Lecture4-Processescpoellab/teaching/cse30341/Lecture4... · 2016-01-22 · Process Switching...

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CSE30341Opera,ngSystemPrinciples

Lecture4–Processes

Recap–LastLecture

•  Opera,ngSystemServices•  SystemCallsandInterrupts•  SystemPrograms•  Opera,ngSystemDesignandImplementa,on•  SystemLayering•  VirtualMachines

CSE30341-Opera,ngSystemPrinciples 2

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Overview

•  ProcessConcepts•  ProcessScheduling•  Opera,onsonProcesses•  Inter-processCommunica,on


ProcessConcept

•  Job/Process/Taskusedinterchangeably•  Aprocessisaninstanceofaprogram(“programinexecu,on”)

•  Program:“pieceofcode”•  Process:code+data+moredata+controlstructures+…


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ProcessMemoryLayout


•  Stack:TemporaryData•  Func,onparameters,localvariables,func,oncallframes.

•  Heap:Dynamicallyallocatedmemory.

•  Data:Globalvariables&constantstrings.

•  Text:Programcode.

Process“LifeCycle”


NEW

READY RUNNING

TERMINATED

WAITING

admibed

Interrupt

exit

Schedulerdispatch

I/OoreventwaitI/Ooreventcomple,on

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ProcessState

•  Aprocesschangesstateconstantly:– New:beingcreated.– Running:runningonaprocessorcore.– WaiAng:wai,ngforanevent.– Ready:(orrunnable)wai,ngforacore.– Terminated:finished/dead.


OSInforma,onaboutProcesses

•  Memory(Stack,Heap,Code,Sta,cData)•  CurrentState(e.g.,programcounter)•  ProcessID•  SavedRegisters•  OpenFiles•  OtherbookkeepingdataThisiscalledtheprocesscontrolblock(PCB)or

taskcontrolblock(TCB).


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ProcessControlBlock


PCBonLinux


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ProcessSwitching(“ContextSwitch”)


Process“Wait”Queues

•  JobQueueisallprocessesonthesystem.•  ReadyQueue(RunQueue)containsprocesseswai,ngtorun,orwaiAngfortheCPU!

•  DeviceQueueprocesseswai,ngforadeviceevent(“blocked”devices).

•  “Other”Queuescontainprocesseswai,ngforotherprocessestofinish,sleepingfor,me,etc.


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Process“Wait”Queues


Schedulers

•  Long-termschedulerwhichprocessesshouldberuninthefuture?– Degreeofmul,programming!

•  Short-termschedulerwhichprocessshouldberunnext?– Managesqueuesandquicklydecidesnextprocesstorun.


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SchedulingConcerns

•  IsenoughRAMavailabletosa,sfyrunningprocesses?

•  IsdevicethroughputabletosupportmoreIO-boundprocesses?

•  IsthereenoughCPU,meavailabletosa,sfyallprocesses?(long)HowdoIschedulefairly?(short)

•  Aretherebenefitsforsleeping(swapping)aprocessforanextendedAme?(long)


Schedulers

•  Short-term:invokedfrequently(milliseconds);mustbefast

•  Long-term:infrequently(seconds)•  I/O-boundprocess:spendsmore,medoingI/Othanprocessing(CPUburstscanbefrequent,butareshort)

•  CPU-bound:spendsmore,medoingcomputa,ons(verylongCPUbursts)


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“Medium”Scheduling


ProcessCrea,on

•  Aprocessisalwayscreatedviaaparent,exceptforprocess1,/sbin/init.

•  Aparentcanhavemul,plechildren.En,restructureisatree.

•  Eachprocesshasauniqueiden,fier,theprocessidenAfier,orpid(getthepidofaprocessusingthegetpid()systemcall).


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ProcessCrea,on•  Resourcesharing

1.  Parentandchildrenshareallresources2.  Childrensharesubsetofparent’sresources3.  Parentandchildsharenoresources

•  ExecuAon1.  Parentandchildrenexecuteconcurrently2.  Parentwaitsun,lchildrenterminate

•  Addressspace1.  Childisduplicateofparent2.  Childhasaprogramloadedintoit


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UNIXProcessCrea,on

•  UNIXexamples–  forksystemcallcreatesnewprocess– execsystemcallusedaperaforktoreplacetheprocess’memoryspacewithanewprogram


ExampleCode#include <sys/types.h> #include <stdio.h> #include <unistd.h> int main() { pid_t pid; /* fork another process */ pid = fork(); if (pid < 0) { /* error occurred */ fprintf(stderr, "Fork Failed"); return 1; } else if (pid == 0) { /* child process */ execlp("/bin/ls", "ls", NULL); } else { /* parent process */ /* parent will wait for the child */ wait (NULL); printf ("Child Complete"); } return 0;

}


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ProcessTermina,on•  Process executes last statement and then asks the operating

system to delete it using the exit() system call.–  Returns status data from child to parent (via wait())–  Process’ resources are deallocated by operating system

•  Terminate a process from another process: SIGKILL (kill -9 pid)

•  Parent may terminate the execution of children processes using the abort() system call. Some reasons for doing so:–  Child has exceeded allocated resources–  Task assigned to child is no longer required–  The parent is exiting and the operating systems does not allow

a child to continue if its parent terminates


ProcessTermina,on•  What happens if a process “dies”?

–  The parent process may wait for termination of a child process by using the wait()system call. The call returns status information and the pid of the terminated process: pid = wait(&status);

–  Parent “collects” child’s resources –  If no parent waiting (did not invoke wait()) process is a

zombie•  What happens if parent “dies”?

–  If parent terminated without invoking wait, process is an orphan

–  May receive “new parent” (grandparent, init process, etc.)–  Cascading termination: no orphans allowed; if a process

terminates, all its children must also be terminated


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InterprocessCommunica,on

•  Processescommunicatebysharingdata.•  Whydoprocessescommunicate?– Computa,onspeedup– Modularity–  Informa,onsharing

•  Mechanism:interprocesscommunicaAon(IPC)

•  Twostandardmodels:SharedMemoryandMessagePassing


Communica,onModels


MessagePassing SharedMemory

process A

message queue

kernel

(a) (b)

process A

shared memory

kernel

process B

m0 m1 m2 ...m3 mn

process B

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Producer-ConsumerProblem

•  Oneprocessproducesdata.Thesecondprocessconsumesthedata.

•  Datastoredinabuffer:– Unbounded-Bufferhasnolimitonsize.Growstosizeofmemory.

– Bounded-Bufferhasfixedsize.Createsanewproblem:•  HowdowehandletheproducercreaAngdatatoofast?


SharedMemorySolu,on

Circularbuffer


OUTIN

NUL NUL NUL NUL NUL NUL NUL NUL NUL

IN=OUT->EMPTY

OUT IN

A NUL NUL NUL NUL NUL NUL NUL NUL

OUT IN

NUL NUL C D E F NUL NUL NUL

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SharedMemorySolu,on•  Shared data

#defineBUFFER_SIZE10typedefstruct{...}item;itembuffer[BUFFER_SIZE];intin=0;intout=0;

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


BoundedBuffer-Producer

itemnext_produced;while(true){

/*produceaniteminnextproduced*/while(((in+1)%BUFFER_SIZE)==out) ;/*donothing*/buffer[in]=next_produced;in=(in+1)%BUFFER_SIZE;

}


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BoundedBuffer-Consumer

itemnext_consumed;while(true){

while(in==out) ;/*donothing*/

next_consumed=buffer[out];out=(out+1)%BUFFER_SIZE;

/*consumetheiteminnextconsumed*/

}


MessagePassing

•  Twoprimi,ves:– send(C,message)–messageshavemaximumsize–  receive(P,message)

•  Thinkmailboxes.•  Kernelusuallymanagesthemessagepassingand“mailboxes”.


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MessagePassingConsidera,ons

•  Howisthelinkestablished?– Automa,callyonsend?

•  Canthelinkbeasymmetric?–  Receivingamessage:whoisthesender?

•  Istherealimittothecapacityofthelink?•  Isthemessagesizefixedorvariable?•  Isalinkunidirec,onalorbidirec,onal?•  Cantherebemul,plelinksbetweenapairofcommunica,onprocesses?


MessagePassing

•  DirectCommunica,on– send(P,message)->receiverprocessP–  receive(Q,message)->senderprocessQ

•  IndirectCommunica,on(“mailboxes”)– send(M1,message)->putinmailboxM1–  receive(M1,message)->takefrommailboxM1


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IPCSynchroniza,on

•  Blocking?– Consumerisputinawai,ngschedulerqueueif“mailbox”isempty.

– Producerisputinawai,ngschedulerqueueif“mailbox”isfull.

•  Non-blocking?– NeitherProducernorConsumerblocks;failureisreturnedfrommessagepassingprimi,veinstead.


Buffering

•  Queue of messages attached to the link; implemented in one of three ways:– Zero capacity – no messages are queued on

a link. Sender must wait for receiver (rendezvous)

– Bounded capacity – finite length of n messages. Sender must wait if link full

– Unbounded capacity – infinite length. Sender never waits


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IPC-POSIX

•  POSIXSharedMemoryshm_id = shm_open(name, O CREAT | O RDWR, 0666); ftruncate(shm_id, 4096); shared_memory = (char *) shmat(shm_id, NULL, 0); sprintf(shared_memory, "Writing to shared memory"); Also: shmdt (remove), shmctl (destroy)


IPC-Mach

•  Mach communication is message based– Even system calls are messages– Each task gets two mailboxes at creation- Kernel

and Notify– Only three system calls needed for message

transfer msg_send(), msg_receive(), msg_rpc() – Mailboxes needed for communication, created via port_allocate()


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Recap

•  KeyPoints:– SystemLayering– ConceptofaProcess– Scheduling– ProcessCrea,onandTermina,on–  InterprocessCommunica,on


Lecture4-Processescpoellab/teaching/cse30341/Lecture4... · 2016-01-22 · Process Switching...

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