Operating Systems PDF

Operating Systems 1. Introduction to Operating Systems Operating System: Three Easy Pieces 2 What a happens when a program runs?  A running program executes instructions. 1. The processor fetches an instruction from m...

Operating Systems 1. Introduction to Operating Systems Operating System: Three Easy Pieces 2 What a happens when a program runs?  A running program executes instructions. 1. The processor 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, jump to function, and so forth. 4. The processor moves on to the next instruction and so on. 3 Operating System (OS)  Responsible for  Making it easy to run programs  Allowing programs to share memory  Enabling programs to interact with devices OS is in charge of making sure the system operates correctly and efficiently. 4 Virtualization  The OS takes a physical resource and transforms it into a virtual form of itself.  Physical resource: Processor, Memory, Disk …  The virtual form is more general, powerful and easy-to-use.  Sometimes, we refer to the OS as a virtual machine. 5 System call  System call allows user to tell the OS what to do.  The OS provides some interface (APIs, standard library).  A typical OS exports a few hundred system calls.  Run programs  Access memory  Access devices 6 The OS is a resource manager.  The OS manage resources such as CPU, memory and disk.  The OS allows  Many programs to run  Sharing the CPU  Many programs to concurrently access their own instructions and data  Sharing memory  Many programs to access devices  Sharing disks 7 Virtualizing the CPU  The system has a very large number of virtual CPUs.  Turning a single CPU into a seemingly infinite number of CPUs.  Allowing many programs to seemingly run at once  Virtualizing the CPU 8 Virtualizing the CPU (Cont.) 1 #include 2 #include 3 #include 4 #include 5 6 int main(int argc, char *argv[]) 7 { 8 if (argc != 2) { 9 fprintf(stderr, "usage: cpu \n"); 10 exit(1); 11 } 12 char *str = argv; 13 while (1) { 14 sleep(1); 15 printf("%s\n", str); 16 } 17 return 0; 18 } Simple Example(cpu.c): Code That Loops and Prints 9 Virtualizing the CPU (Cont.)  Execution result 1. prompt> gcc -o cpu cpu.c -Wall prompt>./cpu "A" A A A ˆC prompt> Run forever; Only by pressing “Control-c” can we halt the program 10 Virtualizing the CPU (Cont.)  Execution result 2. prompt>./cpu A &./cpu B &./cpu C &./cpu D & 7353 7354 7355 7356 A B D C A B D C A C B D... Even though we have only one processor, all four of programs seem to be running at the same time! 11 Virtualizing Memory  The physical memory is an array of bytes.  A program keeps all of its data structures in memory.  Read memory (load):  Specify an address to be able to access the data  Write memory (store):  Specify the data to be written to the given address 12 Virtualizing Memory (Cont.)  A program that Accesses Memory (mem.c) 1 #include 2 #include 3 #include 4 5 int main(int argc, char *argv[]) 6 { 7 int *p = malloc(sizeof(int)); // a1: allocate some memory 8 assert(p != NULL); 9 printf("(%d) address of p: %08x\n", 10 getpid(), (unsigned) p); // a2: print out the address of the memmory 11 *p = 0; // a3: put zero into the first slot of the memory 12 while (1) { 13 sleep(1); 14 *p = *p + 1; 15 printf("(%d) p: %d\n", getpid(), *p); // a4 16 } 17 return 0; 18 } 13 Virtualizing Memory (Cont.)  The output of the program mem.c prompt>./mem (2134) memory address of p: 00200000 (2134) p: 1 (2134) p: 2 (2134) p: 3 (2134) p: 4 (2134) p: 5 ˆC  The newly allocated memory is at address 00200000.  It updates the value and prints out the result. 14 Virtualizing Memory (Cont.)  Running mem.c multiple times prompt>./mem &;./mem & 24113 24114 (24113) memory address of p: 00200000 (24114) memory address of p: 00200000 (24113) p: 1 (24114) p: 1 (24114) p: 2 (24113) p: 2 (24113) p: 3 (24114) p: 3...  It is as if each running program has its own private memory.  Each running program has allocated memory at the same address.  Each seems to be updating the value at 00200000 independently. 15 Virtualizing Memory (Cont.)  Each process accesses its own private virtual address space.  The OS maps address space onto the physical memory.  A memory reference within one running program does not affect the address space of other processes.  Physical memory is a shared resource, managed by the OS. 16 The problem of Concurrency  The OS is juggling many things at once, first running one process, then another, and so forth.  Modern multi-threaded programs also exhibit the concurrency problem. 17 Concurrency Example  A Multi-threaded Program (thread.c) 1 #include 2 #include 3 4 volatile int counter = 0; 5 int loops; 6 7 void *worker(void *arg) { 8 int i; 9 for (i = 0; i < loops; i++) { 10 counter++; 11 } 12 return NULL; 13 } 14 15 int main(int argc, char *argv[]) 16 { 17 if (argc != 2) { 18 fprintf(stderr, "usage: threads \n"); 19 exit(1); 20 } 18 Concurrency Example (Cont.) 21 loops = atoi(argv); 22 pthread_t p1, p2; 23 printf("Initial value : %d\n", counter); 24 25 Pthread_create(&p1, NULL, worker, NULL); 26 Pthread_create(&p2, NULL, worker, NULL); 27 Pthread_join(p1, NULL); 28 Pthread_join(p2, NULL); 29 printf("Final value : %d\n", counter); 30 return 0; 31 }  The main program creates two threads.  Thread: a function running within the same memory space. Each thread start running in a routine called worker().  worker(): increments a counter 19 Concurrency Example (Cont.)  loops determines how many times each of the two workers will increment the shared counter in a loop.  loops: 1000. prompt> gcc -o thread thread.c -Wall -pthread prompt>./thread 1000 Initial value : 0 Final value : 2000  loops: 100000. prompt>./thread 100000 Initial value : 0 Final value : 143012 // huh?? prompt>./thread 100000 Initial value : 0 Final value : 137298 // what the?? 20 Why is this happening?  Increment a shared counter  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.  Problem of concurrency happen. 21 Persistence  Devices such as DRAM store values in a volatile.  Hardware and software are needed to store data persistently.  Hardware: I/O device such as a hard drive, solid-state drives(SSDs)  Software:  File system manages the disk.  File system is responsible for storing any files the user creates. 22 Persistence (Cont.)  Create a file (/tmp/file) that contains the string “hello world” 1 #include 2 #include 3 #include 4 #include 5 #include 6 7 int main(int argc, char *argv[]) 8 { 9 int fd = open("/tmp/file", O_WRONLY | O_CREAT | O_TRUNC, S_IRWXU); 10 assert(fd > -1); 11 int rc = write(fd, "hello world\n", 13); 12 assert(rc == 13); 13 close(fd); 14 return 0; 15 } open(), write(), and close() system calls are routed to the part of OS called the file system, which handles the requests 23 Persistence (Cont.)  What OS does in order to write to disk?  Figure out where on disk this new data will reside  Issue I/O requests to the underlying storage device  File system handles system crashes during write.  Journaling or copy-on-write  Carefully ordering writes to disk 24 Design Goals  Build up abstraction  Make the system convenient and easy to use.  Provide high performance  Minimize the overhead of the OS.  OS must strive to provide virtualization without excessive overhead.  Protection between applications  Isolation: Bad behavior of one does not harm other and the OS itself. 25 Design Goals (Cont.)  High degree of reliability  The OS must also run non-stop.  Other issues  Energy-efficiency  Security  Mobility 26 Main frame computer M360 Virtual Machine 27 Mini computer PDP-11 Unix Shell, pipe, signal 28  DOS, Windows, MAC 29 30 31 What lies all these behind… 32 What does OS deal with  CPU  Execute code with data  Memory  Read and write code and data  Storage  Persistently store the code and data 33 34