CS 3214 Introduction to Computer Systems Lecture 12 Godmar Back

Announcements Read Chapter 8 Exercise 8 due Oct 11 Recommended Reading: David M. Beazley, Brian D. Ward, Ian R. Cooke, "The Inside Story on Shared Libraries and Dynamic Loading," Computing in Science and Engineering, vol. 3, no. 5, pp. 90-97, Sep./Oct. 2001, doi:10.1109/5992.947112 Midterm: Tuesday Oct 20 CS 3214 Fall 2009 4/18/2018

Part 1 Threads and Processes CS 3214 Fall 2009 4/18/2018

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 2009 4/18/2018

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 2009 4/18/2018

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 transitions between user & kernel mode int main() { asm(“hlt”); } CS 3214 Fall 2009 4/18/2018

Mode Switching User  Kernel mode External (aka hardware) interrupt: 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 2009 4/18/2018

A Context Switch Scenario Timer interrupt: P1 is preempted, context switch to P2 I/O device interrupt: P2’s I/O complete switch back to P2 Process 1 Process 2 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 Kernel CS 3214 Fall 2009 4/18/2018

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 user mode kernel mode Kernel switch_threads: (in) (saves caller’s state) switch_threads: (out) (restores caller’s state) (kernel stack switch) CS 3214 Fall 2009 4/18/2018

System Calls 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. Process 1 user mode kernel mode Kernel Kernel’s System Call Implementation CS 3214 Fall 2009 4/18/2018

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, sizeof msg); } /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 ff cmp $0xfffff001,%eax 8050074: 0f 83 56 1e 00 00 jae 8051ed0 <__syscall_error> 805007a: c3 ret CS 3214 Fall 2009 4/18/2018

Kernel Threads 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 user mode kernel mode Kernel Kernel Thread Careful: “kernel thread” not the same as kernel-level thread (KLT) – more on KLT later CS 3214 Fall 2009 4/18/2018

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 Colors in previous slide Hardware knows about modes, does not (typically) know about contexts CS 3214 Fall 2009 4/18/2018

Reasoning about Processes: Process States RUNNING READY BLOCKED Process must wait for event Event arrived Scheduler picks process preempted Only 1 process (per CPU) can be in RUNNING state CS 3214 Fall 2009 4/18/2018

Process States RUNNING: READY: BLOCKED: Model is simplified 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 2009 4/18/2018

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 2009 4/18/2018

Process Creation Two common paradigms: Cloning: (Unix) 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 2009 4/18/2018

fork() #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 2009 4/18/2018

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 2009 4/18/2018

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 2009 4/18/2018

fork() vs. exec() fork(): exec(): 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 2009 4/18/2018

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 2009 4/18/2018

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 int waitpid(pid_t pid, int *status, int options) Can say which child to wait for CS 3214 Fall 2009 4/18/2018

If multiple children completed, will take in arbitrary order Can use macros WIFEXITED and WEXITSTATUS to get information about exit status 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); } Wait Example

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 2009 4/18/2018