CS444/CS544 Operating Systems

1. CS444/CS544Operating Systems Processes & Threads 1/31/2006 Prof. Searleman jets@clarkson.edu

2. Outline • Processes and how they communicate and cooperate • Introduction to Threads NOTE: • Thursday’s class will be held in the ITL (Science Center 334) • New due date for Lab#1: Tuesday, Feb. 7 • Read: Chapter 4

3. Process creation Question: How many processes will be created by running the following code? int main (int argc, char **argv) { fork(); fork(); }

4. PCp0 Process hierarchy int main () { 1: fork(); 2: fork(); 3: } Running process: p0 p0 State: before p0 executes statement 1

5. Process hierarchy int main () { 1: fork(); 2: fork(); 3: } p0 PCp0 PCp1 p1 State: after p0 executes statement 1 Who will run next? Depends on the scheduler. Suppose it’s process p1 (requires a context switch from p0 to p1)

6. Process hierarchy int main () { 1: fork(); 2: fork(); 3: } Running process: p1 p0 PCp0 PCp1 p1 State: before p1 executes statement 2

7. Process hierarchy int main () { 1: fork(); 2: fork(); 3: } Running process: p1 p0 PCp0 PCp1 p1 PCp2 State: after p1 executes statement 2 p2 Suppose that process p0 is selected to run next (requires a context switch from p1 to p0)

8. p3 Process hierarchy int main () { 1: fork(); 2: fork(); 3: } Running process: p0 p0 PCp0 PCp1 p1 PCp2 State: after p0 executes statement 2 PCp3 p2 How many processes were created? Answer: 4 (NOTE: other execution sequences are possible.) Question: what happens to the 4 processes?

9. Init process • In last stage of boot process, kernel creates a user level process, init • Init is the parent (or grandparent…) of all other processes • Init does various important housecleaning activities • checks and mounts the filesystems, sets hostname, timezones, etc. • Init reads various “resource configuration files” (/etc/rc.conf, etc) and spawns off processes to provide various services • In multi-user mode, init maintains processes for each terminal port (tty) • Usually runs getty which executes the login program

10. Foreground vs Background int main (int argc, char **argv){ while (1){ int childPid; char* cmdLine = readCommandLine(); if (userChooseExit(cmdLine)){ wait for all background jobs } childPid = fork(); if (childPid == 0){ setSTDOUT_STDIN_STDERR(cmdLine); exec( getCommand(cmdLine)); } else if (runInForeground(cmdLine)){ wait(childPid); } else { Record childPid In list of background jobs} } }

11. Redirecting I/O • Three default file streams in each process • Stdout, stdin, stderr • In a shell, child process may need a different stdout, stdin or stderr than the parent • Before call exec • If there is a “ < infile” then set stdin to the infile • If there is a “> outfile” then set the stdout the outfile

12. Dup2 //after fork, in the child if (command line contains < infile){ int fd = open (infileName, O_RDONLY); if (fd == -1){ redirection failed } else { dup2 (fd, STDIN_FILENO); } } //exec

13. Cooperating Processes • Processes can run independently of each other or processes can coordinate their activities with other processes • To cooperate, processes must use OS facilities to communicate • One example: parent process waits for child • Many others • Files (You’ve Used) • Sockets (Networks) • Pipes (Like Sockets for local machine; Pair of files) • Signals (Today) • Shared Memory • Events • Remote Procedure Call

14. Signals • Processes can register to handle signals with the signal function • void signal (int signum, void (*proc) (int)) • Processes can send signals with the kill function • kill (pid, signum) • System defined signals like SIGHUP (0), SIGKILL (9), SIGSEGV(11) • In UNIX shell, try: “kill –9 pidOfVictimProcess” • Signals not used by system like SIGUSR1 and SIGUSR2 • Note: sigsend/sigaction similar to kill/signal

15. Signals if (signal(SIGUSR1, sig_handler) == SIG_ERR) fprintf(stderr, "Unable to create handler for SIGUSR1\n"); if (signal(SIGUSR2, sig_handler) == SIG_ERR) fprintf(stderr, "Unable to create handler for SIGUSR2\n"); parentPid = getpid(); fprintf(stdout, "Parent process has id %d\n", parentPid); fprintf(stdout, "Parent process forks child...\n"); childPid = fork(); if (childPid == 0){ doChild(); } else { doParent(); }

16. doChild void doChild() { /* I am the child */ myPid = getpid(); assert(myPid != parentPid); fprintf(stdout, "In child (id %d) , Child process has id %d\n", myPid, myPid); /* send a SIG_USR1 to the parent */ fprintf(stdout, "Child process (id %d) sending 1st SIGUSR1 to parent process (id %d)\n", myPid, parentPid); err = kill(parentPid, SIGUSR1); if (err){ fprintf(stderr, "Child process (id %d) is unable to send SIGUSR1 signal to the parent process (id %d)\n", myPid, parentPid); } }

17. doParent void doParent() { myPid = getpid(); assert(myPid == parentPid); fprintf(stdout, "In parent (id %d) , child process has id %d\n", myPid, childPid); fprintf(stdout, "Parent process (id %d) sending 1st SIGUSR2 to child process (id %d)\n", myPid, childPid); err = kill(childPid, SIGUSR2); if (err){ fprintf(stderr, "Parent process (id %d) is unable to send SIGUSR2 signal to the child process (id %d)\n", myPid, childPid); } }

18. sigHandler static void sig_handler(int signo){ switch(signo){ case SIGUSR1: /* incoming SIGUSR1 signal */ handleSIGUSR1(); break; case SIGUSR2: /*incoming SIGUSR2 signal */ handleSIGUSR2(); break; case SIGTERM: /* incoming SIGTERM signal */ handleSIGTERM(); break; } return; }

19. handleSIGUSR1 void handleSIGUSR1(){ numSIGUSR1handled++; if (myPid == parentPid){ fprintf(stdout, "Process %d: Parent Received SIGUSR1 %u\n", myPid, numSIGUSR1handled); } else { fprintf(stdout, "Error: Process %d: Received SIGUSR1, but I am not the parent!!\n", myPid); exit(1); } #if RECEIVE_MORE_THAN_ONE_SIGNAL if (signal(SIGUSR1, sig_handler) == SIG_ERR){ fprintf(stderr, "Unable to reset handler for SIGUSR1\n"); } #endif }

20. Windows Process Creation BOOL CreateProcess( LPCTSTR lpApplicationName, // name of executable module LPTSTR lpCommandLine, // command line string LPSECURITY_ATTRIBUTES lpProcessAttributes, // SD LPSECURITY_ATTRIBUTES lpThreadAttributes, // SD BOOL bInheritHandles, // handle inheritance option DWORD dwCreationFlags, // creation flags LPVOID lpEnvironment, // new environment block LPCTSTR lpCurrentDirectory, // current directory name LPSTARTUPINFO lpStartupInfo, // startup information LPPROCESS_INFORMATION lpProcessInformation // process information );

21. Windows vs Unix • Windows doesn’t maintain quiet the same relationship between parent and child • Later versions of Windows have concept of “job” to mirror UNIX notion of parent and children (process groups) • Waiting for a process to complete? • WaitforSingleObject to wait for completion • GetExitCodeProcess ( will return STILL_ALIVE until process has terminated)

22. Sockets • A socket is an end-point for communication over the network • Create a socket • int socket(int domain, int type, int protocol) • Type = SOCK_STREAM for TCP • Read and write socket just like files • Can be used for communication between two processes on same machine or over the network

23. Pipes • Bi-directional data channel between two processes on the same machine • Created with: • int pipe (int fildes[2]) • Read and write like files

24. Remote Procedure Call (RPC)

25. Processes: Summary • A process includes • Address space (Code, Data, Heap, Stack) • Register values (including the PC) • Resources allocated to the process • Memory, open files, network connections • How to create a process • Initializing the PCB and the address space (page tables) takes a significant amount of time • Interprocess communication • IPC is costly also • Communication must go through OS (“OS has to guard any doors in the walls it builds around processes for their protection”)

26. Problem which needs > 1 independent sequential process? • Some problems are hard to solve as a single sequential process; easier to express the solution as a collection of cooperating processes • Hard to write code to manage many different tasks all at once • How would you write code for “make phone calls while making dinner while doing dishes while looking through the mail” • Can’t be independent processes because share data (your brain) and share resources (the kitchen and the phone) • Can’t do them sequentially because need to make progress on all tasks at once • Easier to write “algorithm” for each and when there is a lull in one activity let the OS switch between them • On a multiprocessor, exploit parallelism in problem

27. Example: Web Server • Web servers listen on an incoming socket for requests • Once it receives a request, it ignore listening to the incoming socket while it services the request • Must do both at once • One solution: Create a child process to handle the request and allow the parent to return to listening for incoming requests • Problem: This is inefficient because of the address space creation (and memory usage) and PCB initialization

28. Observation • There are similarities in the process that are spawned off to handle requests • They share the same code, have the same privileges, share the same resources (html files to return, cgi script to run, database to search, etc.) • But there are differences • Operating on different requests • Each one will be in a different stage of the “handle request” algorithm

29. Idea • Let these tasks share the address space, privileges and resources • Give each their own registers (like the PC), their own stack etc • Process – unit of resource allocation (address space, privileges, resources) • Thread – unit of execution (PC, stack, local variables)

30. Single-Threaded vs Multithreaded Processes

31. Process vs Thread • Each thread belongs to one process • One process may contain multiple threads • Threads are logical unit of scheduling • Processes are the logical unit of resource allocation

32. Stack Pointer PC Address Space Map For Single-Threaded Process Stack (Space for local variables etc. For each nested procedure call) Biggest Virtual Address Heap (Space for memory dynamically allocated e.g. with malloc) Statically declared variables (Global variables) Code (Text Segment) Ox0000

33. Thread 1 stack Thread 2 stack SP (thread 1) PC (thread 2) PC (thread 1) SP (thread 2) Address Space Map For Multithreaded Process Biggest Virtual Address Heap (Space for memory dynamically allocated e.g. with malloc) Statically declared variables (Global variables) Code (Text Segment) Ox0000

34. Kernel support for threads? • Some OSes support the notion of multiple threads per process and others do not • Even if no “kernel threads” can build threads at user level • Each “multi-threaded” program gets a single kernel in the process • During its timeslice, it runs code from its various threads • User-level thread package schedules threads on the kernel level process much like OS schedules processes on the CPU • SAT question? CPU is to OS is to processes like? • Kernel thread is to User-level thread package is to user threads • User-level thread switch must be programmed in assembly (restore of values to registers, etc.)

35. User-level Threads

36. User-level threads • How do user level thread packages avoid having one thread monopolize the processes time slice? • Solve much like OS does • Solution 1: Non-preemptive • Rely on each thread to periodically yield • Yield would call the scheduling function of the library • Solution 2: OS is to user level thread package like hardware is to OS • Ask OS to deliver a periodic timer signal • Use that to gain control and switch the running thread

37. Kernel vs User Threads • One might think, kernel level threads are best and only if kernel does not support threads use user level threads • In fact, user level threads can be much faster • Thread creation , “Context switch” between threads, communication between threads all done at user level • Procedure calls instead of system calls (verification of all user arguments, etc.) in all these cases!

38. Problems with User-level threads • OS does not have information about thread activity and can make bad scheduling decisions • Examples: • If thread blocks, whole process blocks • Kernel threads can take overlap I/O and computation within a process! • Kernel may schedule a process with all idle threads

39. Scheduler Activations • If have kernel level thread support available then use kernel threads *and* user-level threads • Each process requests a number of kernel threads to use for running user-level threads on • Kernel promises to tell user-level before it blocks a kernel thread so user-level thread package can choose what to do with the remaining kernel level threads • User level promises to tell kernel when it no longer needs a given kernel level thread

40. Thread Support • Pthreads is a user-level thread library • Can use multiple kernel threads to implement it on platforms that have kernel threads • Java threads (extend Thread class) run by the Java Virtual Machine • Kernel threads • Linux has kernel threads (each has its own task_struct) – created with clone system call • Each user level thread maps to a single kernel thread (Windows 95/98/NT/2000/XP, OS/2) • Many user level threads can map onto many kernel level threads like scheduler activations (Windows NT/2000 with ThreadFiber package, Solaris 2)

41. Pthreads Interface • POSIX threads, user-level library supported on most UNIX platforms • Much like the similarly named process functions • thread = pthread_create(procedure) • pthread_exit • pthread_wait(thread) Note: To use pthreads library, #include <pthread.h> compile with -lpthread

42. Pthreads Interface (con’t) • Pthreads support a variety of functions for thread synchronization/coordination • Used for coordination of threads (ITC ) – more on this soon! • Examples: • Condition Variables ( pthread_cond_wait, pthread_signal) • Mutexes(pthread_mutex_lock, pthread_mutex_unlock)

43. Performance Comparison In microseconds, on a 700 MHz Pentium, Linux 2.2.16, Steve Gribble, 2001.

44. Windows Threads HANDLE CreateThread( LPSECURITY_ATTRIBUTES lpThreadAttributes, DWORD dwStackSize, LPTHREAD_START_ROUTINE lpStartAddress, DWORD dwCreationFlags, LPVOID lpParameter, DWORD dwCreationFlags, LPDWORD lpThreadId);

45. Windows Thread Synchronization • Windows supports a variety of objects that can be used for thread synchronization • Examples • Events (CreateEvent, SetEvent, ResetEvent, WaitForSingleObject) • Semaphores (CreateSemaphore, ReleaseSemaphore, WaitForSingleObject) • Mutexes (CreateMutex, ReleaseMutex, WaitForSingleObject) • WaitForMultipleObject • More on this when we talk about synchronization

46. Warning: Threads may be hazardous to your health • One can argue (and John Ousterhout did) that threads are a bad idea for most purposes • Anything you can do with threads you can do with an event loop • Remember “make phone calls while making dinner while doing dishes while looking through the mail” • Ousterhout says thread programming to hard to get right

47. Outtakes • Processes that just share code but do not communicate • Wasteful to duplicate • Other ways around this than threads

48. Example: User Interface • Allow one thread to respond to user input while another thread handles a long operation • Assign one thread to print your document, while allowing you to continue editing

49. Benefits of Concurrency • Hide latency of blocking I/O without additional complexity • Without concurrency • Block whole process • Manage complexity of asynchronous I/O (periodically checking to see if it is done so can finish processing) • Ability to use multiple processors to accomplish the task • Servers often use concurrency to work on multiple requests in parallel • User Interfaces often designed to allow interface to be responsive to user input while servicing long operations

50. Thread pools • What they are and how they avoid thread creation overhead