操作系统课件：02_process_thread

1、1Processes and ThreadsCreation and TerminationStatesUsageImplementationsWhat is a process? A task created by the OS, running in a restricted virtual machine environment a virtual CPU, virtual memory environment, interface to the OS via system calls The unit of execution The unit of scheduling Thread

2、 of execution + address space Is a program in execution Sequential, instruction-at-a-time execution of a program.The same as “job” or “task” or “sequential process”What is a program?A program consists of: Code: machine instructions Data: variables stored and manipulated in memory initialized variabl

3、es (globals) dynamically allocated variables (malloc, new) stack variables (C automatic variables, function arguments) DLLs: libraries that were not compiled or linked with the program containing code & data, possibly shared with other programs mapped files: memory segments containing variables

4、(mmap() used frequently in database programs A process is a executing programPreparing a Programsourcefilecompiler/assembler.o filesLinkerExecutable file(must follow standard format,such as ELF on Linux, Microsoft PE on Windows)HeaderCodeInitialized dataBSSSymbol tableLine numbersExt. refsstatic lib

5、raries(libc, streams)Running a program OS creates a “process” and allocates memory for it The loader: reads and interprets the executable file sets processs memory to contain code & data from executable pushes “argc”, “argv”, “envp” on the stack sets the CPU registers properly & calls “_star

6、t()” Part of CRT0 Program start running at _start(), which calls main() we say “process” is running, and no longer think of “program” When main() returns, CRT0 calls “exit()” destroys the process and returns all resourcesProcess != ProgramHeaderCodeInitialized dataBSSSymbol tableLine numbersExt. ref

7、sCodeInitialized dataBSSHeapStackDLLsmapped segmentsExecutableProcess address spaceProgram is passive Code + dataProcess is running program stack, regs, program counterExample:We both run IE:- Same program - Separate processesC Language in Address SpaceCodeInitialized dataBSSHeapStackDLLsmapped segm

8、entsProcess address space#include int a=10;int c;main(int argc, char* argv) int *b; b=(int *)malloc (sizeof(int); *b=a; 8Processes Program in execution (cf. recipe vs. cooking) Multiprogramming - pseudo-parallelism(vs. true hardware parallelism of multiprocessor systems)9The Process Model Multiprogr

9、amming of four programs Conceptual model of 4 independent, sequential processes Only one program active at any instant10Process CreationPrincipal events that cause process creation1. System initialization (background a daemon processes)2. Execution of a process creation system call (data from networ

10、k)3. User request to create a new process4. Initiation of a batch jobUNIX: fork system call (+ execve)Windows: CreateProcess function callCreating a Process - Fork Creating a process and executing a program are two different things in UNIX Fork duplicates a process so that instead on one process you

11、 get two- But the code being executed doesnt change! Fork returns 0 if child -1 if fork fails Childs PID if parent process Child gets new program counter, stack, file descriptors, heap, globals, pid!Examplemain(int argc, char *argv) char *myName = argv1; int cpid = fork(); if (cpid = 0) printf(“The

12、child of %s is %dn”, myName, getpid(); exit(0); else printf(“My child is %dn”, cpid); exit(0); What does this program print?Bizarre But Reallace:tmp cc a.clace:tmp ./a.out foobarThe child of foobar is 23874My child is 23874ParentChildOperating Systemfork()retsysv0=0v0=23874UNIX Example (Example 3.6,

13、 p.65)#include #include #include int main(void) pid_t parentpid; pid_t childpid; Reference: Kay Robbins, Steven Robbins, UNIX Systems Programming, Communication, Concurrency, and Threads,人民邮电出版社, ISBN 7-115-14984-4if (childpid = fork() = -1) perror(cant create a new process);exit(1); else if (childp

14、id = 0) /* child process executes */printf(“child: childpid = %d, parentpid = %d n”, getpid(), getppid();exit(0); else /*parent process executes */printf(“parent: childpid = %d, parentpid = %d n”, childpid, getpid();exit(0);What Happens? (Ex. 3.17, p66)#include #include #include int main(void) pid_t

15、 childpid; pid_t mypid; mypid=getpid(); childpid=fork(); if(childpid=-1) perror(Failed to fork); return 1; if(childpid=0) printf(I am child %ld, ID=%ldn, (long)getpid(),(long)mypid); else printf(I am parent %ld, ID=%ldn,(long)getpid(),(long)mypid); return 0;Chain and FanChildChildParentParentChildCh

16、ildpid_t childpid = 0;for (i=1;in;i+) if (childpid = fork() break;pid_t childpid = 0;for (i=1;in;i+) if (childpid = fork() =0) break;ChainFan19Process TerminationConditions which terminate processes1. Normal exit (voluntary) - (exit, ExitProcess)2. Error exit (voluntary)3. Fatal error (involuntary),

17、 e.g. program bug4. Killed by another process (involuntary) - kill, TerminateProcess20Process Hierarchies Parent creates a child process, child processes can create its own process Forms a hierarchy UNIX calls this a process group”init Windows has no concept of process hierarchy all processes are cr

18、eated equal21Process States (1) Possible process states running blocked ready Transitions between states shown22Process States (2) Lowest layer of process-structured OS handles interrupts, scheduling Above that layer are sequential processes23Implementation of Processes (1)Fields of a process table

19、entry24Implementation of Processes (2)Skeleton of what lowest level of OS does when an interrupt occursMultithreaded ProcessesThreads vs. Processes A thread has no data segment or heap A thread cannot live on its own, it must live within a process Inexpensive creation Inexpensive context switching I

20、f a thread dies, its stack is reclaimedA process has code/data/heap & other segmentsThere must be at least one thread in a processExpensive creationExpensive context switchingIf a process dies, its resources are reclaimed & all threads die27ThreadsProcess = resource grouping (code, data, ope

21、n files, etc.)+execution (program counter, registers, stack)Multithreading: multiple execution takes place in the same process environment co-operation by sharing resources (address space, open files, etc.)28The Thread Model (1)(a) Three processes each with one thread(b) One process with three threa

22、ds29The Thread Model (2) Items shared by all threads in a process Items private to each thread30The Thread Model (3)Each thread has its own stack to keep track execution history (called procedures)31Advantages Pseudo-parallelism with shared address space and data Easier to create and destroy than pr

23、ocesses Better performance for I/O bound applications32Thread Usage (1)A word processor with three threadsWriting a book: interactive and background threads sharing the same file33Thread Usage (2)A multithreaded Web server34Thread Usage (3) Rough outline of code for previous slide(a) Dispatcher thre

24、ad(b) Worker thread35Thread Usage (4)Three ways to construct a server36Implementing Threads in User SpaceA user-level threads package37(Dis)advantages+:no specific OS support neededfaster than kernel instructionsprocess-specific scheduling algorithms-:blocking system calls (select)page faults38Imple

25、menting Threads in the KernelA threads package managed by the kernel39(Dis)advantages+:handling blocking and page faults-:more costly (but recycling threads)40Hybrid Implementations Multiplexing user-level threads onto kernel- level threads41Scheduler Activations Goal: mimic functionality of kernel

26、threads gain performance of user space threads Avoids unnecessary user/kernel transitions Kernel assigns virtual processors to each process lets runtime system allocate threads to processors, upcall Problem: Fundamental reliance on kernel (lower layer) calling procedures in user space (higher layer)

27、42Pop-Up Threads Creation of a new thread when message arrives(a) before message arrives(b) after message arrives (quick)43Making Single-Threaded Code Multithreaded (1)Conflicts between threads over the use of a global variable44Making Single-Threaded Code Multithreaded (2)Threads can have private global variables.But non-reentrant library procedures.POSIX Threads Pthread_create Pthread_exit Pthread_joinCode Example in P.10645Program 12.2, P.414#include #include #include #includ