CS 3214 Computer Systems

1. CS 3214Computer Systems Godmar Back Lecture 12

2. Announcements • Exercise 6 coming up • Project 3 milestone due Oct 8 CS 3214 Fall 2010

3. Part 1 Threads and Processes CS 3214 Fall 2010

4. Processes • Def: An instance of a program in execution • OS provides each process with key abstractions • Logical control flow • 1 flow – single-threaded process • Multiple flows – multi-threaded process • Private address space • Abstracted resources: e.g., stdout/stdin file descriptors • These abstractions create the illusion that each process has access to its own • CPU (or CPUs for multi-threaded processes) • Memory • Devices: e.g., terminal CS 3214 Fall 2010

5. Context Switching • Historical motivation for processes was introduction of multi-programming: • Load multiple processes into memory, and switch to another process if current process is (momentarily) blocked • This required protection and isolation between these processes, implemented by a privileged kernel • Time-sharing: switch to another process periodically to make sure all processes make equal progress • Switch between processes is called a context switch CS 3214 Fall 2010

6. Dual-Mode Operation • Two fundamental modes: • “kernel mode” – privileged • aka system, supervisor or monitor mode • Intel calls its PL0, Privilege Level 0 on x86 • “user mode” – non-privileged • PL3 on x86 • Bit in CPU – controls operation of CPU • Privileged operations can only be performed in kernel mode. Example: hlt • Must carefully control transition between user & kernel mode int main() { asm(“hlt”); } CS 3214 Fall 2010

7. Mode Switching • User  Kernel mode • For reasons external or internal to CPU • External (aka hardware) interrupt: • timer/clock chip, I/O device, network card, keyboard, mouse • asynchronous (with respect to the executing program) • Internal interrupt (aka software interrupt, trap, or exception) • are synchronous • can be intended (“trap”): for system call (process wants to enter kernel to obtain services) • or unintended (usually): (“fault/exception”) (division by zero, attempt to execute privileged instruction in user mode, memory access violation, invalid instruction, alignment error, etc.) • Kernel  User mode switch on iret instruction CS 3214 Fall 2010

8. Timer interrupt: P1 is preempted, context switch to P2 I/O device interrupt:P2’s I/O completeswitch back to P2 user mode kernel mode System call: (trap): P2 starts I/O operation, blocks context switch to process 1 Timer interrupt: P2 still has time left, no context switch A Context Switch Scenario Process 1 Process 2 Kernel CS 3214 Fall 2010

9. user mode kernel mode Context Switching, Details intr_entry: (saves entire CPU state) (switches to kernel stack) intr_exit: (restore entire CPU state) (switch back to user stack) iret Process 1 Process 2 Kernel switch_threads: (in) (saves caller’s state) switch_threads: (out) (restores caller’s state) (kernel stack switch) CS 3214 Fall 2010

10. user mode kernel mode User processes access kernel services by trapping into the kernel, executing kernel code to perform the service, then returning – very much like a library call. Unless the system call cannot complete immediately, this does not involve a context switch. System Calls Process 1 Kernel Kernel’s System Call Implementation CS 3214 Fall 2010

11. Syscall example: write(2) /* gcc -static -O -g -Wall write.c -o write */ #include <unistd.h> int main() { const char msg[] = "Hello, World\n"; return write(1, msg, sizeofmsg); } /usr/include/asm/unistd.h: …. #define __NR_write 4 …. 0805005a <__write_nocancel>: 805005a: 53 push %ebx 805005b: 8b 54 24 10 mov 0x10(%esp),%edx#arg2 805005f: 8b 4c 24 0c mov 0xc(%esp),%ecx# arg1 8050063: 8b 5c 24 08 mov 0x8(%esp),%ebx# arg0 8050067: b8 04 00 00 00 mov $0x4,%eax # syscall no 805006c: cd 80 int $0x80 805006e: 5b pop %ebx 805006f: 3d 01 f0 ff ffcmp $0xfffff001,%eax 8050074: 0f 83 56 1e 00 00 jae 8051ed0 <__syscall_error> 805007a: c3 ret CS 3214 Fall 2010

12. user mode kernel mode KernelThreads Most OS support kernel threads that never run in user mode – these threads typically perform book keeping or other supporting tasks. They do not service system calls or faults. Process 1 Process 2 Kernel Kernel Thread Careful: “kernel thread” not the same as kernel-level thread (KLT) – more on KLT later CS 3214 Fall 2010

13. Context vs Mode Switching • Mode switch guarantees kernel gains control when needed • To react to external events • To handle error situations • Entry into kernel is controlled • Not all mode switches lead to context switches • Kernel decides when – subject of scheduling policies • Mode switch does not change the identity of current process/thread • See blue/yellow colors in slide on ctxt switch details • Hardware knows about modes, does not (typically) know about contexts CS 3214 Fall 2010

14. RUNNING Scheduler picks process Process must wait for event Process preempted BLOCKED READY Event arrived Reasoning about Processes:Process States • Only 1 process (per CPU) can be in RUNNING state CS 3214 Fall 2010

15. Process States • RUNNING: • Process is on CPU, its instructions are executed • READY: • Process could make progress if a CPU were available • BLOCKED: • Process cannot make progress even if a CPU were available because it’s waiting for something (e.g., a resource, a signal, a point in time, …) • Model is simplified • OS have between 5 and 10 states typically • Terminology not consistent across OS: • E.g., Linux calls BLOCKED “SLEEPING” and READY “RUNNING” CS 3214 Fall 2010

16. User View • If process’s lifetimes overlap, they are said to execute concurrently • Else they are sequential • Default assumption is concurrently • Exact execution order is unpredictable • Programmer should never make any assumptions about it • Any interaction between processes must be carefully synchronized CS 3214 Fall 2010

17. Process Creation • Two common paradigms: • Cloning vs. spawning • Cloning: (Unix) • “fork()” clones current process • child process then loads new program • Spawning: (Windows) • “exec()” spawns a new process with new program • Difference is whether creation of new process also involves a change in program CS 3214 Fall 2010

18. #include <unistd.h> #include <stdio.h> int main() { int x = 1; if (fork() == 0) { // only child executes this printf("Child, x = %d\n", ++x); } else { // only parent executes this printf("Parent, x = %d\n", --x); } // parent and child execute this printf("Exiting with x = %d\n", x); return 0; } fork() Child, x = 2 Exiting with x = 2 Parent, x = 0 Exiting with x = 0 CS 3214 Fall 2010

19. The fork()/join() paradigm • After fork(), parent & child execute in parallel • Unlike a fork in the road, here we take both roads • Used in many contexts • In Unix, ‘join()’ is called wait() • Purpose: • Launch activity that can be done in parallel & wait for its completion • Or simply: launch another program and wait for its completion (shell does that) Parent: fork() Parent process executes Child process executes Child process exits Parent:join() OS notifies CS 3214 Fall 2010

20. fork() #include <sys/types.h> #include <unistd.h> #include <stdio.h> int main(int ac, char *av[]) { pid_t child = fork(); if (child < 0) perror(“fork”), exit(-1); if (child != 0) { printf ("I'm the parent %d, my child is %d\n", getpid(), child); wait(NULL); /* wait for child (“join”) */ } else { printf ("I'm the child %d, my parent is %d\n", getpid(), getppid()); execl("/bin/echo", "echo", "Hello, World", NULL); } } CS 3214 Fall 2010

21. fork() vs. exec() • fork(): • Clone most state of parent, including memory • Inherit some state, e.g. file descriptors • Keeps program, changes process • Called once, returns twice • exec(): • Overlays current process with new executable • Keeps process, changes program • Called once, does not return (if successful) CS 3214 Fall 2010

22. exit(3) vs. _exit(2) • exit(3) destroys current processes • OS will free resources associated with it • E.g., closes file descriptors, etc. etc. • Can have atexit() handlers • _exit(2) skips them • Exit status is stored and can be retrieved by parent • Single integer • Convention: exit(EXIT_SUCCESS) signals successful execution, where EXIT_SUCCESS is 0 CS 3214 Fall 2010

23. wait() vs waitpid() • int wait(int *status) • Blocks until any child exits • If status != NULL, will contain value child passed to exit() • Return value is the child pid • Can also tell if child was abnormally terminated • intwaitpid(pid_tpid, int *status, int options) • Can say which child to wait for CS 3214 Fall 2010

24. Wait Example void fork10() { pid_t pid[N]; int i; int child_status; for (i = 0; i < N; i++) if ((pid[i] = fork()) == 0) exit(100+i); /* Child */ for (i = 0; i < N; i++) { pid_t wpid = wait(&child_status); if (WIFEXITED(child_status)) printf("Child %d terminated with exit status %d\n", wpid, WEXITSTATUS(child_status)); else printf("Child %d terminate abnormally\n", wpid); } } If multiple children completed, will take in arbitrary order • Can use macros WIFEXITED and WEXITSTATUS to get information about exit status CS 3214 Fall 2010

25. Observations on fork/exit/wait • Process can have many children at any point in time • Establishes a parent/child relationship • Resulting in a process tree • Zombies: processes that have exited, but their parent hasn’t waited for them • “Reaping a child process” – call wait() so that zombie’s resources can be destroyed • Orphans: processes that are still alive, but whose parent has already exited (without waiting for them) • Become the child of a dedicated process (“init”) who will reap them when they exit • “Run Away” processes: processes that (unintentionally) execute an infinite loop and thus don’t call exit() or wait() CS 3214 Fall 2010