Scheduling & Signals

1. Scheduling & Signals • CS241 Discussion Section • Fall 2007 • Week 5

2. Today we’ll… • Review the basic scheduling algorithms • Play with signals (as time permits) • This section will focus on examples.

3. CPU Scheduling • The CPU Scheduler decides which thread should be in the running state. • It is called when: • A new thread is created • A thread finishes • A clock interrupt occurs • An I/O interrupt occurs • A thread yields

4. Algorithms • First-Come First-Serve (FCFS) • Shortest Job First (SJF) • Priority • Round Robin (RR)

5. CPU Scheduling • Scheduling algorithms can be preemptive or non-preemptive • Non-Preemptive: Each thread chooses when to yield the processor • system call • thread finishes • Preemptive: The scheduler can force the thread to yield • time quantum expires • other thread preempts current one

6. Metrics • Response Time • The time from the submission of a thread until the thread begins to execute • Waiting Time • Total time that a thread spends waiting • Turnaround Time • Thread finish time – thread entry time

7. Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 First Come First Serve (FCFS) • At t=0, P1 starts

8. Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 FCFS Example • At t=6, P1 finishes, and P2 starts P1 0 6

9. Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 FCFS Example • At t=14, P2 finishes, and P3 starts P1 P2 0 14 6

10. Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 FCFS Example • At t=21, P3 finishes, and P4 starts P1 P2 P3 0 14 21 6

11. Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 FCFS Example • At t=24, P4 finishes P1 P2 P3 P4 0 14 21 24 6

12. Metrics for FCFS Example P1 P2 P3 P4 0 14 21 24 6 Average response time (ART): (0+6+14+21)/4 = 10.25 Average waiting time (AWT): (0+6+14+21)/4 = 10.25 Average turnaround time (ATT): (6+14+21+24)/4 = 16.25 P1 waiting time: 0 P2 waiting time: 6 P3 waiting time: 14 P4 waiting time: 21 P1 turnaround time: 6 P2 turnaround time: 14 P3 turnaround time: 21 P4 turnaround time: 24

13. Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 Shortest Job First (SJF) • At t=0, P4 starts

14. Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 SJF Example • At t=3, P4 finishes, and P1 starts P4 0 3

15. Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 SJF Example • At t=9, P1 finishes, and P3 starts P4 P1 0 3 9

16. Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 SJF Example • At t=16, P3 finishes, and P2 starts P4 P1 P3 0 3 9 16

17. Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 SJF Example • At t=24, P2 finishes P4 P1 P3 P2 0 3 9 16 24

18. Metrics for SJF Example P4 P1 P3 P2 0 3 9 16 24 Average response time (ART): (0+3+9+16)/4 = 7 Average waiting time (AWT): (0+3+9+16)/4 = 7 Average turnaround time (ATT): (3+9+16+24)/4 = 13 P4 waiting time: 0 P1 waiting time: 3 P3 waiting time: 9 P2 waiting time: 16 P4 turnaround time: 3 P1 turnaround time: 9 P3 turnaround time: 16 P2 turnaround time: 24

19. Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 Priority Scheduling Example • At t=0, P2 starts

20. Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 Priority Scheduling Example • At t=8, P2 finishes, and P4 starts P2 0 8

21. Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 Priority Scheduling Example • At t=11, P4 finishes, and P3 starts P2 P4 0 8 11

22. Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 Priority Scheduling Example • At t=18, P3 finishes, and P1 starts P2 P4 P3 0 8 11 18

23. Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 Priority Scheduling Example • At t=24, P1 finishes P2 P4 P3 P1 24 0 8 11 18

24. Metrics for Priority Example P2 P4 P3 P1 24 0 8 11 18 Average response time (ART): (0+8+11+18)/4 = 9.25 Average waiting time (AWT): (0+8+11+18)/4 = 9.25 Average turnaround time (ATT): (8+11+18+24)/4 = 15.25 P2 waiting time: 0 P4 waiting time: 8 P3 waiting time: 11 P1 waiting time: 18 P2 turnaround time: 8 P4 turnaround time: 11 P3 turnaround time: 18 P1 turnaround time: 24

25. Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 Round Robin Example • Each thread runs for 1 second, and then yields to the next thread. P1 P2 P3 P4 0

26. Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 Round Robin Example • At t=12, P4 finishes (P1, P2, and P3 left in queue) P1 P2 P3 P4 0 12

27. Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 Round Robin Example • At t=19, P1 finishes (P2 and P3 left in queue) P1 P2 P3 P4 0 12 19

28. Round Robin Example • At t=23, P3 finishes (P2 left in queue) Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 P1 P2 P3 P4 0 12 19 23

29. Round Robin Example • At t=24, P2 finishes Process Duration Priority # Arrival Time P1 6 4 0 P2 8 1 0 P3 7 3 0 P4 3 2 0 P1 P2 P3 P4 0 12 19 24

30. Metrics for Round Robin Example P1 P2 P3 P4 0 12 19 24 P1 waiting time: 13 P2 waiting time: 16 P3 waiting time: 16 P4 waiting time: 9 Average response time (ART): (0 + 1 + 2 + 3) / 4 = 1.5 Average waiting time (AWT): (13+16+16+9)/4 = 13.5 Average turnaround time (ATT): (12+24+23+12)/4 = 17.75 P1 turnaround time: 19 P2 turnaround time: 24 P3 turnaround time: 23 P4 turnaround time: 12

31. Comparison of Metrics Method ART AWT ATT FCFS 10.25 10.25 16.25 SJF 7 7 13 Priority 9.25 9.25 15.25 RR 1.5 13.5 17.75

32. Problem 1: Schedules (in pairs) • Say that these three jobs are going to arrive at our scheduler 1 • time unit apart from each other (i.e. one job at time 0, one at time • 1, and one at time 2). Assume that the jobs do not block. • Which arrival order(s) guarantee that the jobs finish • in the order J1 then J2 then J3 if the scheduler uses: • FCFS? • non-preemptive SJF? • preemptive SJF? (use remaining time; break ties with earliest arriving) • RR-1? (assume jobs that arrive during a round are placed on the ready queue immediately) • non-preemptive Priority? (assume higher priority # is more important) • preemptive Priority?

33. Abrupt change of topic…

34. Signals • A signal is a software notification to a process of an event. • Signals are asynchronous, so the process can handle them no matter where it is in the program.

35. User-triggered signals • Ctrl+C triggers SIGINT • Ctrl+Z triggers SIGSTOP • Ctrl+D triggers SIGQUIT • etc. • Run stty -a to see others.

36. Sending signals from the command line • The Unix kill command sends signals. • The usage is • kill -<signal> <pid> • Run kill -l to see available signals. • The command is called kill because in many cases the default behavior is for the process receiving the signal to die.

37. Sending signals from inside the program • Use the kill() function to send signals from one process to another. • #include <sys/types.h> • #include <signal.h> • int kill(pid_t pid, int sig); • Examples: • kill(getpid(),SIGTERM); • kill(getppid(),SIGKILL);

38. Example: d5-example1.c • This example takes in input one character at a time, and shifts the letters forward by one. • Run this program, and send it signals using: • Ctrl+C, Ctrl+Z, etc. • The kill command, from another window

39. Signal blocking • We can block some signals, so they don’t reach the process right away. • d5-example2.c contains code for blocking SIGINT. • The calls used are: • sigemptyset(&newsigset); • sigaddset(&newsigset,SIGINT); • sigprocmask(SIG_BLOCK,&newsigset,NULL);

40. Example: d5-example2.c • Run this program. Note that unlike d5-example1.c , this program does not respond to Ctrl+C. You will need to send it a SIGKILL signal to kill it. • This is because the SIGINT signal has been blocked. • (Note: some signals, such as SIGKILL, cannot be blocked.)

41. Signal sets • A sigset_t is a set of signals, represented as a bit array. • The functions sigemptyset, sigfillset, sigaddset, and sigdelset set and change the signal set. • Call sigemptyset or sigfillset first (else operations are undefined).

42. Signal Mask • The signal mask is the set of signals that are currently blocked. sigprocmask changes the signal mask: • sigprocmask(SIG_BLOCK,&newset,&oldset) blocks all signals in newset. • sigprocmask(SIG_UNBLOCK,&newset,&oldset) unblocks all signals not in newset. • sigprocmask(SIG_SETMASK,&newset,&oldset) blocks all signals in newset, and unblocks all signals not in newset.

43. Signal handlers • We can also change what happens when a process receives a signal. • In d5-example3.c, every time the process receives the signal SIGUSR1, it calls the change_shift function, which changes the shift value. • act.sa_handler = change_shift; • act.sa_flags = 0; • sigemptyset(&act.sa_mask); • sigaction(SIGUSR1, &act, NULL);

44. Example: d5-example3.c • Run this program. • From another window, use kill to send this process a SIGUSR1 signal a few times. • Notice how the output changes each time.

45. Signal Actions • In the struct sigactionact: • sa_handler is the function called upon receiving the signal • sa_flags are special flags • sa_mask are the additional signals to block while handling this signal • sigaction(SIGUSR1, &act1, NULL) associates this handler with SIGUSR1.