Chapter 4: Threads

1. Chapter 4: Threads

2. Chapter 4: Threads • Overview • Multithreading Models • Threading Issues • Pthreads • Windows XP Threads • Linux Threads • Java Threads

3. Threaded Applications • Web browsers: display and data retrieval • Web servers • Many others

4. Threads • What is a thread ? • Lightweight Process (LWP)? • Basic unit of CPU utilization • Contains • Thread ID • Program counter • Register set • Stack • Why multithreading ? • Creating processes are expensive • Other advantages

5. Single and Multithreaded Processes

6. Benefits • Responsiveness • Resource Sharing • share memory and resources of the process they belong to • Sharing code and data allow different threads of activity within the same address space • Economy • Processes are expensive to create, and do context-switch • In Solaris • Process creating is about 30 times slower • Context-switch is about 5 times slower • Utilization of MP Architectures • A single-threaded process can only run on one CPU

7. User Threads • Thread management (creation, scheduling) done by user-level threads library • Drawback • Blocking system call suspends other threads in the same process • Three primary thread libraries: • POSIX Pthreads • Win32 threads • Java threads

8. Kernel Threads • Supported by the Kernel • Advantages • Non-blocking thread execution • Multi-processors (threads on different processors) • Drawback • Slower to create and manage than user-level • Examples • Windows XP/2000 • Solaris • Linux • Tru64 UNIX • Mac OS X

9. Multithreading Models • Many-to-One • One-to-One • Many-to-Many

10. Many-to-One • Many user-level threads mapped to single kernel thread • Thread management is done by thread lib. in user space; so, it is efficient. But, • a thread making a blocking system call block the entire process • Multiple threads cannot run in parallel on MP computers (only one thread can access the kernel at a time) • Used on systems that do not support kernel threads. • Examples: • Solaris Green Threads • GNU Portable Threads

11. Many-to-One Model

12. One-to-One • Each user-level thread maps to a kernel thread • More concurrency than many-to-one: allowing another thread to run when a thread makes a blocking system call; allowing multiple threads running on MP computers as well • Overhead: creating a kernel thread upon a user thread • Examples • Windows NT/XP/2000 • Linux • Solaris 9 and later

13. One-to-one Model

14. Many-to-Many Model • Allows many (K) user level threads to be mapped to many (M) kernel threads: M<=K • Allows the operating system to create a sufficient number of kernel threads without overburdening the system • Solaris prior to version 9 • Windows NT/2000 with the ThreadFiber package

15. Many-to-Many Model

16. Two-level Model • Similar to M:M, except that it allows a user thread to be bound to kernel thread • Examples • IRIX • HP-UX • Tru64 UNIX • Solaris 8 and earlier

17. Two-level Model

18. Threading Issues Due to multithreading: • Semantics of fork() and exec() system calls • Thread cancellation • Signal handling • Thread pools • Thread specific data • Scheduler activations

19. Semantics of fork() and exec() • Does fork() duplicate only the calling thread (single-threaded process) or all threads? • It depends on applications • Example: if call exec() after fork?

20. Thread Cancellation • Terminating a thread before it has finished • Examples • Multiple threads are concurrently doing the same task • Cancel web browser’s on-going tasks • Two general approaches: • Asynchronous cancellation terminates the target thread immediately • Deferred cancellation allows the target thread to periodically check if it should be cancelled

21. Signal Handling • Signals are used in UNIX systems to notify a process that a particular event has occurred • A signal handler (user-defined handler overrides default handler) is used to process signals • Signal is generated by particular event • Signal is delivered to a process • Signal is handled • Depends on signal type • Synchronous signals (e.g., division by 0, illegal memory access) delivered to the thread causing the signal • Asynchronous signals have options • Options: • Deliver the signal to the thread to which the signal applies • Deliver the signal to every thread in the process, e.g, ctrl-c • Deliver the signal to certain threads in the process: kill(aid, signal) • Assign a specific thread to receive all signals for the process

22. Thread Pools • Create a number of threads in a pool where they await work • Advantages: • Usually slightly faster to service a request with an existing thread than create a new thread • Allows the number of threads in the application(s) to be bound to the size of the pool

23. Thread Specific Data • Threads belonging to a process share the data of the process • Allows each thread to have its own copy of data • Useful when you do not have control over the thread creation process (i.e., when using a thread pool)

24. Scheduler Activations • Both M:M and Two-level models require communication to maintain the appropriate number of kernel threads allocated to the application by an immediate data structure called LWP (light-weight process), a virtual processor • LWP runs a user thread; LWP maps to a kernel thread which the OS schedules to run on the physical processor • Scheduler activations provide upcalls - a communication mechanism from the kernel to the thread library; upcall handler perform the task, mapping a user thread to a new LWP, or removing a user thread being blocked from a LWP • The kernel provides a LWP for a user thread • This communication allows an application to maintain the correct number kernel threads

25. Pthreads • A POSIX standard (IEEE 1003.1c) API for thread creation and synchronization • API specifies behavior of the thread library, implementation is up to development of the library • Common in UNIX operating systems (Solaris, Linux, Mac OS X)

26. Windows XP Threads • Implements the one-to-one mapping • Each thread contains • A thread id • Register set • Separate user and kernel stacks • Private data storage area • The register set, stacks, and private storage area are known as the context of the threads • The primary data structures of a thread include: • ETHREAD (executive thread block) • KTHREAD (kernel thread block) • TEB (thread environment block)

27. Linux Threads • Linux refers to them as tasks rather than threads • Thread creation is done through clone() system call • clone() allows a child task to share the address space of the parent task (process)

28. Java Threads • Java threads are managed by the JVM • Java threads may be created by: • Extending Thread class • Implementing the Runnable interface

29. Java Thread States

30. Project1: Unix Shell with History Feature • Goals • Descriptions • Methodology • Submission

31. Goals • Understand how a simple shell works. • Understand systems calls, such as fork, read, wait, execvp, and etc. • Understand signal handling mechanisms

32. Descriptions • Demo • command> ls • commnad> cat proj1.c • command> ctr-c • command> ctr-d • Input: commands from keyboard • Fork a child process to perform the command • Store the past commands in a buffer • Given a signal, display the most recent commands in the buffer • Ctrl-C terminates the shell

33. Methodology • How to get the command from the keyboard? • Use system call read() with STDIN_FILENO • Implement a setup() void setup(char inputBuffer[], char *args[], int *background) setup() reads in the next command line, separating it into distinct tokens using whitespace as delimiters. setup() sets the args parameter as a null-terminated string. Also set background =1 if & is met If “ctrl-d” is met, just simply call exit(0);

34. Methodology • How to execute the command? • while (1){ /* Program terminates normally inside setup */ • background = 0; • printf(" COMMAND->\n"); • setup(inputBuffer,args,&background); /* get next command */ • /* the steps are: • (1) fork a child process using fork() • (2) the child process will invoke execvp() • (3) if background == 1, the parent will wait, • otherwise returns to the setup() function. */ • }

35. Methodology • How to display recent commands? • Use signal handler: CTRL-C is the SIGINT signal • /* the signal handler function */ • void handle_SIGINT() { • write(STDOUT_FILENO,buffer,strlen(buffer)); • exit(0); • } • int main(int argc, char *argv[]) • { • /* set up the signal handler */ • struct sigaction handler; • handler.sa_handler = handle_SIGINT; • sigaction(SIGINT, &handler, NULL); • strcpy(buffer,"Caught <ctrl><c>\n"); • /* wait for <control> <C> */ • while (1); • return 0; • }

36. Methodology • How to keep track of past commands? • Limited-size buffer, why not use circular buffer? • Modify setup() to store the current command which may overwrite the oldest command in the buffer • Implement SININT signal handler to display the 10 most recent commands

37. Suggested Steps • Step 1: implement setup() • Step 2: execute the command from setup() • Step 3: add the history feature

38. Submission • Email to zhuy@seattleu.edu • All source files • A readme file that describes each file, how to compile the file(s), and how to run the file. If there is any problem running the file, please state it here as well. • Makefile may be a good option • Due: 10/10/2006, Tuesday 1:30PM

39. Questions

40. End of Chapter 4