Transcript os-1
Operating Systems Part I: Introduction “I think that there is a world market for five computers” - Thomas J. Watson (1945) Why Study Operating Systems? We want to have an efficient O/S because it – – – – consumes more resources than any other program. is the most complex program. is necessary for any use of the computer. is used by many users. Efficiency is measured through – – – Functionality Performance: Time and Utilization Convenience and Cost Goals of This Course Understand what an operating system is Understand the key components of an operating system Have a deeper understanding of common operating systems in the market (e.g. Windows, Unix, MS-DOS) and the issues associated with them To be able to use performance measures What is an O/S? •A layer of abstraction between the HW and SW •A resource coordinator •Virtual machine •Reactive system Operating Systems: A Definition A collection of programs that integrate the hardware resources of the computer and make those resources available to the user in a productive, timely, and efficient manner Operating Systems Ease the Pain Performs the interface task with the hardware (file operations, memory paging, etc.) which should have been done by the user if the OS did not exist High-level interface (GUI, command line a.k.a. CUI) The O/S’s capability for multiuser and multitasking utilize the hardware efficiently Makes visible the “virtual” component of the system Allows program interaction Why are Operating Systems Difficult to Create and Maintain? Size – Too big for one person; current systems have millions of lines of code and involve 10-100 man years to build Lifetime – Operating systems remain longer than the programmers who originally wrote them. Code is written and rewritten and original intent is forgotten (Unix designed to be cute, small system - now several volumes thick!) Why are Operating Systems Difficult to Create and Maintain? Complexity – Multitasking – The system must do difficult things -- deal with ugly I/O devices, multiplexing/juggling act, handle errors Must do several things at once. General purpose A Brief History: Early 1950’s, Mainframes Rule! Early systems – – No O/S! Programmer is also operator Large machines run from a console; programs loaded through switches and card readers Simple batch systems were the first real OS – – – Setup time was a problem -> hire an operator Operator ran related jobs together O/S was a simple program stored in one part of memory Loads a single job from card reader into memory Transfers control from one job to the next Offline Processing Allowed jobs to be read ahead of time onto tape Card Reader Line printer CPU On-line processing Card Reader Tape Drive Tape Drive CPU Off-line processing Tape Drive Line printer Tape Drive History: Spooling Allowed jobs to be read ahead onto disk Spool (Simultaneous Peripheral Operation OnLine) disk Card Reader CPU Line printer Multiprogrammed Systems Multiprogrammed batch systems provided increased utilization – – – – Keeps several jobs in memory simultaneously I/O processing of one job overlaps with computation of another Analogy: Lawyer working on several cases; while waiting to go to trial on one, can work on another Needs CPU scheduling Timesharing/Multitasking Systems Timesharing supported interactive use – – – Each user feels as if he/she has the entire machine Tries to optimize response time Based on time-slicing; divide CPU equally among others Desktop Systems First appeared in the 1970s More popularly known as personal computers (PCs) Breakthroughs in hardware allowed downsizing from expensive mainframes Multiprocessor Systems Also known as parallel systems or tightly-coupled systems Three main advantages – – – Increased throughput (more CPUs = more work in less time) Economy of scale (saves money, CPUs share peripherals) Increased reliability (provides redundancy and fault tolerance) Symmetric multiprocessing (SMP): All CPUs do the same thing Asymmetric multiprocessing: each CPU has specific role (usually master-slave) Distributed (Loosely-Coupled) Systems Facilitates use of geographically distributed computing resources Supports communications between parts of a job or different jobs Supports sharing of distributed resources, both hardware and software Client-server systems vs. Peer-to-peer systems Clustered Systems Makes several CPUs work together to accomplish computational task Most likely share storage and linked through a local area network (LAN) Possible clustering schemes: – – Symmetric mode (two or more hosts running applications and monitoring each other) Asymmetric clustering (one is in hot standby mode while another is running applications; switches to backup if active fails) Real Time Systems Used for specialized applications: subway systems, flight-control, factories, power plants Basic idea: O/S guarantees response to physical events will occur within a fixed amount of time Problem faced : Schedule activities so as to meet all time constraints Hard real time system – – Deadline is critical Typically used to control a device Soft real time system – – Deadline is important but not critical Example : Video applications (Use in PC environment) Handheld Systems Used in PDAs and cellular phones Common concerns: – – – Limited main memory Processor speed Small display screens General Structure of an O/S Resident Programs Programs which are critical to the operation of the system KERNEL Non-resident Programs Loaded into memory only when needed An Example: MS-DOS Structure Memory resident components – – – Command interface shell (eg. ver, dir, date, time) : COMMAND.COM Set of I/O routines which control each I/O devices (drivers) -- e.g. BIOS : IO.SYS File Management System : MSDOS.SYS Non-resident components – FORMAT.COM, XCOPY.EXE, LABEL.EXE, etc. How MS-DOS Programs are Loaded in Main Memory At System Start-up Running a Program Operating System Components Process Management – – – – Process: a program in execution Keeps track of each process and it’s state Create, delete, suspend, resume processes; synchronize process communications, handle deadlocks Possibly support threads (executable parts of a process) Operating System Components Main Memory Management – – – Keep track of which parts of memory are in use Allocates and deallocates memory as needed Decides which processes must be loaded in main memory when space becomes available Operating System Components File Management – – – – Keeps tracks of available space on the system Maintains directory structure and hierarchy Supports file manipulation commands Keeps track of file information (inode, name, timestamp) I/O System Management – – Allows for a standard methodology for access of each device Use of device drivers for modularity Operating System Components Secondary Storage Management – – – Free space management Storage allocation Disk scheduling Networking – – Transfer protocols Routing and connection strategies Operating System Components Protection System – – Provide mechanism for controlling access to programs, processes, or users Essential in multitasking and multiuser systems Command Interpreter System – – GUI vs. Command Line Interface Redirection, Piping, and Parameter Passing Operating Systems Architectures Monolithic Layered Virtual Machine Microkernel Monolithic Easy to implement “The Big Mess” – virtually no structure! Kernel is one large structure Each procedure is visible to every other procedure Not used anymore Layered Not easy to implement because some functionalities are mutually dependent. Inefficient because it requires a high number of traversals of interfaces Virtual Machine Non-virtual Machine Virtual Machine Each user has a “virtual machine” and he can choose which OS to run on that machine Elegant, but does not deal with questions of resource management and responsiveness Microkernel Used in Mac/OSF/NT Takes out as much functionality as possible from kernel -- allows modularity and portability across platforms Interactions between processes involve the kernel, thereby requiring high efficiency in transfer