1. Chapter 4: Threads

2. 4.2CSCI 380 Operating Systems Chapter 4: Threads Overview Multithreading Models Threading Issues Pthreads Windows XP Threads Linux Threads Java Threads

3. 4.3CSCI 380 Operating Systems Single and Multithreaded Processes

4. 4.4CSCI 380 Operating Systems Benefits Responsiveness Resource Sharing Economy Utilization of MP Architectures What’s the difference between a thread and a process? Advantages and Disadvantages of threads vs. processes?

5. 4.5CSCI 380 Operating Systems User Threads Thread management done by user-level threads library Three primary thread libraries: POSIX Pthreads Win32 threads Java threads These are libraries that you will include in your code Look on page 133 for an example Define a set of tasks that get switched to by manipulating the stack pointer Usually each thread gives up the CPU by calling a switch Problems? Monopolize the timeslice Can’t take advantage of MultiProcessor systems, why?

6. 4.6CSCI 380 Operating Systems Kernel Threads Supported by the Kernel Examples Windows XP/2000 Solaris Linux Tru64 UNIX Mac OS X Less likely to hog the CPU because the kernel scheduler controls it So what’s the difference between user and kernel threads? Why have kernel threads?

7. 4.7CSCI 380 Operating Systems system call to access kernel threads int clone(int (*fn)(), void **stack, int flags, int argc,... /* args */); fn - The thread routine Stack - The thread's stack Flags - specifies level of shareness, see table on p. 145 argc - number of remaining parameters /* args */ - the parameters expected by the thread.

8. 4.8CSCI 380 Operating Systems Common Problems with User Threads What happens if a thread calls fork()? Dups only the thread or executes and external program? stops all threads Concurrency Complexity Understanding chopped up tasks Answering, what can be threaded up? Locking What issues can happen here? Why don’t process have this problem?

9. 4.9CSCI 380 Operating Systems Multithreading Models Relationship between User and Kernel threads options: Many-to-One One-to-One Many-to-Many Which one do you think is implemented in most operating systems? Advantages / Disadvantages of each?

10. 4.10CSCI 380 Operating Systems Many-to-One Many user-level threads mapped to single kernel thread Examples: Solaris Green Threads GNU Portable Threads

11. 4.11CSCI 380 Operating Systems Many-to-One Model

12. 4.12CSCI 380 Operating Systems One-to-One Each user-level thread maps to kernel thread Examples Windows NT/XP/2000 Linux Solaris 9 and later

13. 4.13CSCI 380 Operating Systems One-to-one Model

14. 4.14CSCI 380 Operating Systems Many-to-Many Model Allows many user level threads to be mapped to many kernel threads Allows the operating system to create a sufficient number of kernel threads Solaris prior to version 9 Windows NT/2000 with the ThreadFiber package

15. 4.15CSCI 380 Operating Systems Many-to-Many Model

16. 4.16CSCI 380 Operating Systems 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. 4.17CSCI 380 Operating Systems Two-level Model

18. 4.18CSCI 380 Operating Systems Threading Issues Semantics of fork() and exec() system calls Thread cancellation Signal handling Thread pools Thread specific data Scheduler activations Semantics of fork() and exec() Does fork() duplicate only the calling thread or all threads?

19. 4.19CSCI 380 Operating Systems Thread Cancellation Terminating a thread before it has finished Two general approaches: Asynchronous cancellation terminates the target thread immediately Deferred cancellation allows the target thread to periodically check if it should be cancelled

20. 4.20CSCI 380 Operating Systems Signal Handling Signals are used in UNIX systems to notify a process that a particular event has occurred A signal handler is used to process signals  Signal is generated by particular event  Signal is delivered to a process  Signal is handled Options: Deliver the signal to the thread to which the signal applies Deliver the signal to every thread in the process Deliver the signal to certain threads in the process Assign a specific thread to receive all signals for the process

21. 4.21CSCI 380 Operating Systems 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

22. 4.22CSCI 380 Operating Systems Thread Specific Data 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)

23. 4.23CSCI 380 Operating Systems Scheduler Activations Scheduler activations provide upcalls - a communication mechanism from the kernel to the thread library This communication allows an application to maintain the correct number kernel threads

24. 4.24CSCI 380 Operating Systems 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) The API you will use to implement threads on a linux OS For windows there is the Win32 API

25. 4.25CSCI 380 Operating Systems 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)

26. 4.26CSCI 380 Operating Systems 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) Specify the level of “shareness” with clone

27. 4.27CSCI 380 Operating Systems Java Threads Java threads are managed by the JVM Java threads may be created by: Extending Thread class Implementing the Runnable interface Java is OO programming, so you need to implement classes that create instances of other classes that implement the thread

28. 4.28CSCI 380 Operating Systems Java Thread States

29. End of Chapter 4