Threads, SMP, and Microkernels

1. Threads, SMP, and Microkernels Chapter 4

2. Process • Resource ownership - process includes a virtual address space to hold the process image • Scheduling/execution- follows an execution path that may be interleaved with other processes • These two characteristics are treated independently by the operating system

3. Process • Dispatching is referred to as a thread or lightweight process • Resource of ownership is referred to as a process or task

4. Multithreading（多线程） • Operating system supports multiple threads of execution within a single process • MS-DOS supports a single thread • UNIX supports multiple user processes but only supports one thread per process • Windows, Solaris, Linux, Mach, and OS/2 support multiple threads

6. Process the unit of resource allocation(资源分配) and the unit of protection • Have a virtual address space which holds the process image • Protected access to processors, other processes, files, and I/O resources

7. Thread • An execution state (running, ready, etc.) • Saved thread context when not running • Has an execution stack • Some per-thread static storage for local variables • Access to the memory and resources of its process • all threads of a process share this

8. 1.每个线程有独立的栈和控制信息 2.所有线程共享进程的状态和资源

9. Benefits of Threads • Takes less time to create a new thread than a process • Less time to terminate a thread than a process • Less time to switch between two threads within the same process • Since threads within the same process share memory and files, they can communicate with each other without invoking the kernel

10. Threads • Suspending a process involves suspending all threads of the process since all threads share the same address space • Termination of a process, terminates all threads within the process

11. Thread States • The key states for a thread are Running, Ready and Blocked. • If a process is swapped out, all of its threads are necessarily swapped out because they all share the address space of the process. So, the suspend states of thread is not necessary.

12. Thread States • States associated with a change in thread state • Spawn(产生) • Spawn another thread • Block(阻塞) • Unblock(非阻塞) • Finish • Deallocate(释放) register context and stacks

13. Multithreading

14. The categories of threads • User-level threads(ULTs, 用户级线程) • Kernel-level threads(KLTs,内核级线程)

15. User-Level Threads • All thread management is done by the application • The kernel is not aware of the existence of threads • Using threads library to create and destroy threads, pass message and data between threads, schedule thread execution, and to save and restore thread contexts

16. User-Level Threads

18. The advantages of ULTs • Thread switching does not require kernel mode privileges. • Scheduling can be application specific. • ULTs can run on any operating system.

19. The disadvantages of ULTs • When a threads are blocked on system call, all of the threads within the process are blocked. • A kernel assigns one process to only one processor at a time, so a multithreaded application cannot take advantage of multiprocessing.

20. Kernel-Level Threads • Windows is an example of this approach • Kernel maintains context information for the process and the threads • Scheduling is done on a thread basis

21. Kernel-Level Threads

22. The advantages of KLTs • The kernel can simultaneously schedule multiple threads from the same process on multiple processors. • If one thread in a process is blocked, the kernel can schedule another thread of the same process

23. The disadvantages of KLTs • The transfer of control from one thread to another within the same process requires a mode switch to the kernel.

24. Combined Approaches • Example is Solaris • Thread creation done in the user space • Bulk of scheduling and synchronization of threads within application • The multiple ULTs from a single application are mapped onto some (smaller or equal) number of KLTs.

25. Combined Approaches