Process Management

1. Process Management Operating Systems Lecture 3, 27 March 2003 Mr. Greg Vogl Uganda Martyrs University

2. Overview • Monolithic kernel vs. microkernel (lecture 1) • Program, Process, Thread • Interrupts • Process States • Process Scheduling • Processes in UNIX • Processes in Windows Operating Systems: Process Management

3. 1. Monolithic Kernel Operating Systems: Process Management

4. Microkernel Operating Systems: Process Management

5. 2. Program • Series of commands • Instructions only (usually not data) • Stored on disk, copied into memory • Can be in use by many users and processes at the same time • Also called software • Includes shell scripts, batch files Operating Systems: Process Management

6. Process • Variables point to instructions + data • Loaded in memory • context contains all process state info. • stored in Process Control Block • One user and one program per process • Also called task • (or job for batch processes) Operating Systems: Process Management

7. Process Context • program counter • register indicating next program instruction to run • other registers • accumulator, index/address, status, general • stack of subroutine return addresses • subroutines call other subroutines • values of local and global variables • pointers to open data files • user and terminal number (in multi-user OS) Operating Systems: Process Management

8. Examples of Processes • User Processes • Shells • Text editors, databases • Background jobs (end with & in UNIX) • System Processes • Memory management, process scheduling • daemons (system background processes) • Mail and print servers Operating Systems: Process Management

9. Thread • Also called sub- or lightweight process • Little private memory; memory is shared • Subdivides work of the process • Threads are managed by the process • Reduces high overhead for creating processes and context switching • Java was designed to write thread-based programs Operating Systems: Process Management

10. Thread Components • At minimum, every thread has its own • program counter • stack • Program text shared with other threads • Each procedure has a frame to hold its local variables • Heap of objects is shared by all threads Operating Systems: Process Management

11. Uses of Threads • Servers (database, mail, print, etc.) • one thread per client request • Network server • one thread per connection • Time-sharing • one thread per user • Real-time factory control • one thread per device Operating Systems: Process Management

12. 3. Interrupt • Signal hardwareCPU requesting services • CPU postpones its work to handle interrupt • Interrupt handler routines are part of OS code • OS switches modes (usersupervisor) • Interrupts are prioritised • Stack used to store multiple interrupt levels • Programs can mask (ignore) some interrupts • Others unmaskable (to avoid losing data) Operating Systems: Process Management

13. Examples of Interrupts • Key pressed • Disk or other I/O task finished • System clock Operating Systems: Process Management

14. Software Interrupts • Also called traps or exceptions • Processor interrupted by programs • Examples • Division by 0 error • Bus error • Array out of bounds • Buffer overflow Operating Systems: Process Management

15. Context Switching • Preserve state of current process • Start or re-start another process • Performed by dispatcher (OS component) • When to change context? • Overhead cost (takes CPU time) • Processes prioritised by properties Operating Systems: Process Management

16. 4. Process States • Runnable • Running (only one per processor) • Ready (waiting for its turn to run) • Blocked • Explicit e.g. wait() until a child terminates • Implicit e.g. read() • Blocked by another process • New (not yet allowed to wait its turn) • Suspended (e.g. swapped to virtual memory) • Terminated (finished but still using resources) Operating Systems: Process Management

17. Process State DiagramRitchie p. 62 entry (HLS) I/O completion ready blocked (LLS)dispatch timeout I/O wait (MLS)suspend running resume suspend resume suspend termination blocked suspended ready suspended I/O completion Operating Systems: Process Management

18. 5. Scheduling • Assign each process time to use CPU • Determine sequence (order), timing (when) • Conflicting objectives  need compromise • Scheduling levels • High: whether to admit a new process • Medium: suspend/resume a process • Low: dispatch (run) a ready process Operating Systems: Process Management

19. Scheduling Objectives • Maximise throughput • Give all users a “fair” (not equal) chance • Provide tolerable performance • Response time for on-line user • Turnaround time for batch users • Degrade performance gracefully • OK to be slow but avoid complete collapse • Be consistent, predictable over time Operating Systems: Process Management

20. Scheduling Criteria • Priority which may be either/both: • Assigned to job by user • Determined by properties of the job • Class of job (real-time > on-line > batch) • Resources needed (CPU time, memory) • I/O or CPU bound (the aim is a balance) • Resources already used • Time already waited Operating Systems: Process Management

21. Types of Scheduling Policies • Preemptive (Used by most OS today) • OS stops one process to run another • Non-preemptive • process runs until termination or I/O wait • Cooperative (Used by Windows 3.11) • Programs must voluntarily give up CPU • Not managed by OS; trusts programmers • If a process hangs the whole PC hangs  Operating Systems: Process Management

22. Scheduling Policies • First come first served (FCFS/FIFO) • Shortest job first (SJF) • Shortest remaining time (SRT) • Highest response ratio next • Round robin (RR) • Multi-level feedback queues (MFQ) Operating Systems: Process Management

23. First come first served • Also called first in first out (FIFO) • Process waiting longest is first in queue • Favours long jobs • high run-time/wait-time ratio • Favours CPU-bound jobs • I/O devices underused Operating Systems: Process Management

24. Shortest job first (next) • Run job with shortest estimated run time • Long jobs may be delayed indefinitely • JCL commands can specify run time Operating Systems: Process Management

25. Shortest remaining time • Run job w/ shortest est. remaining time • Highly favours short jobs • Long jobs will be delayed indefinitely • Requires estimating total run time • Requires measuring elapsed run time Operating Systems: Process Management

26. Highest response ratio next • P = (time waiting + run time) / run time • Priority based on two factors • Favours shorter jobs • Guarantees a job cannot be starved Operating Systems: Process Management

27. Round Robin • Each process given a set time slice • Pre-emption at end of time quantum • Hardware timer generates interrupts • After running, go to back of queue • Used in most interactive operating systems • How large should each time slice be? • Small  high context switch overhead • Large  user response time is reduced • In practice, about 10-20 ms Operating Systems: Process Management

28. Multi-level feedback queues • Separate queues for different priorities • New process  level 1 FIFO (highest) • After timeout  level 2 FIFO, etc. • Lowest level is Round Robin • Adapts to past process behaviours • high CPU usage  reduced access level • Long processes may starve • if wait time is long, promote/increase slice Operating Systems: Process Management

29. 6. UNIX process creation • PID 0: sched (process scheduler) • PID 1: init (ancestor of other processes) • daemons (automatic, for sys. admin.) • getty (one per terminal) • login (lets users log in) • shell (lets users run programs) • user-initiated processes (run from shell) Operating Systems: Process Management

30. UNIX fork() • fork() produces new “child” process • Child is exact copy of parent • Starts in same program, at same place • Exact copy of memory space • (globals, stack, heap object) • Both calls to fork() return a value • Parent fork() returns process ID of child • Child fork() returns 0 Operating Systems: Process Management

31. UNIX exec() • Execute (load, run) another program • Overwrite calling process in memory • Uses same PID; not a new process • Used after fork() to start new process • Parent waits for child process to finish • Run in background to not hold up shell • myprogram & Operating Systems: Process Management

32. UNIX scheduling • Varies with the flavour of UNIX • Often dynamic priority, round robin, MFQ • Processes have number priority values • 0=highest, 60=lowest, 20=initial/default • Users can reduce priority using nice • nice -10 myprog # makes priority 30 • Only superuser can increase priority • nice --10 myprog # makes priority 10 Operating Systems: Process Management

33. UNIX ps • Command to display process status • ps # list PIDs, TTYs, CPU time, name • ps -a # all terminal-created processes • ps -e # everything (incl. daemons) • ps -f # full list (more details) • ps -l # long list (ppid, priority, size, nice) Operating Systems: Process Management

34. Other UNIX commands • top – process list, continuously updated • jobs – list jobs running in current shell • bg, fg – send jobs to back/foreground • at – run batch job at specified time(s) • kill – send signal to terminate process • sleep – pause for some seconds Operating Systems: Process Management

35. 7. Processes in Windows • Every process treated as thread • Process can create additional threads • Scheduler operates over all threads • Each process has virtual address space • No parent-child process relationship • Environment copied to new processes Operating Systems: Process Management

36. Dynamic Link Libraries (DLLs) • Executable code routines • Linked into application only at run time • Can be shared by several programs Operating Systems: Process Management

37. Starting Programs • Type program name (maybe full path) • MS-DOS prompt, run dialog box, Explorer • Double-click icon • Desktop, Explorer, My Computer • Click shortcut • Start or Programs menu, QuickLaunch • Start when the computer starts • autoexec.bat, config.sys, Startup menu Operating Systems: Process Management

38. Managing Tasks • Ways to close programs • Ctrl-C; File, Exit; Close button (X); Alt-F4 • Press Ctrl-Alt-Del to view task manager • End Task: stop non-responding program • Ctrl-Alt-Del again to shut down/restart Operating Systems: Process Management

39. WinMe System Information • Loaded modules (exe, dll, etc.) • Name, version, size, file date, mfg., path • Running tasks • Name, path, PID, priority, version, size • Startup programs • Program, command, user name, location • Print jobs Operating Systems: Process Management