Lecture 4: Processes
Fall 2019 Jason Tang
Slides based upon Operating System Concept slides, http://codex.cs.yale.edu/avi/os-book/OS9/slide-dir/index.html Copyright Silberschatz, Galvin, and Gagne, 2013 1 Topics
• Process Concept
• Process Lifecycle
• Scheduling
• Process Creation and Termination
2 Process Concept
• An OS executes processes (programs in execution)
• Text section: executable code
• Program counter and processor registers: current state of CPU
• Stack: function parameters, return addresses, local variables
• Heap: dynamically allocated variables
• Data section: global variables
3 Process Concept
• Program is passive, stored as an executable file on disk, while a process is active
• Program becomes a process when executable file loaded into memory
• Execution of program begins with a double-click, tap an icon, etc.
• Single program can spawn multiple several processes
• Such as by multiple users executing the same program
4 Process Memory Layout
For most operating systems and architectures, stack grows downwards (towards address 0).
5 Process Lifecycle
• A process transitions through several states during its lifetime
• New: process is being created, memory allocated and initialized
• Running: process is using the CPU to execute instructions
• Waiting: OS has suspended process because it is waiting for I/O, timer, or some other event to occur
• Ready: process has all necessary resources, but something else is occupying CPU currently
• Terminated: process has finished execution; OS is cleaning it up
6 Process Lifecycle
7 Process Control Block
• OS represents each process within a Process Control Block (PCB) (also known as a task)
• Process state
• Unique process identifier
• Program counter
• Scheduling information - task priority
• Memory management - memory allocated to process
8 Process Control Block
• Accounting information - CPU time spent, time elapsed
• I/O status - list of open files, network connections
• Security information - owner of process
• CPU registers - copy of registers while process is suspended
9 CPU Switch from Process to Process
10 Threads
• A process with multiple program counters, one per thread
• Threads may be simultaneously executing on multiprocessor system
• PCB contains separate sections for each thread
11 Processes in Linux
• Represented by C structure task_struct struct task_struct { … /* -1 unrunnable, 0 runnable, >0 stopped: */ volatile long state;
/* * This begins the randomizable portion of task_struct. Only * scheduling-critical items should be added above here. */ randomized_struct_fields_start
void *stack; atomic_t usage; /* Per task flags (PF_*), defined further below: */ unsigned int flags; unsigned int ptrace; … /* Filesystem information: */ struct fs_struct *fs;
/* Open file information: */ struct files_struct *files; … }
https://elixir.bootlin.com/linux/latest/source/ 12 include/linux/sched.h Process Scheduling
• While one process is waiting, OS runs another process
• Process scheduler: mechanism for how OS chooses which ready process to run next
• Scheduling queue: queues of processes, categorized by process state
• Job queue: set of all processes in system
• Ready queue: processes waiting to execute
• Device queue: processes waiting for I/O to finish
13 Process Scheduling
• Queueing diagram represents queues, resources, flows
14 Schedulers
• Short-term scheduler (or CPU scheduler) - selects which ready process to execute next
• Invoked frequently (every few milliseconds / microseconds)
• Invoked after handling an interrupt or a system call
• Long-term scheduler (or job scheduler) - selects which processes to bring into ready queue, used for batch processing
• Invoked infrequently (every few seconds or even minutes)
• Not present in all operating systems
15 Schedulers
• Medium-term scheduler - moves suspended processes from main memory to secondary storage (swap out), or vice versa (swap in)
• Sometimes combined with long-term scheduler
16 Choosing Scheduling Algorithms
• Generally, processes are of one type or another:
• I/O-bound - spends majority of time waiting for I/O or user interaction; many short CPU bursts, followed by long waiting times
• CPU-bound - spends majority of time performing calculations; few but long CPU bursts
• Schedulers must balance time spent running CPU-bound processes with interactiveness expected by I/O-bound processes
17 Context Switch
• Context switch: when CPU switches to another process, it saves state of original process (registers, program counter) to main memory, and then loads from memory the next process’s state (registers, program counters)
• Context of process represented by its PCB
• During a context switch, system is not doing any useful work
• Time to make context switch based upon hardware; some architectures faster than others
• More complex OSes have more complex context switches
• Larger context switch means slower computer response time
18 Process Creation
• Parent process creates child processes, which create grandchild processes, forming a process tree
• Each process has a unique process identifier (PID)
• Resource sharing options:
• Parent and children share all resources, or children share subset, or children share no resources
• Execution options:
• Parent and children execute concurrently, or parent waits for children to terminate
19 Example Linux Process Tree
$ pstree -h -p -G systemd(1)!"!ModemManager(344)!"!{ModemManager}(373) # $!{ModemManager}(377) %!NetworkManager(340)!"!dhclient(12487) # %!{NetworkManager}(413) # $!{NetworkManager}(416) … %!systemd(483)!"!(sd-pam)(484) # %!at-spi-bus-laun(733)!"!dbus-daemon(738) # # %!{at-spi-bus-laun}(734) # # %!{at-spi-bus-laun}(735) # # $!{at-spi-bus-laun}(736) … # %!gnome-shell-cal(813)!"!{gnome-shell-cal}(814) # # %!{gnome-shell-cal}(816) # # %!{gnome-shell-cal}(827) # # %!{gnome-shell-cal}(828) # # $!{gnome-shell-cal}(1036) # %!gnome-terminal-(1277)!"!bash(1299)!!!pstree(28960)
20 Unix Process Creation
• Typical pattern is fork() and then exec()
• fork - OS creates child process, that is duplicate of parent’s PCB
• Child PCB gains its own PID, and starts running at the parent’s PC
• execute - OS then replaces the memory space (text and data sections) with new program
21 Example Fork/Exec Pattern (Unix)
#include
int main(void) { pid_t pid = fork(); if (pid < 0) { // error occurred fprintf(stderr, "fork failed\n"); exit(EXIT_FAILURE); } else if (pid == 0) { // child process execlp("/bin/ls", "ls", NULL); fprintf(stderr, "exec failed\n"); exit(EXIT_FAILURE); } // parent process waitpid(pid, NULL, 0); printf("Child process done\n"); exit(EXIT_SUCCESS); }
22 Process Termination
• Process ends when it executes last statement, it prematurely quits (such as exit() function), or some other process forcibly terminates it (such as by sending it a kill() signal)
• OS stores return status data from child, so that parent can retrieve it later (via wait() or equivalent)
• OS releases the rest of process’s resources
• Closes files, deallocates memory, other cleanup activities
• In Linux, this process becomes a zombie until parent reaps the return value
23 Process Termination
• For some OSes, if a parent process terminates, the OS will automatically terminate its child processes (and recursively grandchildren, etc.)
• For other OSes, a parent can terminate without causing this cascading termination
• If a parent terminates without reaping its children, those child process become orphans
• In Linux, the init process (typically PID 1) becomes the orphans’ new parent
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