course: operating systems instructor: umar kalim nust institute of information technology, pakistan ...
DESCRIPTION
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 executingTRANSCRIPT
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
Course: Operating SystemsInstructor: Umar Kalim
NUST Institute of Information Technology, Pakistanhttp://www.niit.edu.pk
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
Course: Operating SystemsInstructor: Umar Kalim
NUST Institute of Information Technology, Pakistanhttp://www.niit.edu.pk
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
Course: Operating SystemsInstructor: Umar Kalim
NUST Institute of Information Technology, Pakistanhttp://www.niit.edu.pk
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
Course: Operating SystemsInstructor: Umar Kalim
NUST Institute of Information Technology, Pakistanhttp://www.niit.edu.pk
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
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