SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong (jinkyu@skku.edu)

Processes

Jinkyu Jeong (jinkyu@skku.edu)

Computer Systems Laboratory

Sungkyunkwan University

http://csl.skku.edu

SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong (jinkyu@skku.edu) 2

OS Internals

Hardware

System Call Interface

shellshell

lsps

Hardware Control (Interrupt handling, etc.)

File SystemManagement

I/O Management(device drivers)

MemoryManagement

ProcessManagement P

rote

ction

Kernelspace

Userspace trap

scheduler

IPC

synchronization

SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong (jinkyu@skku.edu) 3

What is a Process?

• An instance of a program in execution

• The basic unit of protection

• A process is identified using its process ID

(PID)

• A process includes

– CPU context (registers)

– OS resources (address space, open files, etc.)

– Other information (PID, state, owner, etc.)

SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong (jinkyu@skku.edu) 4

Process vs. Program

Stack

Heap

program

Code

Data

code

data

PC

SP

Memory Disk

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Running a Process

Code

Data

Fetch I Mem[PC]

Decode I

Execute I

Update PC

PC

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Running Multiple ProcessesCode A

Data A

Code C

Data C

Code B

Data B

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Interleaving Multiple Processes

Code A

Data A

Code C

Data C

Code B

Data B

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Virtualizing the CPU

Code A

Data A

OS Code

OS Data

Code B

Data B

OS creates the illusion

that each process has its

own CPU (and memory)

SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong (jinkyu@skku.edu) 9

Example#include <sys/types.h>#include <unistd.h>

int main(){

int pid;

if ((pid = fork()) == 0)/* child */printf (“Child of %d is %d\n”,

getppid(), getpid());else

/* parent */printf (“I am %d. My child is %d\n”,

getpid(), pid);}

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Example Output

% ./a.out

I am 31098. My child is 31099.

Child of 31098 is 31099.

% ./a.out

Child of 31100 is 31101.

I am 31100. My child is 31101.

SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong (jinkyu@skku.edu) 11

Process Hierarchy

• Parent-child relationship– One process can create another

process

– Unix calls the hierarchy a “process group”

– Windows has no concept of process hierarchy

• Browsing a list of processes:– ps in Unix

– Task Manager (taskmgr) in Windows

sh

$ cat file1 | wc

cat wc

SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong (jinkyu@skku.edu) 12

Process Creation

• fork()– Creates a new process cloning the parent process• Parent inherits most of resources and privileges: open files, UID,

etc.• Child also duplicates the parent’s address space

– Parent may either wait for the child to finish (using wait()), or it may continue in parallel

– Shells or GUIs use this system call internally

• exec()– Replaces the current process image with a new program

– Windows: CreateProcess() = fork() + exec()

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Process Termination

• Normal exit (c)

• Error exit (voluntary)

• Fatal error (involuntary)– Segmentation fault – illegal memory access

– Protection fault

– Exceed allocated resources, etc.

• Killed by another process (involuntary)– By receiving a signal

• Zombie process: terminated, but not removed

SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong (jinkyu@skku.edu) 14

Process APIs

• exit()– Caller process terminates its execution

– Return from main() is identical

– Parameter denotes whether this exit is normal (0) or error (1)

• kill()– Send signal to a process

– Parameters are directives to go to sleep, to die, and other useful imperatives

• wait()– Wait for state changes in any child of the calling process• Usually termination of a child

– Release resources associated with the terminated child

– waitpid() for a specific child

SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong (jinkyu@skku.edu) 15

Simplified Shellint main(void){char cmdline[MAXLINE];char *argv[MAXARGS];pid_t pid;int status;

while (getcmd(cmdline, sizeof(buf)) >= 0) {parsecmd(cmdline, argv);if (!builtin_command(argv)) {if ((pid = fork()) == 0) {

if (execv(argv[0], argv) < 0) {printf(“%s: command not found\n”, argv[0]);exit(0);

}}waitpid(pid, &status, 0);

}}

}

SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong (jinkyu@skku.edu) 16

Process State Transitions

RunningReady

Scheduled

Time slice exhausted

I/O or event wait

Blocked

I/O or event completion

Created exit

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Process State – Linux Example

RunnableSleepingTraced or StoppedUninterruptible SleepZombie

R:S:T:

D:

Z:

High-priority taskLow-priority taskSession leaderIn the foregroundprocess groupMulti-threaded

<:N: s:+:

l:

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Implementing Processes

• PCB (Process Control Block) or Process Descriptor

– Each PCB represents a process

– Contains all of the information about a process

• CPU registers

• PID, PPID, process group, priority, process state, signals

• CPU scheduling information

• Memory management information

• Accounting information

• File management information

• I/O status information

• Credentials

– task_struct in Linux

• 3248 bytes as of Linux 3.2.0

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Context Switch

• The act of switching CPU from one process to another

• Administrative overhead– Saving and restoring registers and memory maps

– Flushing and reloading the memory cache

– Updating various tables and lists, etc.

• The overhead depends on hardware support– Multiple register sets in UltraSPARC

– Advanced memory management techniques may require extra data to be switched with each context

• 100s or 1000s of switches/sec typically

SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong (jinkyu@skku.edu) 20

Context Switch – Linux Example• Linux example– Total 1,693,515,228 ticks = 4704 hours = 196 days

– Total 4,066,419,922 context switches

– Roughly 240 context switches / sec

SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong (jinkyu@skku.edu) 21

Performing Context Switch

handle the trapcall switch() routinesave regs(A) to PCB(A)restore regs(B) from PCB(B)switch to k-stack(B)return-from-trap (into B)

timer interruptsave regs(A) to k-stack(A)move to kernel modejump to trap handler

Process A…

Kernel Hardware Process

restore regs(B) from k-stack(B)move to user modejump to B’s IP

Process B…

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Process State Queues

• The OS maintains a collection of queues that

represent the state of all processes in the system

– Ready queue (or run queue)

– Wait queue(s): one queue for each type of event (device,

timer, message, …)

• Each PCB is queued onto a state queue according

to its current state

– As a process changes state, its PCB is migrated between

the various queues

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Process State Queues

ready queue

disk I/O queue

time slice expired

fork a child

keyboard I/O queue

mutex wait queue

exitscheduled

disk interrupt

keyboard interrupt

mutex acquired

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fork()

• Implementing fork()

– Creates and initializes a new PCB

– Creates and initializes a new address space

– Initializes the address space with a copy of the entire

contents of the address space of the parent

– Initializes the kernel resources to point to the resources

used by the parent (e.g. open files)

– Places the new PCB on the ready queue

– Returns the child’s PID to the parent, and zero to the child

int fork()

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exec()

• Implementing exec()– Wipe the process’ address space and release most of the

resources allocated to the process

– Loads the new program “prog” into the its address space

– Initializes hardware context and “args” for the new program

– exec() does not create a new process

– What does it mean for exec() to return?

• Variants– exec(): execl(), execle(), execlp(), execv(), and execvp()

int execv(char *prog, char *argv[])

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Inter-Process Communication

• Cooperating processes need to communicate

with each other for

– Information sharing

– Computation speedup

– Modularity

– Convenience

• Two models of IPC

– Shared memory

– Message passing

SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong (jinkyu@skku.edu)

IPC Models

• Message passing • Shared memory

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Policy vs. Mechanism

• Policy

– What should be done?

– Policy decisions must be made for all resource

allocation and scheduling problems

– e.g. What is the next process to run?

• Mechanism

– How to do something?

– The tool for implementing a set of policies

– e.g. How to make multiple processes run at once?

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Separating Policy from Mechanism

• A key principle in operating system design

• Policies are likely to change depending on workloads and also across places or over time

• A general mechanism, separated from policy, is more desirable

• Allows to build a more modular OS

• Enables extensible systems – User-specific policies?