Operating system 17 process management
- 1. Operating System 17 Process Management Prof Neeraj Bhargava Vaibhav Khanna Department of Computer Science School of Engineering and Systems Sciences Maharshi Dayanand Saraswati University Ajmer
- 2. Multiprogramming and Multitasking • Multiprogramming needed for efficiency – Single user cannot keep CPU and I/O devices busy at all times – Multiprogramming organizes jobs (code and data) so CPU always has one to execute – A subset of total jobs in system is kept in memory – One job selected and run via job scheduling – When it has to wait (for I/O for example), OS switches to another job
- 3. Multiprogramming and Multitasking • Timesharing (multitasking) is logical extension in which CPU switches jobs so frequently that users can interact with each job while it is running, creating interactive computing – Response time should be < 1 second – Each user has at least one program executing in memory process – If several jobs ready to run at the same time CPU scheduling – If processes don’t fit in memory, swapping moves them in and out to run – Virtual memory allows execution of processes not completely in memory
- 4. Memory Layout for Multiprogrammed System
- 5. 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
- 6. Process Management Activities • 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 The operating system is responsible for the following activities in connection with process management:
- 7. Process Concept • An operating system executes a variety of programs: – Batch system – jobs – Time-shared systems – user programs or tasks • Textbook uses the terms job and process almost interchangeably • Process – a program in execution; process execution must progress in sequential fashion • Multiple parts – The program code, also called text section – Current activity including program counter, processor registers – Stack containing temporary data • Function parameters, return addresses, local variables – Data section containing global variables – Heap containing memory dynamically allocated during run time
- 8. Process Concept (Cont.) • Program is passive entity stored on disk (executable file), process is active – Program becomes process when executable file loaded into memory • Execution of program started via GUI mouse clicks, command line entry of its name, etc • One program can be several processes – Consider multiple users executing the same program
- 10. 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
- 11. Diagram of Process State
- 12. 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
- 13. CPU Switch From Process to Process
- 14. 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 • See next chapter
- 15. Assignment 1. What is process concept? Explain Process Management in OS 2. Draw and describe the process states