Unix System Kernel

1. Unix System Kernel Instructors: Fu-Chiung Cheng (鄭福炯) Associate Professor Computer Science & Engineering Tatung Institute of Technology

2. Unix: Introduction • Operating System: a system that manages the resources • of a computer. • Resources: CPUs, Memory, I/O devices, Network • Kernel: the memory resident portion of Unix system • File system and process control system are two major • components of Unix Kernel.

3. Architecture of Unix System emacs • OS interacts directly with • the hardware • Such OS is called • system kernel sh who date kernel cpp hardware ed as cc wc ld grep nroff Other apps

4. Unix System Kernel • Three major tasks of kernel: • Process Management • Device Management • File Management • Three additional Services for Kernel: • Virtual Memory • Networking • Network File Systems • Experimental Kernel Features: • Multiprocessor support • Lightweight process (thread) support

5. Block Diagram of System Kernel User Programs Libraries User Level Kernel Level System Call Interface File Subsystem Inter-process communication Process control subsystem Scheduler Device drivers Memory management hardware control hardware Hardware Level

6. Process Control Subsystem • Process Synchronization • Interprocess communication • Memory management: • Scheduler: process scheduling • (allocate CPU to Processes)

7. File subsystem • A file system is a collection of files and directories on • a disk or tape in standard UNIX file system format. • Kernel’s file sybsystem regulates data flow between • the kernel and secondary storage devices.

8. Hardware Control • Hardware control is responsible for handling interrupts • and for communicating with the machine. • Devices such as disks or terminals may interrupt the • CPU while a process is executing. • The kernel may resume execution of the interrupted • process after servicing the interrupt.

9. Processes • A program is an executable file. • A process is an instance of the program in execution. • For example: create two active processes $ emacs & $ emacs & $ ps PID TTY TIME CMD 12893 pts/4 0:00 tcsh 12581 pts/4 0:01 emacs 12582 pts/4 0:01 emacs $

10. Processes • A process has • text: machine instructions (may be shared by other processes) • data • stack • Process may execute either in user mode and in kernel • mode. • Process information are stored in two places: • Process table • User table

11. User mode and Kernel mode • At any given instant a computer running the Unix system • is either executing a process or the kernel itself is running • The computer is in user mode when it is executing • instructions in a user process and it is in kernel mode • when it is executing instructions in the kernel. • Executing System call ==> User mode to Kernel mode • perform I/O operations • system clock interrupt

12. Process Table • Process table: an entry in process table has the following • information: • process state: A. running in user mode or kernel mode B. Ready in memory or Ready but swapped C. Sleep in memory or sleep and swapped • PID: process id • UID: user id • scheduling information • signals that is sent to the process but not yet handled • a pointer to per-process-region table • There is a single process table for the entire system

13. User Table (u area) • Each process has only one private user table. • User table contains information that must be accessible • while the process is in execution. • A pointer to the process table slot • parameters of the current system call, return values error codes • file descriptors for all open files • current directory and current root • process and file size limits. • User table is an extension of the process table.

14. Process table Kernel address space user address space Active process resident swappable Region table text u area data stack Per-process region table

15. Shared Program Text and Software Libraries • Many programs, such as shell, are often being • executed by several users simultaneously. • The text (program) part can be shared. • In order to be shared, a program must be compiled using • a special option that arranges the process image so that • the variable part(data and stack) and the fixed part (text) • are cleanly separated. • An extension to the idea of sharing text is sharing • libraries. • Without shared libraries, all the executing programs • contain their own copies.

16. Region table Process table text data stack Active process Reference count = 2 text data stack Per-process region table

17. System Call • A process accesses system resources through system call. • System call for • Process Control: fork: create a new process wait: allow a parent process to synchronize its execution with the exit of a child process. exec: invoke a new program. exit: terminate process execution • File system: • File: open, read, write, lseek, close • inode: chdir, chown chmod, stat fstat • others: pipe dup, mount, unmount, link, unlink

18. System call: fork() • fork: the only way for a user to create a process in Unix • operating system. • The process that invokes fork is called parent process • and the newly created process is called child process. • The syntax of fork system call: • newpid = fork(); • On return from fork system call, the two processes have • identical copies of their user-level context except for the • return value pid. • In parent process, newpid = child process id • In child process, newpid = 0;

19. $ cc forkEx1.c -o forkEx1 $ forkEx1 Before forking ... Child Process fpid=0 After forking fpid=0 Parent Process fpid=14707 After forking fpid=14707 $ /* forkEx1.c */ #include <stdio.h> main() { int fpid; printf("Before forking ...\n"); fpid = fork(); if (fpid == 0) { printf("Child Process fpid=%d\n", fpid); } else { printf("Parent Process fpid=%d\n", fpid); } printf("After forking fpid=%d\n", fpid); }

20. $ forkEx2 Before forking ... PID TTY TIME CMD 14759 pts/9 0:00 tcsh 14778 pts/9 0:00 sh 14777 pts/9 0:00 forkEx2 PID TTY TIME CMD 14781 pts/9 0:00 sh 14759 pts/9 0:00 tcsh 14782 pts/9 0:00 sh 14780 pts/9 0:00 forkEx2 14777 pts/9 0:00 forkEx2 After forking fpid=14780 $ PID TTY TIME CMD 14781 pts/9 0:00 sh 14759 pts/9 0:00 tcsh 14780 pts/9 0:00 forkEx2 After forking fpid=0 /* forkEx2.c */ #include <stdio.h> main() { int fpid; printf("Before forking ...\n"); system("ps"); fpid = fork(); system("ps"); printf("After forking fpid=%d\n", fpid); } $ ps PID TTY TIME CMD 14759 pts/9 0:00 tcsh $

21. System Call: getpid() getppid() • Each process has a unique process id (PID). • PID is an integer, typically in the range 0 through 30000. • Kernel assigns the PID when a new process is created. • Processes can obtain their PID by calling getpid(). • Each process has a parent process and a corresponding • parent process ID. • Processes can obtain their parent’s PID by calling • getppid().

22. /* pid.c */ #include <stdio.h> #include <sys/types.h> #include <unistd.h> main() { printf("pid=%d ppid=%d\n",getpid(), getppid()); } $ cc pid.c -o pid $ pid pid=14935 ppid=14759 $

23. /* forkEx3.c */ #include <stdio.h> #include <sys/types.h> #include <unistd.h> main() { int fpid; printf("Before forking ...\n"); fpid = fork(); if (fpid == 0) { printf("Child Process fpid=%d pid=%d ppid=%d\n", fpid, getpid(), getppid()); } else { printf("Parent Process fpid=%d pid=%d ppid=%d\n", fpid, getpid(), getppid()); } printf("After forking fpid=%d pid=%d ppid=%d\n", fpid, getpid(), getppid()); }

24. $ cc forkEx3.c -o forkEx3 $ forkEx3 Before forking ... Parent Process fpid=14942 pid=14941 ppid=14759 After forking fpid=14942 pid=14941 ppid=14759 $ Child Process fpid=0 pid=14942 ppid=1 After forking fpid=0 pid=14942 ppid=1 $ ps PID TTY TIME CMD 14759 pts/9 0:00 tcsh

25. System Call: wait() • wait system call allows a parent process to wait • for the demise of a child process. • See forkEx4.c

26. #include <stdio.h> #include <sys/types.h> #include <unistd.h> main() { int fpid, status; printf("Before forking ...\n"); fpid = fork(); if (fpid == 0) { printf("Child Process fpid=%d pid=%d ppid=%d\n", fpid, getpid(), getppid()); } else { printf("Parent Process fpid=%d pid=%d ppid=%d\n", fpid, getpid(), getppid()); } wait(&status); printf("After forking fpid=%d pid=%d ppid=%d\n", fpid, getpid(), getppid()); }

27. $ cc forkEx4.c -o forkEx4 $ forkEx4 Before forking ... Parent Process fpid=14980 pid=14979 ppid=14759 Child Process fpid=0 pid=14980 ppid=14979 After forking fpid=0 pid=14980 ppid=14979 After forking fpid=14980 pid=14979 ppid=14759 $

28. System Call: exec() • exec() system call invokes another program by replacing • the current process • No new process table entry is created for exec() program. • Thus, the total number of processes in the system isn’t • changed. • Six different exec functions: • execlp, execvp, execl, execv, execle, execve, • (see man page for more detail.) • exec system call allows a process to choose its successor.

29. /* execEx1.c */ #include <stdio.h> #include <unistd.h> main() { printf("Before execing ...\n"); execl("/bin/date", "date", 0); printf("After exec\n"); } $ execEx1 Before execing ... Sun May 9 16:39:17 CST 1999 $

30. /* execEx2.c */ #include <sys/types.h> #include <unistd.h> #include <stdio.h> main() { int fpid; printf("Before execing ...\n"); fpid = fork(); if (fpid == 0) { execl("/bin/date", "date", 0); } printf("After exec and fpid=%d\n",fpid); } $ execEx2 Before execing ... After exec and fpid=14903 $ Sun May 9 16:47:08 CST 1999 $

31. Handling Signal • A signal is a message from one process to another. • Signal are sometime called “software interrupt” • Signals usually occur asynchronously. • Signals can be sent • A. by one process to anther (or to itself) • B. by the kernel to a process. • Unix signals are content-free. That is the only thing that • can be said about a signal is “it has arrived or not”

32. Handling Signal • Most signals have predefined meanings: • A. sighup (HangUp): when a terminal is closed, the • hangup signal is sent to every process in control terminal. • B. sigint (interrupt): ask politely a process to terminate. • C. sigquit (quit): ask a process to terminate and produce a • codedump. • D. sigkill (kill): force a process to terminate. • See signEx1.c

33. #include <stdio.h> #include <sys/types.h> #include <unistd.h> main() { int fpid, *status; printf("Before forking ...\n"); fpid = fork(); if (fpid == 0) { printf("Child Process fpid=%d pid=%d ppid=%d\n", fpid, getpid(), getppid()); for(;;); /* loop forever */ } else { printf("Parent Process fpid=%d pid=%d ppid=%d\n", fpid, getpid(), getppid()); } wait(status); /* wait for child process */ printf("After forking fpid=%d pid=%d ppid=%d\n", fpid, getpid(), getppid()); }

34. $ cc sigEx1.c -o sigEx1 $ sigEx1 & Before forking ... Parent Process fpid=14989 pid=14988 ppid=14759 Child Process fpid=0 pid=14989 ppid=14988 $ ps PID TTY TIME CMD 14988 pts/9 0:00 sigEx1 14759 pts/9 0:01 tcsh 14989 pts/9 0:09 sigEx1 $ kill -9 14989 $ ps ...

35. Scheduling Processes • On a time sharing system, the kernel allocates the CPU to • a process for a period of time (time slice or time quantum) • preempts the process and schedules another one when • time slice expired, and reschedules the process to continue • execution at a later time. • The scheduler use round-robin with multilevel feedback • algorithm to choose which process to be executed: • A. Kernel allocates the CPU to a process for a time slice. • B. preempts a process that exceeds its time slice. • C. feeds it back into one of the several priority queues.

36. Process Priority Processes Priority Levels swapper wait for Disk IO wait for buffer wait for inode ... Kernel Mode wait for child exit User Mode User level 0 User level 1 ... User level n

37. Process Scheduling (Unix System V) • There are 3 processes A, B, C under the following • assumptions: • A. they are created simultaneously with initial priority 60. • B. the clock interrupt the system 60 times per second. • C. these processes make no system call. • D. No other process are ready to run • E. CPU usage calculation: CPU = decay(CPU) = CPU/2 • F. Process priority calculation: priority = CPU/2 + 60. • G. Rescheduling Calculation is done once per second.

38. Process A Priority CPU count Process B Priority CPU count Process C Priority CPU count 0 60 0 … 60 75 30 67 15 63 7 … 67 76 33 60 0 60 0 … 60 75 30 67 15 63 7 ... 60 0 60 0 60 0 … 60 75 30 67 15 1 2 3 4

39. Unix System Kernel Instructors: Fu-Chiung Cheng (鄭福炯) Associate Professor Computer Science & Engineering Tatung Institute of Technology

40. Booting • When the computer is powered on or rebooted, a short • built-in program (maybe store in ROM) reads the first • block or two of the disk into memory. These blocks • contain a loader program, which was placed on the disk • when disk is formatted. • The loader is started. The loader searches the root • directory for /unix or /root/unix and load the file into • memory • The kernel starts to execute.

41. The first processes • The kernel initializes its internal data structures: • it constructs linked list of free inodes, regions, page table • The kernel creates u area and initializes slot 0 of process • table • Process 0 is created • Process 0 forks, invoking the fork algorithm directly • from the Kernel. Process 1 is created. • In kernel mode, Process 1 creates user-level context • (regions) and copy code (/etc/init) to the new region. • Process 1 calls exec (executes init).

42. init process • The init process is a process dispatcher:spawning • processes, allow users to login. • Init reads /etc/inittab and spawns getty • when a user login successfully, getty goes through a login • procedure and execs a login shell. • Init executes the wait system call, monitoring the death • of its child processes and the death of orphaned processes • by exiting parent.

43. Init fork/exec a getty progrma to manage the line When the shell dies, init wakes up and fork/exec a getty for the line Getty prints “login:” message and waits for someone to login The shell runs programs for the user unitl the user logs off The login process prints the password message, read the password then check the password

44. File Subsystem • A file system is a collection of files and directories on • a disk or tape in standard UNIX file system format. • Each UNIX file system contains four major parts: • A. boot block: • B. superblock: • C. i-node table: • D. data block: file storage

45. File System Layout Block 0: bootstrap Block 1: superblock Block 2 Block 2 - n:i-nodes ... Block n Block n+1 Block n+1 - last:Files ... The last Block

46. Boot Block • A boot block may contains several physical blocks. • Note that a physical block contains 512 bytes • (or 1K or 2KB) • A boot block contains a short loader program for • booting • It is blank on other file systems.

47. Superblock • Superblock contains key information about a file system • Superblock information: • A. Size of a file system and status: • label: name of this file system • size: the number of logic blocks • date: the last modification date of super block. • B. information of i-nodes • the number of i-nodes • the number of free i-nodes • C. information of data block: free data blocks. • The information of a superblock is loaded into memory.

48. I-nodes • i-node: index node (information node) • i-list: the list of i-nodes • i-number: the index of i-list. • The size of an i-node: 64 bytes. • i-node 0 is reserved. • i-node 1 is the root directory. • i-node structure: next page

49. mode I-node structure owner timestamp Data block Data block Size Data block Data block Reference count ... ... Block count Direct blocks 0-9 Data block Data block Indirect block ... Single indirect Indirect block Double indirect Indirect block Indirect block Triple indirect

50. I-node structure • mode: A. type: file, directory, pipe, symbolic link • B. Access: read/write/execute (owner, group,) • owner: who own this I-node (file, directory, ...) • timestamp: creation, modification, access time • size: the number of bytes • block count: the number of data blocks • direct blocks: pointers to the data • single indirect: pointer to a data block which • pointers to the data blocks (128 data blocks). • Double indirect: (128*128=16384 data blocks) • Triple indirect: (128*128*128 data blocks)