Killing Zombies, Working, Sleeping, and Spawning Children

Transcription

1 Killing Zombies, Working, Sleeping, and Spawning Children CS 333 Prof. Karavanic (c) 2015 Karen L. Karavanic 1

2 The Process Model The OS loads program code and starts each job. Then it cleans up afterwards, freeing memory, clearing data structures, etc." Process an instance of a program during execution" Today we will discuss;" How to setup and start a process" How to stop a process" How to protect processes from each other" How to protect the Operating System from all processes " These questions pertain to The Process Model"

3 A simple program (part 1) 1 #include <stdio.h> " 2 #include <stdlib.h> " 3 #include <sys/time.h> " 4 #include <assert.h> " 5 #include "common.h" " "

4 A simple program (part 2) 7 int " 8 main(int argc, char *argv[]) " 9 { " 10 if (argc!= 2) { 11 "fprintf(stderr, "usage: cpu <string>\n"); 12 "exit(1); " 13 } " 14 char *str = argv[1]; " 15 while (1) { " 16 Spin(1); " 17 " printf("%s\n", str); " 18 } " 19 return 0; " 20 } "

5 The Process Model prompt>./cpu A & ;./cpu B & ;./cpu C & ;./cpu D & [1] 7353 [2] 7354 [3] 7355 " [4] 7356 " A B D " C " A " B " D " C " A " C " B " D " "

6 The Process Model Let s think: What information do we need to handle processes?" Start" Interrupt / Restart" Schedule"

7 The Process Abstraction A Process = a program in execution" You may see the terms job and process: process is correct, job is historical" A process includes:" program counter (PC): what instruction to execute next? " Stack: one stack frame per function called" Register values: what values currently held in hardware registers?" Program is passive entity, process is active " Program becomes process when executable file loaded into memory" Operating System Concepts Essentials 8 th Edition! 3.7! Silberschatz, Galvin and Gagne 2011!

8 The Process Abstraction The Process Address Space includes:" text section: the program code" program counter: next instruction to execute" Values of processor registers: smallest, fastest memory" Stack containing temporary data" Function parameters, return addresses, local variables" Data section containing global variables" Heap containing memory dynamically allocated during run time" 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" Operating System Concepts Essentials 8 th Edition! 3.8! Silberschatz, Galvin and Gagne 2011!

9 Process States 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" Operating System Concepts Essentials 8 th Edition! 3.9! Silberschatz, Galvin and Gagne 2011!

10 Process States Operating System Concepts Essentials 8th Edition! 3.10! Silberschatz, Galvin and Gagne 2011!

11 CPU Switch From Process to Process Operating System Concepts Essentials 8 th Edition! 3.11! Silberschatz, Galvin and Gagne 2011!

12 process == task Processes key idea: the OS virtualizes the CPU: we can have more than one program executing but each one is written as if it has the CPU to itself unit of execution. in timeshare system, each user had a separate shell. in multiprocessor server or multicore chip, user has > 1 task running concurrently process has context: structures in system that constitute that process (c) 2000 Mario Marchand and (c) 2014 Karen L. Karavanic 12

13 Process Scheduling Maximize CPU use, quickly switch processes onto CPU for time sharing" Process scheduler selects among available processes for next execution on CPU" Maintains scheduling queues of processes" Job queue set of all processes in the system" Ready queue set of all processes residing in main memory, ready and waiting to execute" Device queues set of processes waiting for an I/O device" Processes migrate among the various queues" Operating System Concepts Essentials 8 th Edition! 3.13! Silberschatz, Galvin and Gagne 2011!

14 Processes The Process Model Multiprogramming of four programs Conceptual model of 4 independent, sequential processes Only one program active at any instant (c) 2000 Mario Marchand and (c) 2014 Karen L. Karavanic 14

15 Process Representation in Linux Represented by the C structure task_struct: pid t pid; /* process identifier */ long state; /* state of the process */ unsigned int time slice /* scheduling information */ struct task struct *parent; /* this process s parent */ struct list head children; /* this process s children */ struct files struct *files; /* list of open files */ struct mm struct *mm; /* address space of this pro */ Operating System Concepts Essentials 8 th Edition! 3.15! Silberschatz, Galvin and Gagne 2011!

16 Ready Queue And Various I/O Device Queues Operating System Concepts Essentials 8th Edition! 3.16! Silberschatz, Galvin and Gagne 2011!

17 Representation of Process Scheduling Operating System Concepts Essentials 8 th Edition! 3.17! Silberschatz, Galvin and Gagne 2011!

18 Schedulers Long-term scheduler (or job scheduler) selects which processes should be brought into the ready queue" Ex: Large Scientific Linux Cluster user may wait days from job submission to job scheduling" Short-term scheduler (or CPU scheduler) selects which process on the Ready Queue should be executed next, and allocates the CPU" Sometimes the only scheduler in a system" Operating System Concepts Essentials 8 th Edition! 3.18! Silberschatz, Galvin and Gagne 2011!

19 Schedulers (Cont.) 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! Processes can be described as either:" I/O-bound process spends more time doing I/O than computations, many short CPU bursts" CPU-bound process spends more time doing computations; few very long CPU bursts" Operating System Concepts Essentials 8 th Edition! 3.19! Silberschatz, Galvin and Gagne 2011!

20 Addition of Medium Term Scheduling Operating System Concepts Essentials 8th Edition! 3.20! Silberschatz, Galvin and Gagne 2011!

21 Context Switch 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" The more complex the OS and the PCB -> longer the context switch" Context switch time is dependent on hardware support" Ex: Some hardware provides multiple sets of registers per CPU -> multiple contexts loaded at once" Operating System Concepts Essentials 8 th Edition! 3.21! Silberschatz, Galvin and Gagne 2011!

22 Process Creation Principal events that cause process creation:" System initialization" Execution of a process creation system " User request to create a new process" Initiation of a batch job" (c) 2000 Mario Marchand and (c) 2014 Karen L. Karavanic 22

23 Operating System Concepts Essentials 8 th Edition! Process Creation Parent process creates child processes, which, in turn create grandchild processes, forming a tree of processes" Generally, process identified and managed via a process identifier (pid)" Resource sharing different models" Parent and children share all resources" Children share subset of parent s resources" Parent and child share no resources" Execution" " Parent and children execute concurrently" Parent waits until children terminate" 3.23! Silberschatz, Galvin and Gagne 2011!

24 Process Creation (Cont.) Address space" Child is created as a duplicate of parent:" Same program loaded into text segment" Same values in data segment" Same stack" Same register values" Same PC value" UNIX/Linux examples" fork system call creates new process" exec system call used after a fork to replace the process memory space with a new program" Operating System Concepts Essentials 8 th Edition! 3.24! Silberschatz, Galvin and Gagne 2011!

25 Process Creation Operating System Concepts Essentials 8 th Edition! 3.25! Silberschatz, Galvin and Gagne 2011!

26 Process Termination Process executes last statement and asks the operating system to delete it (exit)" Output data from child to parent (via wait)" Process resources are deallocated by operating system" Parent may terminate execution of children processes (abort)" Child has exceeded allocated resources" Task assigned to child is no longer required" If parent is exiting" Some operating system do not allow child to continue if its parent terminates" All children terminated - cascading termination! Operating System Concepts Essentials 8 th Edition! 3.26! Silberschatz, Galvin and Gagne 2011!

27 Process Termination Conditions which terminate processes 1. Normal exit (voluntary) 2. Error exit (voluntary) 3. Fatal error (involuntary) 4. Killed by another process (involuntary) The kill command: kill [pid] can any process kill any other process? (c) 2000 Mario Marchand and (c) 2014 Karen L. Karavanic 27

28 Unix System Calls for Process Management s: an error code pid: a process ID status: Did the process exit normally? Yes: No: (c) 2000 Mario Marchand and (c) 2014 Karen L. Karavanic 28

29 What is a zombie process?? (c) 2000 Mario Marchand and (c) 2014 Karen L. Karavanic 29

30 POSIX signals - simple IPC IPC - interprocess communication Also used for kernel to process communication software interrupts for application you arrange for asynchronous call to a function that will handle the signal signals are events we can t predict what line of code will be executing when they occur can be caught, sent, or ignored (SIG_IGN) use kill(1) and kill(2) to send them (some of them) (c) 2000 Mario Marchand and (c) 2014 Karen L. Karavanic 30

31 Example Signal Functions SIGINT - catch control-c, exit program SIGSEGV - o.s. (mmu) forces program to exit because of memory violation SIGHUP - signal sent to daemon to tell it to exit system cleanly (shutdown) or to reread control file SIGSTOP, SIGCONT, job control of processes (SIGSTOP goes to background) SIGKILL - go away NOW... % kill -9 <pid> (c) 2000 Mario Marchand and (c) 2014 Karen L. Karavanic 31

32 POSIX Signals The signals required by POSIX. (c) 2000 Mario Marchand and (c) 2014 Karen L. Karavanic 32

33 Unix shells Shell is not part of the kernel, but makes heavy use of it User logs in over terminal, network, gets shell command line Each command is executed in a child process Each process at startup has 3 open file descriptors: stdin, stdout, stderr (fd=0, 1, 2) Child inherits these from parent (c) 2000 Mario Marchand and (c) 2014 Karen L. Karavanic 33

34 shellgame... shells handles redirection and pipes with certain metacharacters (>,<, ) date > file sort < file1 > file2 ls more can execute programs in background too ls & (c) 2000 Mario Marchand and (c) 2014 Karen L. Karavanic 34

35 Steps in executing ls at the Command line (c) 2000 Mario Marchand and (c) 2014 Karen L. Karavanic 35

36 Announcements If you missed Lab1 Wed morning: Come half hour early to lab this week to learn the setup and get your lab account Complete takehome version of lab to be graded Everyone should have attended Wed eve and Thurs eve Let me know if not The mailing list is not yet setup First Problem Set will be posted this week 36