2. Introduction to Operating Systems

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1 2. Introduction to Oerating Systems Oerating System: Three Easy Pieces 1

2 What a haens when a rogram runs? A running rogram executes instructions. 1. The rocessor fetches an instruction from memory. 2. Decode: Figure out which instruction this is 3. Execute: i.e., add two numbers, access memory, check a condition, jum to function, and so forth. 4. The rocessor moves on to the next instruction and so on. 2

3 Oerating System (OS) Resonsible for w Making it easy to run rograms w Allowing rograms to share memory w Enabling rograms to interact with devices OS is in charge of making sure the system oerates correctly and efficiently. 3

4 Virtualization The OS takes a hysical resource and transforms it into a virtual form of itself. Physical resource: Processor, Memory, Disk w The virtual form is more general, owerful and easy-to-use. w Sometimes, we refer to the OS as a virtual machine. 4

5 System call System call allows user to tell the OS what to do. w The OS rovides some interface (APIs, standard library). w A tyical OS exorts a few hundred system calls. Run rograms Access memory Access devices 5

6 The OS is a resource manager. The OS manage resources such as CPU, memory and disk. The OS allows w Many rograms to run à Sharing the CPU w Many rograms to concurrently access their own instructions and data à Sharing memory w Many rograms to access devices à Sharing disks 6

7 Virtualizing the CPU The system has a very large number of virtual CPUs. w Turning a single CPU into a seemingly infinite number of CPUs. w Allowing many rograms to seemingly run at once à Virtualizing the CPU 7

8 Virtualizing the CPU (Cont.) 1 #include <stdio.h> 2 #include <stdlib.h> 3 #include <sys/time.h> 4 #include <assert.h> 5 #include "common.h" 6 7 int 8 main(int argc, char *argv[]) 9 { 10 if (argc!= 2) { 11 frintf(stderr, "usage: cu <string>\n"); 12 exit(1); 13 } 14 char *str = argv[1]; 15 while (1) { 16 Sin(1); // Reeatedly checks the time and returns once it has run for a second 17 rintf("%s\n", str); 18 } 19 return 0; 20 } Simle Examle(cu.c): Code That Loos and Prints 8

9 Virtualizing the CPU (Cont.) Execution result 1. romt> gcc -o cu cu.c -Wall romt>./cu "A" A A A ˆC romt> Run forever; Only by ressing Control-c can we halt the rogram 9

10 Virtualizing the CPU (Cont.) Execution result 2. romt>./cu A & ;./cu B & ;./cu C & ;./cu D & [1] 7353 [2] 7354 [3] 7355 [4] 7356 A B D C A B D C A C B D... Even though if we have only one rocessor, all four of rograms seem to be running at the same time! 10

11 Virtualizing Memory The hysical memory is an array of bytes. A rogram kees all of its data structures in memory. w Read memory (load): Secify an address to be able to access the data w Write memory (store): Secify the data to be written to the given address 11

12 Virtualizing Memory (Cont.) A rogram that Accesses Memory (mem.c) 1 #include <unistd.h> 2 #include <stdio.h> 3 #include <stdlib.h> 4 #include "common.h" 5 6 int 7 main(int argc, char *argv[]) 8 { 9 int * = malloc(sizeof(int)); // a1: allocate some memory 10 assert(!= NULL); 11 rintf("(%d) address of : %08x\n", 12 getid(), (unsigned) ); // a2: rint out the address of the memory 13 * = 0; // a3: ut zero into the first slot of the memory 14 while (1) { 15 Sin(1); 16 * = * + 1; 17 rintf("(%d) : %d\n", getid(), *); // a4 18 } 19 return 0; 20 } 12

13 Virtualizing Memory (Cont.) The outut of the rogram mem.c romt>./mem (2134) memory address of : (2134) : 1 (2134) : 2 (2134) : 3 (2134) : 4 (2134) : 5 ˆC w The newly allocated memory is at address w It udates the value and rints out the result. 13

14 Virtualizing Memory (Cont.) Running mem.c multile times romt>./mem &;./mem & [1] [2] (24113) memory address of : (24114) memory address of : (24113) : 1 (24114) : 1 (24114) : 2 (24113) : 2 (24113) : 3 (24114) : 3... w It is as if each running rogram has its own rivate memory. Each running rogram has allocated memory at the same address. Each seems to be udating the value at indeendently. 14

15 Virtualizing Memory (Cont.) Each rocess accesses its own rivate virtual address sace. w The OS mas address sace onto the hysical memory. w A memory reference within one running rogram does not affect the address sace of other rocesses. w Physical memory is a shared resource, managed by the OS. 15

16 The roblem of Concurrency The OS is juggling many things at once, first running one rocess, then another, and so forth. Modern multi-threaded rograms also exhibit the concurrency roblem. 16

17 Concurrency Examle A Multi-threaded Program (thread.c) 1 #include <stdio.h> 2 #include <stdlib.h> 3 #include "common.h" 4 5 volatile int counter = 0; 6 int loos; 7 8 void *worker(void *arg) { 9 int i; 10 for (i = 0; i < loos; i++) { 11 counter++; 12 } 13 return NULL; 14 }

18 Concurrency Examle (Cont.) 16 int 17 main(int argc, char *argv[]) 18 { 19 if (argc!= 2) { 20 frintf(stderr, "usage: threads <value>\n"); 21 exit(1); 22 } 23 loos = atoi(argv[1]); 24 thread_t 1, 2; 25 rintf("initial value : %d\n", counter); Pthread_create(&1, NULL, worker, NULL); 28 Pthread_create(&2, NULL, worker, NULL); 29 Pthread_join(1, NULL); 30 Pthread_join(2, NULL); 31 rintf("final value : %d\n", counter); 32 return 0; 33 } w The main rogram creates two threads. Thread: a function running within the same memory sace. Each thread start running in a routine called worker(). worker(): increments a counter 18

19 Concurrency Examle (Cont.) loos determines how many times each of the two workers will increment the shared counter in a loo. w loos: romt> gcc -o thread thread.c -Wall -thread romt>./thread 1000 Initial value : 0 Final value : 2000 w loos: romt>./thread Initial value : 0 Final value : // huh?? romt>./thread Initial value : 0 Final value : // what the?? 19

20 Why is this haening? Increment a shared counter à take three instructions. 1. Load the value of the counter from memory into register. 2. Increment it 3. Store it back into memory These three instructions do not execute atomically. à Problem of concurrency haen. 20

21 Persistence Devices such as DRAM store values in a volatile. Hardware and software are needed to store data ersistently. w Hardware: I/O device such as a hard drive, solid-state drives(ssds) w Software: File system manages the disk. File system is resonsible for storing any files the user creates. 21

22 Persistence (Cont.) Create a file (/tm/file) that contains the string hello world 1 #include <stdio.h> 2 #include <unistd.h> 3 #include <assert.h> 4 #include <fcntl.h> 5 #include <sys/tyes.h> 6 7 int 8 main(int argc, char *argv[]) 9 { 10 int fd = oen("/tm/file", O_WRONLY O_CREAT O_TRUNC, S_IRWXU); 11 assert(fd > -1); 12 int rc = write(fd, "hello world\n", 13); 13 assert(rc == 13); 14 close(fd); 15 return 0; 16 } oen(), write(), and close() system calls are routed to the art of OS called the file system, which handles the requests 22

23 Persistence (Cont.) What OS does in order to write to disk? w Figure out where on disk this new data will reside w Issue I/O requests to the underlying storage device File system handles system crashes during write. w Journaling or coy-on-write w Carefully ordering writes to disk 23

24 Design Goals Build u abstraction w Make the system convenient and easy to use. Provide high erformance w Minimize the overhead of the OS. w OS must strive to rovide virtualization without excessive overhead. Protection between alications w Isolation: Bad behavior of one does not harm other and the OS itself. 24

25 Design Goals (Cont.) High degree of reliability w The OS must also run non-sto. Other issues w Energy-efficiency w Security w Mobility 25

26 Disclaimer: This lecture slide set is used in AOS course in University of Cantabria. Was initially develoed for Oerating System course in Comuter Science Det. at Hanyang University. This lecture slide set is for OSTEP book written by Remzi and Andrea Araci- Dusseau (at University of Wisconsin) 26

TCSS 422: OPERATING SYSTEMS

TCSS 422: OPERATING SYSTEMS TSS 422: OPERTING SYSTEMS OJETIVES Introduce operating systems INTROUTION Wes J. Lloyd Institute of Technology University of Washington - Tacoma Management of resources oncepts of virtualization/abstraction