CE01000-3 Operating Systems

1. CE01000-3 Operating Systems Lecture 11 Windows – Object manager and process management

2. Overview of lecture In this lecture we will be looking at: Windows object management Threads and processes in Windows Thread priority and scheduling Thread synchronisation Interprocess communication Internal process and thread representation

3. Windows structure overview

4. System resources and data • In any operating system the system needs to keep track of all the different resources that exist in the system e.g. files, I/O devices, memory, system processes • It needs to keep track of all the various processes (running programs) that use those resources • To do this it needs to keep information about these resources and their use.

5. System resources and data (Cont.) • In many operating systems the information is maintained in various data structures which the operating system code manipulates • However in Windows the code that manipulates the data is kept together with the data in objects i.e. it uses an object oriented approach to managing the system resources and information about them

6. System resources and data (Cont.) • ALL entities and services on the system are represented internally by objects e.g. files are represented by objects, as are user programs when they are running on the computer, hardware resources like printers, communication ports, etc. • Motivation behind this is to provide a very modular structure which it is easy to update, extend, etc.

7. System resources and data (Cont.) • By hiding details of implementation of each object’s class, changes to internal implementation of an object's class can occur without impacting upon the code implementation of other object classes in the system provided the interface remains unchanged • It also makes it easier to add new features and facilities to the system in the form of new object classes

8. Object manager • The function of the object manager is to provide services that manage all the objects in the system • It provides mechanisms by which the various other components in the Windows Executive and the environmental subsystems can create, use, and destroy objects

9. Object manager (Cont.) • Object manager provides a uniform set of services to the other components of the Executive - thus it simplifies object access, etc, because the same mechanism is used in each case

10. Object Handles and Handle Table • When a process (running program) wants to use an object it calls the object manager open service to get a handle to that object • handles are like pointers in C that allow you to access the methods and data of an object • Processes must have an object handle for every resource/item that it uses e.g. files, I/O devices, etc.

11. Object Handles and Handle Table (Cont.) • Each process has an Object Handle Table that contains all the handles to the various objects that the process has opened

12. Object Handles and Handle Table (Cont.)

13. Object structure

14. Object structure (Cont.) • Objects in the system have object headers and object bodies • Object header contains information such as the object's name, the hierarchy to which it belongs, security information (we will discuss that in more detail later), quota information, number of handles opened on object, pointer to a list of processes that have handles opened on object, pointer to an object that contains information about the object's type

15. Object structure (Cont.) • Object body contains information that is specific to the object • Type object contains information that is common to all objects of a particular type (class) and includes the code for all the methods that belong to the object class

16. Object services • The object manager provides a set of services that are common to all objects: • create - creates object and returns a handle to it • open - returns an object handle to calling process • close - destroys handle • delete - deletes object • duplicate handle • query information held in object

17. Processes and threads In Windows a process consists of program code, execution context ( the address space of the process plus such things as the access token) resources allocated to the process i.e. handles, one or more threads Threads are the units of execution – they execute program code using the processes context and resources

18. Processes and threads (Cont.) A thread has its own context (register values including instruction pointer, execution stack, and such things as scheduling priority) Threads are scheduled onto the processor, not a process – this is different from Linux which for scheduling purposes treats threads as a separate process that happens to share address space with another thread [Note – change to handout]

19. Processes and threads (Cont.) Processes and threads are managed by 2 components in Windows: the process and thread manager and the kernel

20. kernel The kernel is responsible for: Thread scheduling Interrupt and exception handling Low level processor synchronisation Recovery after power failure

21. kernel (Cont.) • In normal Operating Systems the kernel refers to all the operating systems components that run in kernel mode. In Windows as we have seen this applies to all the Windows Executive components (everything below the line in the diagram). • However, perversely, Windows applies the name kernel to just one component - a low level layer of the OS that manages much of the execution of threads within processes

22. kernel (Cont.) The kernel uses objects to represent and manage low level threads and processes However these kernel objects are different from those managed by the Object Manager They are lower level and provide support for the higher level objects used by the Object Manager

23. Process and thread manager Process manager is part of the Windows Executive and implements the representation of processes and threads that are available to other parts of the Executive and provides high level services to manage and control processes and threads

24. Process and thread manager (Cont.) • Process and thread manager provides functions to • Create and destroy processes • Control resource allocation to processes • Keep track of information about processes and threads • Processes are created by other processes by making a CreateProcess system call

25. Process and thread manager (Cont.) Unlike Unix/Linux the parent and child process are independent of each other – remember fork in Unix/Linux creates the child as a copy of the parent and hence has a copy of the parents address space In Windows child process has completely separate address space, etc. Hence Windows does not keep track of process hierarchies

26. Thread scheduling Windows maintains a list of threads that are in the system Threads may be in one of several different states Ready – thread can execute but waiting for a processor Running – running on a processor Standby - selected to run next on a processor Waiting – unable to execute until some event occurs (typically I/O) Terminated

27. Thread scheduling (Cont.) All processes have at least one thread known as the base thread – created when the process is created

28. Thread scheduling (Cont.) The kernel implements the dispatcher code, which determines which thread to run next The dispatcher implements pre-emptive scheduling among threads The dispatcher schedules threads without reference to the process they belong to – hence a process that has more threads will if everything else is equal have a greater share of the processor

29. Thread scheduling (Cont.) The scheduling algorithm is based on a multilevel priority queue approach with each thread having a priority and hence belonging to a given queue A ready thread is placed in the queue which represents its assigned priority There are 32 priority queue levels designated by numbers with 31 highest priority and 0 lowest

30. Thread scheduling (Cont.) • Dispatcher starts with highest priority queue and schedules threads in order in queue in round robin fashion • Each thread is given a time quantum in which to execute and it either blocks itself waiting on some I/O event or synchronisation event or the quantum expires

31. Thread scheduling (Cont.) • Once a given queue is empty, the dispatcher then proceeds to the next lowest priority queue and so on until the queues are all empty or a higher level priority thread enters a ready state i.e. one of the higher level queues is now no longer empty – and dispatcher pre-empts lower priority running thread

32. Thread scheduling (Cont.) Highest priority levels (16-31) is for real-time threads (needing immediate responsiveness) – the real-time priorities are static Lower priority levels (0-15) are for dynamic priority threads A processes base thread is given a base priority – which is the minimum priority a thread can have

33. Thread scheduling (Cont.) Each process has: a base priority class (a range of priority levels which the define the possible range of base priorities) and a base priority level which specifies the relative base priority a threads should have in the base priority class

34. Thread scheduling (Cont.) Each thread then takes on priority values dynamically i.e. it changes over time e.g. if the thread is delayed waiting on an I/O event, when the I/O event occurs and the thread becomes ready again, its priority is increased temporarily. The size of the increase depends on the length of the wait – the longer the wait the greater the increase in priority

35. Traps and exception handling Kernel implements a trap handler which deals with hardware interrupts and processor exceptions The trap handler disables interrupts, determines the cause of the interrupt, saves processor state, re-enables interrupts and dispatches code to deal with type of interrupt/exception found (Interrupt service routine)

36. Thread synchronisation Windows provides a set of dispatcher objects which can be used for synchronisation of thread behaviour These dispatcher objects can be in a signalled state (when the event that the thread is waiting for occurs) or a unsignaled state (when the event has not yet occurred)

37. Thread synchronisation (Cont.) Event objects represent events such as I/O events – when the relevant event occurs the object manager sets the event object to the signalled state Mutex objects provide mutual exclusion – only one thread can have a mutex object – it does this by setting the object into an unsignaled state. Once the thread completes its activity it sets the mutex object to the signalled state Waitable timer objects – these objects remain unsignaled until a given time has elapsed

38. Interprocess communication Threads (and hence also processes) can communicate using a variety of methods Pipes – work very similar to Unix – unidirectional communication via a shared buffer Sockets – similar to pipes but usually connect processes and threads on different machines Remote procedure calls – allows a thread in one process to invoke the execution of code in different processes address space File sharing is also implemented

39. Internal process and thread representation • Processes are represented within the NT Executive by a process control block (Executive Process Control or EPROCESS block). • It stores information that other executive components use when manipulating a process – process id, pointer to processes handle table, pointer to processes access token (used to determine security in access), etc.

40. Internal process and thread representation (Cont.) • Also contains a pointer to a kernel level process block (KPROCESS block). This contains information when used in scheduling a process, such as priority, time quantum to be used, etc. • EPROCESS blocks are held in a linked list • Thread information at the NT Executive level is held in an Executive Thread block (ETHREAD block)

41. Internal process and thread representation (Cont.) • As with processes the ETHREAD block includes a pointer to a kernel level kernel thread block (KTHREAD block)

42. References • Operating System Concepts. Chapter 22.