course: operating systems instructor: umar kalim nust institute of information technology, pakistan ...

Course: Operating SystemsInstructor: Umar Kalim

NUST Institute of Information Technology, Pakistanhttp://www.niit.edu.pk

Operating Systems

Agenda ~ Process Management• Processes

− Process Concept− Process Scheduling− Operations on Processes− Cooperating Processes− Inter-process Communication

Process Concept• An operating system executes a variety of programs:

− Batch system – jobs− Time-shared systems – user programs or tasks− Textbook uses the terms job and process almost

interchangeably• Process – a program in execution; a unit of work;

process execution must progress in sequential fashion− Earlier OSes ~ only one program to be executed

• Past: program had complete control over the system• Now: multiple programs loaded and executing concurrently

− Finer control and compartmentalization is required• Resources to be allocated

− CPU time, memory, files and I/O Devices− These resources are allocated to the process either when

it is created or while it is executing

Process in Memory• A process is more than

program code (a.k.a. text section)

• It includes:− program counter − stack− data section

• global variables− heap

• Variables required for the scope of the program ~ allocated dynamically; at runtime

Process is not same as a program• A program is a passive entity, a process is an

active entity− With a program counter specifying the next

instruction to execute and a set of associated resources

• Program ~ *.exe• Although two processes may be associated with

the same program, they are nevertheless considered two separate execution sequences− Several users may be running different copies of the

mail program− The same user may invoke many copies of the editor

program− Each of these is a separate process

• Text sections are equivalent, the data sections carry

Process State• As a process executes, it changes state

− new: The process is being created− running: Instructions are being executed− waiting: The process is waiting for some event to

occur− ready: The process is waiting to be assigned to a

processor− terminated: The process has finished execution

The ps Command• Shows the snapshot of the current processes

− bash$ ps− To view documentation

• bash$ man ps− To view the status of current processes, execute

• bash$ ps -S• Look for entries under STAT header PID TTY STAT TIME COMMAND 3361 pts/2 Ss 0:00 /bin/bash 3589 pts/2 R+ 0:00 \_ ps -Sf

ps -S (status codes)Here are the different values that the s, stat and state output specifiers(header "STAT" or "S") will display to describe the state of a process.D Uninterruptible sleep (usually IO)R Running or runnable (on run queue)S Interruptible sleep (waiting for an event to complete)T Stopped, either by a job control signal or because it is being traced.W paging (not valid since the 2.6.xx kernel)X dead (should never be seen)Z Defunct ("zombie") process, terminated but not reaped by its parent.

For BSD formats and when the stat keyword is used, additional characters may

be displayed:< high-priority (not nice to other users)N low-priority (nice to other users)L has pages locked into memory (for real-time and custom IO)s is a session leaderl is multi-threaded (using CLONE_THREAD, like NPTL pthreads do)+ is in the foreground process group

ps -S• Try

− ps -s− ps -as− pstree− X− ps -A S

[umar@dexter ~]$ ps

PID TTY TIME CMD

3361 pts/2 00:00:00 bash

3836 pts/2 00:00:00 ps

[umar@dexter ~]$ ps -S PID TTY STAT TIME COMMAND

3361 pts/2 Ss 0:01 /bin/bash

3611 pts/3 Ss 0:00 /bin/bash

3630 pts/3 S+ 0:00 man ps

3633 pts/3 S+ 0:00 sh -c (cd /usr/share/man && (echo ".pl 1100i"; /usr/b

3634 pts/3 S+ 0:00 sh -c (cd /usr/share/man && (echo ".pl

1100i"; /usr/b

3641 pts/3 S+ 0:00 /usr/bin/less -is 3764 pts/4 Ss+ 0:00 /bin/bash

3837 pts/2 R+ 0:00 ps -S

All system processes • [umar@dexter ~]$ ps -A S• PID TTY STAT TIME COMMAND• 1 ? S 0:10 init [5]• 2 ? SN 0:00 [ksoftirqd/0]• 3 ? S 0:00 [watchdog/0]• 4 ? S< 0:00 [events/0]• 5 ? S< 0:00 [khelper]• 6 ? S< 0:00 [kthread]• 8 ? S< 0:00 [kacpid]• 99 ? S< 0:00 [kblockd/0]• 148 ? S 0:00 [pdflush]• 149 ? S 0:00 [pdflush]• 151 ? S< 0:00 [aio/0]• 150 ? S 0:00 [kswapd0]• 102 ? S 0:00 [khubd]• 238 ? S 0:00 [kseriod]• 459 ? S 0:00 [kjournald]• 988 ? S<s 0:05 udevd• 1382 ? S< 0:00 [khsfd/modem]• 1602 ? S< 0:00 /usr/bin/perl /usr/sbin/hsfdcpd 0• 1612 ? S 0:00 [shpchpd_event]

Process Control Block (PCB)Information associated with

each process• Process state• Process number• Program counter• CPU registers• CPU scheduling information• Memory-management

information• Accounting information• I/O status information

Top command[umar@dexter ~]$ toptop - 19:52:13 up 54 min, 5 users, load average: 0.36, 0.44, 0.41Tasks: 96 total, 1 running, 95 sleeping, 0 stopped, 0 zombieCpu(s): 2.0% us, 0.7% sy, 0.0% ni, 96.7% id, 0.0% wa, 0.7% hi, 0.0% siMem: 450176k total, 385664k used, 64512k free, 19508k buffersSwap: 554200k total, 0k used, 554200k free, 211736k cached

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 3060 root 15 0 94996 16m 2428 S 1.3 3.7 6:00.22 X 3360 umar 15 0 33564 17m 13m S 0.3 4.1 0:05.87 konsole 3485 umar 15 0 289m 106m 46m S 0.3 24.3 4:24.84 soffice.bin 4135 umar 16 0 2020 984 784 R 0.3 0.2 0:00.24 top 1 root 16 0 1748 568 492 S 0.0 0.1 0:01.07 init 2 root 34 19 0 0 0 S 0.0 0.0 0:00.00 ksoftirqd/0 3 root RT 0 0 0 0 S 0.0 0.0 0:00.00 watchdog/0 4 root 10 -5 0 0 0 S 0.0 0.0 0:00.06 events/0 5 root 11 -5 0 0 0 S 0.0 0.0 0:00.02 khelper 6 root 10 -5 0 0 0 S 0.0 0.0 0:00.00 kthread

CPU Switch From Process to Process• 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



Process Scheduling

Process Scheduling Queues• Multiprogramming ~ Process scheduler

• Usually implemented as linked lists

• 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

• Processes migrate among the various queues

Ready Queue And Various I/O Device Queues

Representation of Process Scheduling

Schedulers• A process migrates between various scheduling

queues throughout its lifetime− The OS must select processes from these

queues in some fashion• Long-term scheduler (or job scheduler) –

selects which processes should be brought into the ready queue− Long-term scheduler is invoked very infrequently

(seconds, minutes) (may be slow) ~ when process exits

• Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU− Short-term scheduler is invoked very frequently

(milliseconds) (must be fast)

Schedulers (Cont.)• 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

• Long-term scheduler should select a good process mix of IO bound and CPU bound processes− If all processes are IO bound, the ready queue will

almost always be empty, short term scheduler will have little to do

− If all processes are CPU bound, the IO waiting queue will almost always be empty, devices will go unused

Addition of Medium Term Scheduling• Medium term scheduler

− Removes processes form memory (and active CPU contention) (Swapping)

− Reduces the degree of multiprogramming



Operations on Processes

Process Creation• Parent process 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

• Restricting a child process to a subset of the parent’s resources prevents any process from overloading the system by creating many sub-processes

• 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

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);

}}

A tree of processes on a typical Solaris

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

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

− All children terminated - cascading termination



Inter-process Communication

Cooperating Processes• Independent process cannot affect or be

affected by the execution of another process• Cooperating process can affect or be affected

by the execution of another process• Advantages of process cooperation

− Information sharing − Computation speed-up− Modularity

• Convenience

Communications Models

Producer-Consumer Problem• One method could be to use shared memory

• Paradigm for cooperating processes, producer process produces information that is consumed by a consumer process− unbounded-buffer places no practical limit on the

size of the buffer− bounded-buffer assumes that there is a fixed

buffer size

Bounded-Buffer – Shared-Memory Solution• Shared data

#define BUFFER_SIZE 10typedef struct {. . .

} item;

item buffer[BUFFER_SIZE];int in = 0; /*next free location*/int out = 0; /*first full location*/

• Solution is correct, but can only use BUFFER_SIZE-1 elements

Bounded-Buffer – Insert() Method

while (true) { /* Produce an item */

while (((in = (in + 1) % BUFFER SIZE count) == out)

; /* do nothing – no free buffers */

buffer[in] = item; in = (in + 1) % BUFFER SIZE;

}

Bounded Buffer – Remove() Methodwhile (true) {

while (in == out) ; // do nothing –

nothing to consume

// remove an item from the buffer item = buffer[out]; out = (out + 1) % BUFFER SIZE;return item;

}

Communications Models

Inter-process Communication (IPC)• Mechanism for processes to communicate and

to synchronize their actions• Message system – processes communicate with

each other without resorting to shared variables• IPC facility provides two operations:

− send(message) – message size fixed or variable − receive(message)

• If P and Q wish to communicate, they need to:− establish a communication link between them− exchange messages via send/receive

• Implementation of communication link− physical (e.g., shared memory, hardware bus)− logical (e.g., logical properties)

Implementation Questions• How are links established?• Can a link be associated with more than two

processes?• How many links can there be between every

pair of communicating processes?• What is the capacity of a link?• Is the size of a message that the link can

accommodate fixed or variable?• Is a link unidirectional or bi-directional?

Direct Communication ~ Naming• Processes must name each other explicitly:

− send (P, message) – send a message to process P− receive(Q, message) – receive a message from

process Q• Properties of communication link

− Links are established automatically− A link is associated with exactly one pair of

communicating processes− Between each pair there exists exactly one link− The link may be unidirectional, but is usually bi-

directional

Indirect Communication• Messages are directed and received from

mailboxes (also referred to as ports)− Each mailbox has a unique id− Processes can communicate only if they share a

mailbox• Properties of communication link

− Link established only if processes share a common mailbox

− A link may be associated with many processes− Each pair of processes may share several

communication links− Link may be unidirectional or bi-directional

Indirect Communication• Operations

− create a new mailbox− send and receive messages through mailbox− destroy a mailbox

• Primitives are defined as:send(A, message) – send a message to mailbox Areceive(A, message) – receive a message from mailbox A

Indirect Communication• Mailbox sharing

− P1, P2, and P3 share mailbox A− P1, sends; P2 and P3 receive− Who gets the message?

• Solutions− Allow a link to be associated with at most two

processes− Allow only one process at a time to execute a

receive operation− Allow the system to select arbitrarily the receiver.

Sender is notified who the receiver was.

Synchronization• Message passing may be either blocking or non-

blocking• Blocking is considered synchronous

− Blocking send has the sender block until the message is received

− Blocking receive has the receiver block until a message is available

• Non-blocking is considered asynchronous− Non-blocking send has the sender send the

message and continue− Non-blocking receive has the receiver receive a

valid message or null

Buffering• Queue of messages attached to the link;

implemented in one of three ways1.Zero capacity – 0 messages

Sender must wait for receiver (rendezvous)2.Bounded capacity – finite length of n messages

Sender must wait if link full3.Unbounded capacity – infinite length

Sender never waits



Questions?

•Recommended Reading:− Book ~ 102 - 108− OSRC

• http://www.nondot.org/sabre/os/articles− C. A. R. Hoare, " Communicating Sequential Processes ,"

Communications of the ACM, Vol. 21, No. 8, August 1978, pp. 666-677.

• http://www.csie.fju.edu.tw/~yeh/research/papers/os-reading-list/hoare-cacm78-csp.pdf

− B. N. Bershad, et. al., " User-level Interprocess Communication for Shared Memory Multiprocessors ," ACM Transactions on Computer Systems, Vol. 9, No. 2, May 1991, pp. 175-198.

• http://www.csie.fju.edu.tw/~yeh/research/papers/os-reading-list/bershad-tocs91-uipc.pdf

http://www.csie.fju.edu.tw/~yeh/research/papers/os-reading-list/hoare-cacm78-csp.pdf

http://www.csie.fju.edu.tw/~yeh/research/papers/os-reading-list/hoare-cacm78-csp.pdf

All system processes • 1709 ? S 0:00 [pccardd]• 1716 ? S 0:00 [pccardd]• 1759 ? S 0:00 [khpsbpkt]• 1773 ? S 0:00 [knodemgrd_0]• 2062 ? Ss 0:00 /sbin/cardmgr• 2321 ? Ss 0:00 syslogd -m 0• 2323 ? Ss 0:00 klogd -x• 2333 ? Ss 0:00 portmap• 2351 ? Ss 0:00 rpc.statd• 2366 ? S<sl 0:00 auditd• 2370 ? S< 0:00 [kauditd]• 2394 ? Ss 0:00 rpc.idmapd• 2407 ? Ss 0:00 hcid: processing events• 2411 ? Ss 0:00 sdpd• 2430 ? S< 0:00 [krfcommd]• 2562 ? Ss 0:00 /usr/sbin/automount --timeout=60 /misc file

/etc/auto.misc• 2601 ? Ss 0:00 /usr/sbin/automount --timeout=60 /net program

/etc/auto.net• 2614 ? Ss 0:01 nifd -n• 2646 ? Ssl 0:00 mDNSResponder•

All system processes • 2655 ? Ss 0:00 /usr/sbin/acpid• 2683 ? Ss 0:00 cupsd• 2731 ? Ss 0:00 /usr/sbin/sshd• 2748 ? Ss 0:00 sendmail: accepting connections• 2754 ? Ss 0:00 sendmail: Queue runner@01:00:00 for

/var/spool/clientmqueue• 2763 ? Ss 0:00 gpm -m /dev/input/mice -t imps2• 2771 ? Ss 0:00 crond• 2792 ? Ss 0:00 xfs -droppriv -daemon• 2800 ? SNs 0:00 anacron -s• 2807 ? Ss 0:00 /usr/sbin/atd• 2815 ? Ss 0:00 dbus-daemon --system• 2826 ? Ss 0:00 cups-config-daemon• 2835 ? Ss 0:00 hald --retain-privileges• 2840 ? S 0:00 hald-addon-acpi• 2852 ? S 0:00 hald-addon-storage

All system processes • 2867 tty1 Ss+ 0:00 /sbin/mingetty tty1• 2868 tty2 Ss+ 0:00 /sbin/mingetty tty2• 2869 tty3 Ss+ 0:00 /sbin/mingetty tty3• 2870 tty4 Ss+ 0:00 /sbin/mingetty tty4• 2871 tty5 Ss+ 0:00 /sbin/mingetty tty5• 2872 tty6 Ss+ 0:00 /sbin/mingetty tty6• 2873 ? Ss 0:00 /bin/sh /etc/X11/prefdm -nodaemon• 3016 ? S 0:00 /usr/bin/gdm-binary -nodaemon• 3055 ? S 0:00 /usr/bin/gdm-binary -nodaemon• 3060 ? S 3:59 /usr/X11R6/bin/X :0 -audit 0 -auth /var/gdm/:0.Xauth -

nolisten tcp vt7• 3128 ? Ss 0:00 /bin/sh /usr/bin/startkde• 3173 ? Ss 0:00 /usr/bin/ssh-agent /usr/bin/dbus-launch --exit-with-

session /home/umar/.Xclients• 3176 ? S 0:00 /usr/bin/dbus-launch --exit-with-session

/home/umar/.Xclients• 3177 ? Ss 0:00 dbus-daemon --fork --print-pid 8 --print-address 6 --

session• 3221 ? Ss 1:04 kdeinit Running...• 3224 ? S 0:00 dcopserver [kdeinit] --nosid• 3226 ? S 0:00 klauncher [kdeinit]

All system processes • 3229 ? Sl 0:03 kded [kdeinit]• 3231 ? S 0:01 /usr/libexec/gam_server• 3242 ? S 0:00 /usr/bin/artsd -F 10 -S 4096 -s 60 -m artsmessage -c

drkonqi -l 3 -f• 3244 ? S 0:00 kaccess [kdeinit]• 3245 ? S 0:00 kwrapper ksmserver• 3247 ? S 0:00 ksmserver [kdeinit]• 3248 ? S 0:02 kwin [kdeinit] -session

10138e4e6d1000114155882700000171980000_1141622029_439608• 3250 ? S 0:02 kdesktop [kdeinit]• 3253 ? S 0:20 kicker [kdeinit]• 3259 ? S 0:00 /usr/bin/pam-panel-icon --sm-client-id

10138e4e6d1000114155882900000171980006• 3260 ? S 0:00 /sbin/pam_timestamp_check -d root• 3261 ? S 0:00 eggcups --sm-config-prefix /eggcups-ZAA9c0/ --sm-

client-id 10138e4e6d100011415588300• 3263 ? S 0:01 /usr/bin/autorun -l --interval=1000

--cdplayer=/usr/bin/kscd• 3266 ? S 0:00 /usr/libexec/gconfd-2 14• 3273 ? S 0:04 konqueror [kdeinit] -mimetype inode/directory

file:///home/umar

All system processes • 3277 ? S 0:00 kio_file [kdeinit] file /tmp/ksocket-umar/klauncherB8ipNa.slave-socket /tmp/ksocket-s• 3360 ? S 0:03 konsole [kdeinit]• 3361 pts/2 Ss 0:01 /bin/bash• 3474 ? S 0:00 /bin/sh /usr/lib/openoffice.org1.9.104/program/soffice -draw /home/umar/ch4-

processes• 3485 ? Sl 2:27 /usr/lib/openoffice.org1.9.104/program/soffice.bin -draw /home/umar/ch4-

processes.sxd• 3611 pts/3 Ss 0:00 /bin/bash• 3630 pts/3 S+ 0:00 man ps• 3633 pts/3 S+ 0:00 sh -c (cd /usr/share/man && (echo ".pl 1100i"; /usr/bin/gunzip -c '/usr/share/man/ma• 3634 pts/3 S+ 0:00 sh -c (cd /usr/share/man && (echo ".pl 1100i"; /usr/bin/gunzip -c '/usr/share/man/ma• 3641 pts/3 S+ 0:00 /usr/bin/less -is• 3657 ? S 0:00 knotify [kdeinit]• 3764 pts/4 Ss 0:00 /bin/bash• 3855 pts/4 S 0:00 su• 3858 pts/4 S+ 0:00 bash• 3895 pts/2 R+ 0:00 ps -A S

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

course: operating systems instructor: umar kalim nust institute of information technology, pakistan ...

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