Advanced Operating Systems

1. Advanced Operating Systems Lecture No. 1 (ii) August 07, 2007 Dr. Anwar Majid Mirza anwar.m.mirza@nu.edu.pk National University of Computer & Emerging Sciences NUCES-FAST, A. K. Brohi Rd., H11, Islamabad, Pakistan

2. What is an Operating System? • A computer fresh off the assembly line, with no software in place, can do absolutely nothing. • It cannot accept characters from the keyboard or display characters on screen. • Faced with the raw “iron”, even experienced programmers find it difficult to accomplish much, and non-technical people are completely lost. • Pure hardware presents a most unfriendly interface. Thus, people rarely communicate directly with the hardware. • Instead, users and application programmers deal with the hardware through a system program called the Operating System.

3. Layers and Views of a Computer System End User Programmer Application Programs Operating System Designer Utilities Operating System Computer Hardware

4. Operating System Objectives and Functions • An operating system is a program that controls the execution of application programs and acts as an interface between the user of a computer and the computer hardware. Its three main objectives are: • Convenience: An operating system makes a computer more convenient to use. • Efficiency: An operating system allows the computer system resources to be used in an efficient manner. • Ability to Evolve: An operating system should be constructed in such a way as to permit the effective development, testing and introduction of new system functions without interfering with service.

5. The Operating System as a User/Computer Interface The operating system typically provides the following services: • Program Creation: The operating system provides a variety of facilities and services, such as editors and debuggers, to assist the programmer in creating programs. Typically these services are in the form of utility programs that are not actually part of the operating system but are accessible through the operating system. • Program Execution: A number of tasks need to be performed to execute a program. Instructions and data must be loaded into main memory, input/output devices and files must be initialized, and other resources must be prepared. The OS handles all of this for the user. • Access to I/O Devices: Each I/O device requires its own peculiar set of instructions or control signals for operation. The operating system takes care of the details so that the programmer can think in terms of simple reads and writes.

6. The Operating System as a User/Computer Interface (continued…) • Controlled Access to Files: In the case of files, control must include an understanding of not only the nature of the I/O device (disk drive, CD ROM drive, tape drive) but also the file format on the storage medium. Again, the OS worries about the details. Further, in the case of a system with multiple simultaneous users, the OS can provide protection mechanisms to control access to the files. • System Access: In the case of a shared or public system, the OS controls access to the system as a whole and to specific system resources. The access function must provide protection of resources and data from unauthorized users and must resolve conflicts for resource contention.

7. The Operating System as a User/Computer Interface (continued…) • Error Detection and Response: A variety of errors can occur when a computer system is running. These include internal and external hardware errors, such as a memory error or a device failure or malfunction; and various software errors, such as arithmetic overflow, attempt to access forbidden memory location etc. In each case the OS must make the response that clears the error condition with the least impact on running applications. The response may range from ending the program that caused the error, to retrying the operation, to simple reporting the error to the application • Accounting: A good operating system will collect usage statistics for various resources and monitor performance parameters such as response time. On any system, this information is useful in anticipating the need for future enhancements and in tuning the system to improve performance. On a multi-user system, the information can be used for billing purposes.

8. The Operating System as a Resource Manager • A computer is a set of resources, for the movement, storage and processing of data and for the control of these functions. • By managing the computer’s resources, the operating system is in control of the computer’s basic functions. But this control is exercised in a curious way! • The operating system is nothing more than a computer program. • Like other programs it provides instructions for the processor. • The key difference is in the intent of the program. • The operating system directs the processor in the use of other system resources and in the timing of its execution of other programs. • But in order for the processor to do any of these things, it must cease executing the OS program and execute other programs. • Thus the OS relinquishes control for the processor to do some “useful” work and then resumes control long enough to prepare the processor to do the next piece of work.

9. Ease of Evolution of an Operating System A major operating system will evolve over time for a number of reasons: • Hardware upgrades and new types of hardware: For example, early versions of UNIX and OS/2 did not employ a paging mechanism because they were run on machines without paging hardware. More recent versions have been modified to exploit paging capabilities. Also, the use of graphics terminals and page-mode terminals instead of line-at-a-time scroll mode terminals have affected the operating system design. • New Services: In response to user demand or in response to the needs of the system managers, the operating system will expand to offer new services. For example, if it is found to be difficult to maintain good performance for users with existing tools, new measurements and control tools may be added to the operating system. • Fixes: Any operating system has faults. These are discovered over the course of time, and fixes are made. Of course, the fixes may introduce new faults.

10. Processes • The concept of process is fundamental to the structure of operating systems. • Many definitions have been given for the term process, including: • A program in execution. • The “animated spirit” of a program. • The entity that can be assigned to and executed on a processor. • All multiprogramming operating systems, from single-user systems such as Windows NT to mainframe systems such as MVS (Multiple Virtual Storage) that can support thousands of users, are built around the concept of the process.

11. Processes • Many requirements that the operating system must meet can all be expressed with reference to process: • The operating system must interleave the execution of a number of processes to maximize processor use while providing reasonable response time. • The operating system must allocate resources to a process in conforming to a specific policy (e.g. certain functions or applications are of higher priority) while at the same time avoiding deadlocks. • The operating system may be required to support inter-process communication and user creation of processes, both of which may aid in the structuring of applications.

12. Process States • The principle function of a processor is to execute machine instructions residing in main memory. • From the processor point of view, it will execute instructions from its collection of instructions in some sequence dictated by the changing values in the register known as the program counter (PC)orinstruction pointer. • Over time that pointer may refer to code in different programs that are part of different applications. • The behavior of an individual process can be characterized by listing the sequence of instructions that execute for that process. Such a listing is called a traceof the process. • The behavior of the processor can be characterized by showing the way in which the trace of the various processes are interleaved!

13. Main Memory Program Counter 0K 20K Dispatcher 35K 50K Process A 80K 90K Process B 140K Process C 190K Fig.1: Snapshot of example execution Process States • Figure 1 shows a memory layout of three processes. • We assume no use of “virtual memory” i.e. all three processes are represented by programs that are fully loaded in main memory. • There is a small dispatcher program that moves the processor from one process to another.

14. a+0 a+1 a+2 a+3 a+4 a+5 a+6 a+7 a+8 a+9 a+10 a+11 b+0 b+1 b+2 b+3 g+0 g+1 g+2 g+3 g+4 g+5 g+6 g+7 g+8 g+9 g+10 g+11 (a) Trace of Process A (b) Trace of Process B (c) Trace of Process C a, b and g are the starting addresses of the programs of processes A, B and C respectively. Fig.2: Trace of Processes Process States • Figure 2 shows the traces of the three individual processes during the early part of their execution. • The first 12 instructions executed in processes A and C are shown. • Process B executes 4 instructions and it is assumed that the fourth instruction is an I/O operation for which the process must wait.

15. 1 a+0 • 2 a+1 • 3 a+2 • 4 a+3 • 5 a+4 • 6 a+5 • …Time Out • 7 d+0 • 8 d+1 • 9 d+2 • 10 d+3 • 11 d+4 • d+5 • b+0 • b+1 • b+2 • b+3 • …Time Out • d+0 • 18 d+1 • 19 d+2 • 20 d+3 • 21 d+4 • 22 d+5 • g+0 • g+1 • g+2 • g+3 • g+4 • g+5 • …Time Out • 29 d+0 • 30 d+1 • 31 d+2 • 32 d+3 • d+4 • d+5 • a+6 • 36 a+7 • 37 a+8 • 38 a+9 • 39 a+10 • 40 a+11 • …Time Out • 41 d+0 • 42 d+1 • 43 d+2 • 44 d+3 • d+4 • d+5 • g+6 • g+7 • g+8 • g+9 • g+10 • g+11 • …Time Out Process States • Figure 3 shows the traces from the processor’s point of view. • We have assumed that the OS allows a process to continue execution for a maximum of only six instructions cycles after which it is interrupted; this prevents any single process from monopolizing processor time! Fig.3: Combined Trace of Processes

16. A Two-State Process Model • To be able to “design” the operating system effectively, we need to have a clear model of the behavior of a process. • The first step in designing a program to control processes is to describe the behavior that we would like the processes to exhibit. • The simplest possible model can constructed by observing that at any time, a process is either being executed by a processor or not. • Thus a process may be in one of the two states: RunningandNot-Running

17. Dispatch Enter Exit Not Running Running Pause (a) State transition diagram Queue Dispatch Enter Exit Processor Pause (b) Queuing diagram A Two-State Process Model

18. A Two-State Process Model • When the OS creates a new process, it enters that process into the system in the Not-Running state. • Thus the process exists, is know to the OS, and is waiting for an opportunity to execute. • From time to time, the currently running process will be interrupted, and the dispatcher portion of the OS will select a new process and put in the Running state. • The former process moves from the Running state to the Not-Running state, and one of the other processes moves to the running state. Design Issues / Points: • Each process must be represented in some way so that the OS can keep track of it (e.g. its current state and location in memory). • Those processes that are in Not-Running state, are needed to be kept in some sort of queue, waiting their turn.

19. The Creation and Termination of Processes • Regardless of the model of process behavior used, the life of process is bounded by its creation and termination. Creation of Processes • When a new process is to be added to those that are currently being managed by the OS, the OS • Builds the data structures that are used to manage the process and • Allocates the address space to be used by the process. • These actions constitute the creation of a new process.

20. The Creation and Termination of Processes Reasons for Process Creation • New Batch Job: In a batch environment, a process is created in response to the submission of a job. • Interactive Log On: A user at a terminal logs onto the system • Created by OS to Provide a Service: The OS can create a process to perform a function on behalf of a user program, without the user having to wait e.g. printing. • Spawned by Existing Processes: For purposes of modularity or to exploit parallelism, a user program can dictate the creation of a number of processes. When a process is created by the OS at the explicit request of another process, the action is referred as process spawning. • When one process spawns another process, the spawning process is called the parent process and the spawned process is called the child process. • Typically, the “related” processes need to “communicate” and “cooperate” with each other. Achieving this cooperation is a difficult task for the programmers of the OS.

21. The Creation and Termination of Processes Termination of Process • In any computer system, there must be a means for a process to indicate its completion. • A batch job should indicate a “Halt” instruction, which generates an interrupt to alert the operating system that a process has completed. • For an interactive application, the action of the user will indicate when the process is completed. Reasons for Process Termination • Normal Termination:The process executes an OS service call to indicate that it has completed running. • Time Limit Exceeded:The process has run longer than the specified total time limit. There are a number of possibilities for the type of time that is measured. These include (i) total time elapsed, (ii) amount of time spent executing, and (iii) in case of an interactive process, the amount of time since the user last provided any input.

22. The Creation and Termination of Processes Reasons for Process Termination (continued…) Memory Unavailable:The process requires more memory than the system can provide. Bounds Violation:The process tries to access memory locations that it is not allowed to access. Protection Error:The process attempts to use a resource or a file that it is not allowed to use, or it tries to use it in an improper fashion, such as writing to a read-only file. Arithmetic Error:The process tries a prohibited computation, such as division by zero! Time Overrun:The process has waited longer than a specified maximum for a certain event to occur.

23. The Creation and Termination of Processes Reasons for Process Termination (continued…) An I/O Failure:An error occurs in input or output, such as inability to find a file, failure to read or write after specified number of tries (when, for example, a defective area is encountered on a diskette), or invalid operation (such as reading from the line printer). Invalid Instruction:The process attempts to execute a non-existent instruction (often a result of branching into a data area and attempting to execute the data). Privileged Instruction:The Process attempts to use an instruction reserved for the operating system.

24. The Creation and Termination of Processes Reasons for Process Termination (continued…) Data Misuse:A piece of data is of the wrong type or is not initialized. OS or Operator Intervention:For some reason, the operator or the operating system has terminated the process (e.g. if a deadlock exists). Parent Termination:When a parent terminates, the operating system may be designed to automatically terminate all the offspring of that parent. Parent Request:A parent process typically has the authority to terminate any of its offpring.

25. A Five-State Process Model • The queue in the two state process model, is a first-in-first-out (FIFO) list, and the processor operates in round-robin fashion on the available processes (each process in the queue is given a certain amount of time to execute and then returned to the queue unless blocked). • It must be noticed that some processes in the Not-Running state are “ready to execute”, whereas others are “blocked”, waiting an I/O operation to complete. • Thus, using a single queue, the dispatcher could not just select the process at the oldest end of the queue. • Rather, the dispatcher would have to scan the list looking for the process that is not blocked that has been in the queue the longest. • A more natural way is to split the Not-Running state into two states: Ready and Blocked.

26. Five-State Process Model Dispatch Release Running Enter Ready New Exit Time-out Event Occurs Event Wait Blocked State Transition Diagram

27. A Five-State Process Model The five states in this model are • Running:The process is currently being executed. • Ready: Processes that prepared to execute when given opportunity. • Blocked:A process that cannot execute until some event occurs, such as completion of an I/O operation. • New: A process that has just been created but has not yet been admitted to the pool of executable processes by the operating system. • Exit: A process that has been released from the pool of executable processes by the OS, either because it halted or because it aborted for some reason.

28. Ready Queue Dispatch Admit Release Processor Time-out Event-wait Event Occur Block Queue Five State Process Model Queuing diagram with single blocked queue

29. A Five-State Process Model • In the queuing diagram, now there are two queues: a Ready-Queue and a Blocked-Queue. • When it is time for the operating system to choose another process to run, it selects one from the Ready queue. In the absence of any priority scheme, this can be a simple FIFO queue. • When a running program is removed from execution, it is either terminated or placed in the Ready or Blocked queue, depending on the circumstances. • Finally when an event occurs, all processes in the Blocked queue that waiting on that event are moved to the Ready queue. • This latter arrangement means that, when an event occurs, the operating system must scan the entire Blocked queue, searching for those processes waiting on that event. • In a large operating system, there could be hundreds or even thousands of processes in that queue. • Therefore it would more efficient to have a number of queues, one for each event. • Then, when the event occurs, the entire list of processes in the appropriate queue can be moved to the Ready queue.

30. Ready Queue Dispatch Admit Release Processor Time-out Event 1 - wait Event 1 Occur Block Queue 1 Event 2 - wait Event 2 Occur Block Queue 2 Event n - wait Event n Occur Block Queue n Queuing diagram with multiple blocked queue

31. A Five-State Process Model • Finally: if the dispatching of processes is dictated by a priority scheme, then it is convenient to have a number of Ready queues, one for each priority level. • The operating system can then readily determine which is the highest priority Ready process that has been waiting the longest.

32. Suspended Processes The Need for Swapping • The three principal states that have been described (Ready, Running, Blocked) provide a systematic way of modeling the behavior of processes and guide the implementation of the OS. • Many operating systems are constructed using only these three states. • However, there is good justification for adding additional states to the model. Even with multi-programming, a processor could be idle most of the time. • Consider a system that does not employ virtual memory. Then each process to be executed must be loaded fully into main memory. • Thus, in the multiple blocked queuing model, all the processes in all the queues must be resident in main memory. • Recall, that the reason for all this elaborate machinery is that I/O activities are much slower than computation, and therefore, the processor in a “uni-programming” system is idle most of the time.

33. Suspended Processes The Need for Swapping • But the above arrangement does not entirely solve the problem. • It is true that in this case memory holds multiple processes and that the processor can move to another process when one process is waiting. • But the processor is so much faster than I/O that it will be common for all the processes in the memory for waiting for an I/O. • Thus even with multi-programming, a processor could be idle most of the time.

34. Suspended Processes Solutions: • Expand the main memory • There are two flaws in this approach: • There is a cost associated with memory. • Appetite of programs for memory has grown as fast as the cost of memory has dropped. So larger main memory results in “larges processes” not “more processes” • Swapping • Moving part or all of a process from main memory to disk. • When none of the processes in main memory is in the “Ready state”, the OS swaps one of the “blocked processes” out onto disk, into a “suspend queue”. • Suspend queue is a queue of existing processes that have been temporarily kicked out of the main memory or suspended. • The OS then brings another process from the suspend queue, or it honors a new-process request. Execution then continues with the newly arrived process. • Swapping, however, is an I/O operation, and therefore, there is the potential for making the problem worse, not better. But because, disk I/O is generally the fastest I/O on a system (e.g. compared with a printer I/O) swapping will usually enhance performance.

35. Need for including more states in the process model One additional state must be added in the process model to take into account the swapping. Dispatch Release Running Enter Ready New Exit Time-out Activate Event Occurs Event Wait Blocked Suspend Process state model with one suspend state Suspend • When all the processes in main memory are in the “Blocked state”, the OS can suspend • one process by putting it in the “Suspend state” and transferring it to the disk. • The space that is freed up in main memory can then be used to bring another process.

36. Suspended Processes • When the OS has performed a swapping-out operation, it has two choices for selecting a process to bring it into main memory: • It can admit a newly created process or • It can bring in a previously suspended process. • It would appear that the preference should be given to bring a previously suspended process to provide it with service, rather than increasing the total load on the system. Difficulty The above line of reasoning presents a difficulty. • All the processes that have been suspended were in the Blocked state at the time of suspension. • It clearly, would not do any good to bring a Blocked process back into main memory, because it is still not ready for execution. • Recall, that each process in the suspend state was originally blocked on particular event. When that event occurs, the process is not blocked and is potentially available for execution.

37. Suspended Processes Thus we need the following four states: • Ready:The process is in main memory and available for execution. • Blocked:The process is in main memory and awaiting an event. • Blocked, Suspend: The process is in secondary memory and awaiting an event. • Ready, Suspend: The process is in secondary memory but is available for execution as soon as it is loaded into main memory.

38. Suspended Processes New Suspend Enter Enter Activate Dispatch Release Running Ready Suspend Ready Exit Suspend Time-out Event Occurs Event Occurs Event Wait Activate Blocked Blocked Suspend Suspend Process state model with two suspend states

39. Reasons for Process Suspension • Swapping:The OS need to release sufficient main memory to bring in a process that is ready to execute, • Interactive user request:A user may wish to suspend execution of a program for purposes of debugging or in connection with the use of a resource. • Timing: A process may be executed periodically (e.g. an accounting or system monitoring process) and may be suspended while waiting for the next time interval. • Parent process request: A parent process may wish to suspend execution of a descendent to examine or modify the suspended process, or to coordinate the activity of various descendants. • Other OS reason: The OS may suspend a background or utility process or a process that is suspected of causing a problem.