Processes

Dr. Yingwu Zhu

Process Concept

• Process – a program in execution– What is not a process? -- program on a disk - a

process is an active object, but a program is just a file

• A process includes:– program counter – Text section: program code– Stack: local variables, function params, return

addresses– data section: global variables– Heap (optional): dynamically allocated

memory

Process State

• Processes switch between different states based on internal and external events

• Each process is in exactly one state at a time• As a process executes, it changes state

(Typical States of Processes (varies with OS))– new: The process is being created– running: Instructions are being executed (only

one process per processor may be running)– waiting: The process is waiting for some event

(e.g., I/O, signals) to occur– ready: The process is waiting to be assigned to a

processor– terminated: The process has finished execution

Diagram of Process State

CPU Switch From Process to Process

Process Control Block (PCB)-- PCB Stores all of the information about a processInformation associated with each process• Process state• Program counter• CPU registers: accumulators, index registers, stack

pointers, etc.• CPU scheduling information: priority, etc.• Memory-management information: base/limit,

page tables, or segment tables• Accounting information: CPU, etc• I/O status information: a list of I/O devices

allocated, a list of open files, etc.

Process Control Block (PCB)

Maintaining PCBs

• Keep track of the different processes in the system• Collection of PCBs is called a process table• How to store the process table?• Option 1:

• Problems with Option 1:– hard to find processes– how to fairly select a process

P1 P2 P2 P3 P4 P5

Ready Waiting Waiting ReadyNew Term

Solution: Process Scheduling Queues

• Store processes in queues based on state• Processes migrate among the various queues• Job queue – set of all processes in the system• Ready queue – set of all processes residing in

main memory, ready and waiting to execute• Device queues – set of processes waiting for

an I/O device

Ready Queue And Various I/O Device Queues

Representation of Process Scheduling

Schedulers

• Long-term scheduler (or job scheduler) – selects which processes should be brought

into the memory; controls degree of multiprogramming (# of processes in memory)

• Short-term scheduler (or CPU scheduler) – selects which process should be executed

next and allocates CPU

Addition of Medium-Term Scheduling

Swap in/out processes memory, adaptive to memory status and process status

Schedulers (Cont.)• Short-term scheduler is invoked very frequently

(milliseconds) (must be fast)• Long-term scheduler is invoked very infrequently

(seconds, minutes) (may be slow)• The long-term scheduler controls the degree of

multiprogramming• Processes can be described as either:

– I/O-bound process – spends more time doing I/O than computations, many short CPU bursts

– CPU-bound process – spends more time doing computations; few very long CPU bursts

Short-Term Scheduler• Responsible for:

– saving state into PCB when switching to a new process

– selecting a process to run (from the ready queue)– loading state of another process

• One of the most time critical parts of the OS

Selecting a Process to Run• called scheduling• can simply pick the first item in the queue

– called round-robin scheduling– is round-robin scheduling fair?

• can use more complex schemes– we will study these in the future

• use alarm interrupts to switch between processes– when time is up, a process is put back on the end of the

ready queue– frequency of these interrupts is an important parameter

• typically 3-10ms on modern systems• need to balance overhead of switching vs. responsiveness

Context Switch• When CPU switches to another process, the

system must save the state of the old process and load the saved state for the new process

• Context-switch time is overhead; the system does NO useful work while switching

• Time dependent on hardware support– Depends on

• Memory speed, #-of-registers to copy, special instructions (single instruction to load/save all registers)

• A few milliseconds

Process Creation

• Parent processes create children processes, which, in turn create other processes, forming a tree of processes

• Resource sharing– Parent and children share all resources– Children share subset of parent’s resources– Parent and child share no resources

• Execution– Parent and children execute concurrently– Parent waits until children terminate

Process Creation (Cont.)

• Address space– Child duplicate of parent– Child has a program loaded into it

• UNIX examples– fork system call creates new process– exec system call used after a fork to replace the

process’ memory space with a new program

Forking a New Process• create a PCB for the new process

– copy most entries from the parent– clear accounting fields– buffered pending I/O– allocate a pid (process id for the new process)

• allocate memory for it– could require copying all of the parents segments– however, text segment usually doesn’t change so that

could be shared– might be able to use memory mapping hardware to help

• will talk more about this in the memory management part of the class

• add it to the ready queue

C Program Forking Separate Processint main(){ pid_t pid;

/* fork another process */pid = fork();if (pid < 0) { /* error occurred */

fprintf(stderr, "Fork Failed");exit(-1);

}else if (pid == 0) { /* child process */

execlp("/bin/ls", "ls", NULL);}else { /* parent process */

/* parent will wait for the child to complete */

wait (NULL);printf ("Child Complete");exit(0);

}}

Process Creation

Process Termination• Process executes last statement and asks the operating

system to delete it (exit)– Output data from child to parent (via wait)– Process’ resources are deallocated by operating system

• Parent may terminate execution of children processes (abort)– Child has exceeded allocated resources– Task assigned to child is no longer required– If parent is exiting orphan process

• Some operating system do not allow child to continue if its parent terminates

– All children terminated - cascading termination (VMS)• in UNIX becomes child of the root process

Process Termination - UNIX example

• Kernel– frees memory used by the process– moved PCB to the terminated queue

• Terminated process – signals parent of its death (SIGCHILD)– is called a zombie in UNIX– remains around waiting to be reclaimed

• parent process– wait system call retrieves info about the dead process

• exit status• accounting information

– signal handler is generally called the reaper• since its job is to collect the dead processes

Linux case: controlling processes

part of command Meaning

regular_command Runs this command in the foreground.

command &Run this command in the background (release the

terminal)

jobs Show commands running in the background.

Ctrl+ZSuspend (stop, but not quit) a process running in

the foreground (suspend).

Ctrl+CInterrupt (terminate and quit) a process running

in the foreground.

%n

Every process running in the background gets a number assigned to it. By using the % expression a job can be referred to using its number, for instance fg %2.

bg Reactivate a suspended program in the background.

fg Puts the job back in the foreground.

killEnd a process (also see Shell Builtin Commands in

the Info pages of bash)

Linux: Process Attributes• The process ID or PID: a unique identification number used to refer to

the process.• The parent process ID or PPID: the number of the process (PID) that

started this process.• Nice number: the degree of friendliness of this process toward other

processes (not to be confused with process priority, which is calculated based on this nice number and recent CPU usage of the process).

• Terminal or TTY: terminal to which the process is connected.• User name of the real and effective user (RUID and EUID): the owner of

the process. The real owner is the user issuing the command, the effective user is the one determining access to system resources. RUID and EUID are usually the same, and the process has the same access rights the issuing user would have.

• Real and effective group owner (RGID and EGID): The real group owner of a process is the primary group of the user who started the process. The effective group owner is usually the same, except when SGID access mode has been applied to a file.

• Cmd: ps -af

• ps -ef | grep username – This displays all processes owned by a particular

user

• pstree

Case Study: Unix Processes

• See Supl. Materials online

Exercise

• How many processes for the segment of code int main() { for (int m = 0; m<3; m++) pid_t x = fork(); //assume success all the times

return 0; }