Operating Systems. Week 3 Recitation: Assignment 1 & 2 Review, Assignment 3 Discussion Paul Krzyzanowski. Rutgers University.

Transcription

1 Operating Systems Week 3 Recitation: Assignment 1 & 2 Review, Assignment 3 Discussion Paul Krzyzanowski Rutgers University Spring 2015 February 14, Paul Krzyzanowski 1

2 Assignment 1 Review February 14, Paul Krzyzanowski 2

3 Assignment 1 Question 1 Why does UEFI (the Unified Extensible Firmware Interface) not need to load a first-stage boot loader from the Master Boot Record (MBR) and why did the BIOS need to do so? UEFI can parse a file system and load a multi-block file that contains a bootloader. The BIOS could only read individual disk blocks. It does not know anything about the structure of the file system. It reads a single disk disk block That one disk block contains partition map that identifies different areas of the disk (logical disks) a boot loader that reads N consecutive blocks from a specific partition February 14, Paul Krzyzanowski 3

4 Assignment 1 Question 2 (a) What is the main advantage of the microkernel approach to system design? The design of the kernel is simpler Adding new services does not require modifying the kernel Better controls over security: drivers and other services don't wield supreme power There is also a downside: performance often suffers since many system calls have to go to the kernel, be relayed to a user-level kernel process, back to the kernel, and then back to the user. (b) How do user programs and system services interact in a microkernel architecture? Via interprocess communication mechanisms, such as messages. February 14, Paul Krzyzanowski 4

5 Assignment 1 Question 3 a-b a. By approximately how much did the size of the UNIX system increase when it was rewritten in C in 1973? The code became about 33% bigger. b. What is a path name? A sequence of directory names separated by slashes (/) and ending with a file name. Example: /usr/share/mysql/charsets/index.xml February 14, Paul Krzyzanowski 5

6 Assignment 1 Question 3 c-d c. What are three advantages to treating devices as files within the file system? 1. File and device I/O are as similar as possible. 2. File and device names have the same syntax and meaning. 3. Special files (devices) are subject to the same protection mechanism as regular files. d. What is the purpose of a set-user-id bit in a file's permissions? It allows privileged programs to access files that may be inaccessible to other users. February 14, Paul Krzyzanowski 6

9 Assignment 2 Question 1 Why is the separation of mechanism and policy desirable? Mechanism and policy must be separate to ensure that systems are easy to modify. No two system installations are the same, so each installation may want to tune the operating system to suit its needs. With mechanism and policy separate, the policy may be changed at will while the mechanism stays unchanged. This arrangement provides a more flexible system. February 14, Paul Krzyzanowski 9

10 Assignment 2 Question 2 Explain the role of the init process on UNIX and Linux systems in regard to process termination. When a process terminates, it moves to the zombie state and remains in that state until the parent calls wait(). At that time, the process ID as well as its entry in the process table are both released the exit state is returned with the return from wait and is no longer retained by the kernel. However, if a parent does not invoke wait() the child process remains a zombie as long as the parent remains alive Once the parent process terminates, the init process becomes the new parent of the zombie Periodically, the init process calls wait() which ultimately releases the Process ID and entry in the process table of the zombie process. February 14, Paul Krzyzanowski 10

11 Assignment 2 Question 3 Including the initial parent process, how many processes are created by the following program? #include <unistd.h> #include <stdio.h> int main(int argc, char **argv) { fork(); fork(); fork(); /*... */ } 8 processes. 1 st fork: creates a child process we have 2 processes 2 nd fork: each process creates a child 2 more processes 4 total 3 rd fork: each of 4 processes creates a child 8 total February 14, Paul Krzyzanowski 11

12 Assignment 2 Question 4 a-b a. What mechanism did Linux used to use for invoking system calls (creating the mode switch) on the x86 Intel architecture? In the past, Linux used a software interrupt (trap) int 0x80 to do the mode switch. b. What mechanism is used to invoke system calls since the 2.6 kernel? On Intel architectures, the Linux kernel no longer uses the trap mechanism but the SYSENTER/SYSEXIT instructions February 14, Paul Krzyzanowski 12

13 Assignment 2 Question 4 c c. Why was the system call mechanism changed? Performance. SYSENTER was much faster on Pentium 4 and later processors. More detail: INT accepts a parameter that identifies the entry in the interrupt vector table, which resides in memory. When processing the int instruction, the processor has to read data from the interrupt vector table For example int 0x80 causes the function pointed to by the 128th (128 decimal == 0x80) entry in the interrupt vector table to be executed. SYSENTER does not require consulting a table to take the jump The kernel registers the sysentry function handler by storing the address of the function into the CPU Model Specific Register SYSENTER_EIP_MSR (0x175) when it starts up Whenever the sysenter instruction is executed, the CPU can get the target address directly from a register [ This is a simplification: there are two additional registers that may need to be set for sysenter ] February 14, Paul Krzyzanowski 13

14 Assignment 2 Question 5 Can a multithreaded solution using multiple user-level threads achieve better performance on a multiprocessor system than on a single- processor system? Explain. No. A multithreaded system comprising of multiple user-level threads cannot make use of the different processors in a multiprocessor system simultaneously. The operating system sees only a single process and will not schedule the different threads of the process on separate processors. Consequently, there is no performance benefit associated with executing multiple user-level threads on a multiprocessor system. February 14, Paul Krzyzanowski 14

15 Assignment 3 Discussion February 14, Paul Krzyzanowski 15

16 Assignment 3 Download and build the xv6 operating system Run it under an x86 CPU emulator Add a new system call: i n t t r a c e ( i n t ) Turn system call tracing on/off trace(1): turn tracing on. Each system call from that process only will be printed to the console trace(0): turn tracing off. System calls will no longer be printed to the console Return total number of system calls for the process trace returns the total number of system calls called by the process so far February 14, Paul Krzyzanowski 16

17 Example output When tracing is on, each system call will result in one line of text printed to the console Each line contains The process ID Command name System call number System call name... pid: 2 [sh] syscall(5=read) pid: 2 [sh] syscall(5=read) pid: 2 [sh] syscall(1=fork) pid: 2 [sh] syscall(3=wait) pid: 3 [sh] syscall(12=sbrk) pid: 3 [sh] syscall(7=exec) pid: 3 [try] syscall(20=mkdir) pid: 3 [try] syscall(15=open) pid: 3 [try] syscall(16=write) pid: 3 [try] syscall(21=close) pid: 3 [try] syscall(2=exit) pid: 2 [sh] syscall(16=write) pid: 2 [sh] syscall(16=write)... Sample Output February 14, Paul Krzyzanowski 17

18 What is xv6? UNIX v6 Old version of UNIX from Bell Labs before networking was added to the OS Simple not a lot of features Nicely documented source BUT: written in an obsolete version of C and with PDP-11 specific code xv6 A rewrite of Unix v6 in ANSI C (standard C) for multicore Intel x86 processors Designed at MIT as a tiny OS suitable for teaching Same basic internal design as UNIX v6 Kernel, processes, files & directories, memory protection, Why not Linux? xv6 is a super-simple bare-bones OS about 7K lines of code Linux: >20K lines of code with a lot of complexity due to features & optimizations February 14, Paul Krzyzanowski 18

19 What do you need to do? Follow the instructions in the homework posting! You need: QEMU: open source machine emulator git: to download the xv6 code A Linux system to run it on Search the web for instructions on cross compiling on OS X or Windows if you don t want to use Linux Instructions are provided to install VirtualBox (a virtual machine) and Ubuntu Linux that will ensure you have an environment that works on your machine Rutgers ilab machines already have git; I ll check about QEMU You will run xv6 on a machine emulator You won t have to boot it natively and you can set breakpoints February 14, Paul Krzyzanowski 19

20 Install xv6 Download the source from MIT git clone git://pdos.csail.mit.edu/xv6/xv6.git This will create a directory called xv6 98 files no subdirectories or a separation between user program files and kernel files Running make qemu will Compile everything (user + kernel) Start QEMU emulator Run the OS, which will start init, which will start the shell process The shell will give you a command prompt February 14, Paul Krzyzanowski 20

21 Several options for compiling & running xv6 make qemu Build everything Start QEMU with the VGA console in a new window and the serial console in your terminal Exit by closing the VGA window or ctrl-a x in your terminal make qemu-nox Build everything Run only the serial console Exit by pressing ctrl-a x I use this since I ssh to my Linux systems make qemu-gdb and make qemu-nox-gdb Same as above but waits for a gdb (debugger) connection February 14, Paul Krzyzanowski 21

22 Exiting QEMU This assignment is easy You re unlikely to write code that will cause the kernel to die You won t need to do register-level debugging or setting breakpoints To switch between the console (running xv6) and the QEMU emulator command prompt Type ctrl-a c Then you can type QEMU commands, such as info registers to show CPU registers x/10i $eip show the next 10 instructions at the current instruction pointer system-reset reset & reboot the system quit exit the emulator (quit xv6) Type ctrl-a c again to get back to the console (unless you ran quit) February 14, Paul Krzyzanowski 22

23 Make sure xv6 is working! xv6 supports a hierarchical file system but boots up with only a tiny number of files, all at the root level The only commands that come with the source are: cat print files to the standard output mkdir Create a directory echo print command arguments rm Remove a file forktest test process creation sh The shell grep search for a string in a file stressfs Bunch of writes & reads init The first process starts the shell usertests Tests all system calls kill Send a signal to a process wc Counts pages, lines, chars ln Create a link to a file zombie Create a zombie process ls List files February 14, Paul Krzyzanowski 23

24 How to add a system call syscall.c process system calls syscall(void) entry function for all system calls This is the kernel function that is called for every system call System call number is in the eax register Check the range and call the corresponding function in the syscalls[] table of functions Follow the example of other system calls to add your new one. sysproc.c implements process-related system calls The functions here are called from syscall(). You can add your new function in this file. February 14, Paul Krzyzanowski 24

25 How to add a system call usys.s macros for assembler code for each sys call This simply places the system call number into the eax register and invokes the system call. You need to add an entry to your system call here syscall.h definitions of system call numbers You need to define a unique number for your system call user.h user definition of system calls You need to define the user s interface for your call here February 14, Paul Krzyzanowski 25

26 System call tracing You need to: Keep track of whether system call tracing is on or off for a process Count total number of system calls used by the process (irrespective of whether tracing is on or off) Print a line to the console if tracing is on Storing per-process data xv6 stores its process control blocks in a process table (not a list) struct proc You will need to extend this table to store the info you need Find where the entry is initialized to clear the values Printing a message to the console Use cprintf just like printf! cprintf( Hello, I m number %d\n, num) Look at the proc struct getting the program name is easy. You re on your own for printing the system call name February 14, Paul Krzyzanowski 26

27 Hints The assignment is easy just a few lines of code You have 21 other system calls to use as a guide Write a test program Call a number of different system calls Turn tracing on/off Print system call counts Fork a child process to make sure it does not trace calls even if the parent does There s a detailed description of xv6 online: short link: February 14, Paul Krzyzanowski 27

28 The End February 14, Paul Krzyzanowski 28