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 (fig 3.16) • Scheduling/execution: follows an execution path that may be interleaved with other processes • These two characteristics can be treated independently by the operating system

3. Process • Unit of dispatching is referred to as a thread or lightweight process • Unit of resource ownership is referred to as a process or task

4. Multithreading • 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 • 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

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. Uses of Threads in a Single-User Multiprocessing System • Foreground to background work • Asynchronous processing • Speed of execution • Modular program structure

11. 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

12. Thread States • States associated with a change in thread state • Spawn • Spawn another thread • Block • Unblock • Finish • Deallocate register context and stacks

13. Remote Procedure Call Using Single Thread

14. Remote Procedure Call Using Threads

15. Multithreading

16. Adobe PageMaker

17. User-Level Threads • All thread management is done by the application • The kernel is not aware of the existence of threads

18. User-Level Threads

20. Advantages of User-Level Threads • Thread-switching does not require kernel mode privileges and switching • Thread scheduling can be tailored to application • ULTs can run on any OS. They can be implemented with a thread library that runs within the app.

21. Disadvantages of ULTs • If blocking system call (eg IO call), whole process blocked • can use ‘jacketing’ – call other process to check state of resource • Cannot use multiprocessing since OS sees all threads as a single process.

22. 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

23. Kernel-Level Threads

24. VAX Running UNIX-Like Operating System

25. Combined Approaches • Example is Solaris • Thread creation done in the user space • Bulk of scheduling and synchronization of threads within application

26. Combined Approaches

27. Relationship Between Threads and Processes 1:M and M:N approaches are experimental.

28. Symmetric Multiprocessing (SMP)

29. Symmetric Multiprocessing • Tradition: Single sequence of instructions • von Neumann machine concept • Real machines do instruction pipelining, DMA and other parallel operations • Pipeline explanation and movie • Hyperthreading • With cheaper HW can extend this to multiple processors

30. Categories of Computer Systems • Single Instruction Single Data (SISD) stream • Single processor executes a single instruction stream to operate on data stored in a single memory. • ‘Normal’ computing • Single Instruction Multiple Data (SIMD) stream • Each instruction is executed on a different set of data by the different processors. • Eg DSP or vector processing

31. Categories of Computer Systems • Multiple Instruction Single Data (MISD) stream • A sequence of data is transmitted to a set of processors, each of which executes a different instruction sequence. • Never implemented! • Multiple Instruction Multiple Data (MIMD) • A set of processors simultaneously execute different instruction sequences on different data sets

33. Symmetric Multiprocessing • Kernel can execute on any processor • Typically each processor does self-scheduling from the pool of available process or threads

35. Multiprocessor Operating System Design Considerations • Simultaneous concurrent processes or threads • Re-entrant kernel code, deadlock, etc. • Scheduling (see ch 10 for more detail) • Synchronization • Mutual exclusion, event ordering, locks etc (ch 5) • Memory management • Coordination in paging systems when multiple processors accessing pages (ch 7) • Reliability and fault tolerance • Graceful degradation on processor failure

36. Distributed Processing • Another alternative is distributed processing • Each node has its own CPU, memory and communication channel • Slower communications, more independence • EG. Cluster computing, Beowulf etc. • See Ch 14, 15

37. Microkernels

38. Microkernels • Small operating system core (how small?) • Concept: Contains only essential core operating systems functions • Many services traditionally included in the operating system are now external subsystems • Device drivers • File systems • Virtual memory manager • Windowing system • Security services

40. Benefits of a Microkernel Organization • Uniform interface on request made by a process • Don’t distinguish between kernel-level and user-level services • All services are provided by means of message passing • Extensibility • Allows the addition of new services • Flexibility • New features added • Existing features can be subtracted

41. Benefits of a Microkernel Organization • Portability • Changes needed to port the system to a new processor is changed in the microkernel - not in the other services • Reliability • Modular design • Small microkernel can be rigorously tested • Exhaustive testing vs. module testing

42. Benefits of Microkernel Organization • Distributed system support • Message are sent without knowing what the target machine is • Object-oriented operating system • Components are objects with clearly defined interfaces that can be interconnected to form software

43. Microkernel Design • Low-level memory management • Mapping each virtual page to a physical page frame

44. Microkernel Design • Interprocess communication • I/O and interrupt management

45. MS-Windows Processes • Implemented as objects • An executable process may contain one or more threads • Both processes and thread objects have built-in synchronization capabilities

47. Windows Process Object

48. Windows Thread Object

49. MS-Windows Thread States • Ready • Standby • Running • Waiting • Transition • Terminated