (i.e. mostly , it is hard desk)  Shared resource: a resource allocated to multiple processes at the same time 37