CISC 3595 Operating Systems Part 1

1. CISC 3595 Operating SystemsPart 1 Midterm Review (Chapters 1-5, part of 6)

2. Chapters 1 & 2

3. Operating System Definition OS is a resource allocator Manages all resources Decides between conflicting requests for efficient and fair resource use OS is a control program Controls execution of programs to prevent errors and improper use of the computer

4. Caching Important principle, performed at many levels in a computer (in hardware, operating system, software) Information in use copied from slower to faster storage temporarily Faster storage (cache) checked first to determine if information is there If it is, information used directly from the cache (fast) If not, data copied to cache and used there Cache smaller than storage being cached Cache management important design problem Cache size and replacement policy

5. How a Modern Computer Works A von Neumann architecture

6. Computer-System Architecture Most systems use a single general-purpose processor Most systems have special-purpose processors as well Multiprocessorssystems growing in use and importance Also known as parallel systems, tightly-coupled systems Advantages include: Increased throughput Economy of scale Increased reliability – graceful degradation or fault tolerance Two types: Asymmetric Multiprocessing – each processor is assigned a specific task. Symmetric Multiprocessing – each processor performs all tasks

7. Operating System Structure Multiprogramming (Batch system) needed for efficiency Single user cannot keep CPU and I/O devices busy at all times Multiprogramming organizes jobs (code and data) so CPU always has one to execute A subset of total jobs in system is kept in memory One job selected and run via job scheduling When it has to wait (for I/O for example), OS switches to another job Timesharing (multitasking)is logical extension in which CPU switches jobs so frequently that users can interact with each job while it is running, creating interactive computing Response time should be < 1 second Each user has at least one program executing in memory process If several jobs ready to run at the same time  CPU scheduling If processes don’t fit in memory, swapping moves them in and out to run Virtual memory allows execution of processes not completely in memory

8. Operating-System Operations Interrupt driven (hardware and software) Hardware interrupt by one of the devices Software interrupt (exception or trap): Software error (e.g., division by zero) Request for operating system service Other process problems include infinite loop, processes modifying each other or the operating system

9. Common Functions of Interrupts Interrupt transfers control to the interrupt service routine generally, through the interruptvector, which contains the addresses of all the service routines Interrupt architecture must save the address of the interrupted instruction A trap or exception is a software-generated interrupt caused either by an error or a user request An operating system is interrupt driven

10. Operating-System Operations (cont.) Dual-mode operation allows OS to protect itself and other system components User mode and kernel mode Mode bit provided by hardware Provides ability to distinguish when system is running user code or kernel code Some instructions designated as privileged, only executable in kernel mode System call changes mode to kernel, return from call resets it to user Increasingly CPUs support multi-mode operations i.e. virtual machine manager (VMM) mode for guest VMs

11. Process Management A process is a program in execution. It is a unit of work within the system. Program is a passive entity, process is an active entity. Process needs resources to accomplish its task CPU, memory, I/O, files Initialization data Process termination requires reclaim of any reusable resources Single-threaded process has one program counterspecifying location of next instruction to execute Process executes instructions sequentially, one at a time, until completion Multi-threaded process has one program counter per thread Typically system has many processes, some user, some operating system running concurrently on one or more CPUs Concurrency by multiplexing the CPUs among the processes / threads

12. Process Management Activities Creating and deleting both user and system processes Suspending and resuming processes Providing mechanisms for process synchronization Providing mechanisms for process communication Providing mechanisms for deadlock handling The operating system is responsible for the following activities in connection with process management:

13. Memory Management To execute a program all (or part) of the instructions must be in memory All (or part) of the data that is needed by the program must be in memory. Memory management determines what is in memory and when Optimizing CPU utilization and computer response to users Memory management activities Keeping track of which parts of memory are currently being used and by whom Deciding which processes (or parts thereof) and data to move into and out of memory Allocating and deallocating memory space as needed

14. Storage Management OS provides uniform, logical view of information storage Abstracts physical properties to logical storage unit - file Each medium is controlled by device (i.e., disk drive, tape drive) Varying properties include access speed, capacity, data-transfer rate, access method (sequential or random) File-System management Files usually organized into directories Access control on most systems to determine who can access what OS activities include Creating and deleting files and directories Primitives to manipulate files and directories Mapping files onto secondary storage Backup files onto stable (non-volatile) storage media

15. Mass-Storage Management Usually disks used to store data that does not fit in main memory or data that must be kept for a “long” period of time Proper management is of central importance Entire speed of computer operation hinges on disk subsystem and its algorithms OS activities Free-space management Storage allocation Disk scheduling Some storage need not be fast Tertiary storage includes optical storage, magnetic tape Still must be managed – by OS or applications Varies between WORM (write-once, read-many-times) and RW (read-write)

16. I/O Subsystem One purpose of OS is to hide peculiarities of hardware devices from the user I/O subsystem responsible for Memory management of I/O including buffering (storing data temporarily while it is being transferred), caching (storing parts of data in faster storage for performance), spooling (the overlapping of output of one job with input of other jobs) General device-driver interface Drivers for specific hardware devices

17. Protection and Security Protection – any mechanism for controlling access of processes or users to resources defined by the OS Security – defense of the system against internal and external attacks Huge range, including denial-of-service, worms, viruses, identity theft, theft of service Systems generally first distinguish among users, to determine who can do what User identities (user IDs, security IDs) include name and associated number, one per user User ID then associated with all files, processes of that user to determine access control Group identifier (group ID) allows set of users to be defined and controls managed, then also associated with each process, file Privilege escalation allows user to change to effective ID with more rights

18. Computing Environments - Virtualization Allows operating systems to run applications within other OSes Vast and growing industry Emulation used when source CPU type different from target type (i.e. PowerPC to Intel x86) Generally slowest method When computer language not compiled to native code – Interpretation Virtualization – OS natively compiled for CPU, running guest OSes also natively compiled Consider VMware running WinXP guests, each running applications, all on native WinXP host OS VMM (virtual machine Manager) provides virtualization services

19. Operating System Services (Cont.) • One set of operating-system services provides functions that are helpful to the user (Cont.): • File-system manipulation - The file system is of particular interest. Programs need to read and write files and directories, create and delete them, search them, list file Information, permission management. • Communications – Processes may exchange information, on the same computer or between computers over a network • Communications may be via shared memory or through message passing (packets moved by the OS) • Error detection – OS needs to be constantly aware of possible errors • May occur in the CPU and memory hardware, in I/O devices, in user program • For each type of error, OS should take the appropriate action to ensure correct and consistent computing • Debugging facilities can greatly enhance the user’s and programmer’s abilities to efficiently use the system

20. Operating System Services (Cont.) • Another set of OS functions exists for ensuring the efficient operation of the system itself via resource sharing • Resource allocation - When multiple users or multiple jobs running concurrently, resources must be allocated to each of them • Many types of resources - CPU cycles, main memory, file storage, I/O devices. • Accounting - To keep track of which users use how much and what kinds of computer resources • Protection and security - The owners of information stored in a multiuser or networked computer system may want to control use of that information, concurrent processes should not interfere with each other • Protection involves ensuring that all access to system resources is controlled • Security of the system from outsiders requires user authentication, extends to defending external I/O devices from invalid access attempts

21. A View of Operating System Services

22. System Calls • Programming interface to the services provided by the OS • Typically written in a high-level language (C or C++) • Mostly accessed by programs via a high-level Application Programming Interface (API)rather than direct system call use • Three most common APIs are Win32 API for Windows, POSIX API for POSIX-based systems (including virtually all versions of UNIX, Linux, and Mac OS X), and Java API for the Java virtual machine (JVM) Note that the system-call names used throughout this text are generic

23. Types of System Calls (Cont.) • Information maintenance • get time or date, set time or date • get system data, set system data • get and set process, file, or device attributes • Communications • create, delete communication connection • send, receive messages if message passing model to host name or process name • From client to server • Shared-memory model create and gain access to memory regions • transfer status information • attach and detach remote devices

24. API – System Call – OS Relationship

25. Chapter 3 Processes

26. Process Concept • An operating system executes a variety of programs: • Batch system – jobs • Time-shared systems – user programs or tasks • Textbook uses the terms job and process almost interchangeably • Process – a program in execution; process execution must progress in sequential fashion • Multiple parts • The program code, also called text section • Current activity including programcounter, processor registers • Stackcontaining temporary data • Function parameters, return addresses, local variables • Data sectioncontaining global variables • Heapcontaining memory dynamically allocated during run time

27. Process in Memory

28. Process State • As a process executes, it changes state • new: The process is being created • running: Instructions are being executed • waiting: The process is waiting for some event to occur • ready: The process is waiting to be assigned to a processor • terminated: The process has finished execution

29. Diagram of Process State

30. Process Control Block (PCB) Information associated with each process (also called task control block) • Process state – running, waiting, etc • Process number – unique id • Program counter – location of instruction to next execute • CPU registers – contents of all process-centric registers • CPU scheduling information- priorities, scheduling queue pointers • Memory-management information – memory allocated to the process • Accounting information – CPU used, clock time elapsed since start, time limits • I/O status information – I/O devices allocated to process, list of open files

31. CPU Switch From Process to Process

32. Schedulers • Processes can be described as either: • I/O-bound process– spends more time doing I/O than computations, many short CPU bursts • CPU-bound process – spends more time doing computations; few very long CPU bursts

33. Context Switch • When CPU switches to another process, the system must save the state of the old process and load the saved state for the new process via a context switch • Context of a process represented in the PCB • Context-switch time is overhead; the system does no useful work while switching • The more complex the OS and the PCB  the longer the context switch • Time dependent on hardware support • Some hardware provides multiple sets of registers per CPU  multiple contexts loaded at once

34. Process Creation • Parentprocess create childrenprocesses, which, in turn create other processes, forming a tree of processes • Generally, process identified and managed via aprocess identifier (pid) • Resource sharing options • Parent and children share all resources • Children share subset of parent’s resources • Parent and child share no resources • Execution options • Parent and children execute concurrently • Parent waits until children terminate

35. Process Creation (Cont.) • Address space • Child duplicate of parent • Child has a program loaded into it • UNIX examples • fork()system call creates new process • exec() system call used after a fork() to replace the process’ memory space with a new program

36. C Program Forking Separate Process

37. Creating a Separate Process via Windows API

38. Process Termination • Process executes last statement and then asks the operating system to delete it using the exit() system call. • Returns status data from child to parent (via wait()) • Process’ resources are deallocated by operating system • Parent may terminate the execution of children processes using the abort() system call. Some reasons for doing so: • Child has exceeded allocated resources • Task assigned to child is no longer required • The parent is exiting and the operating systems does not allow a child to continue if its parent terminates

39. Process Termination • Some operating systems do not allow child to exist if its parent has terminated. If a process terminates, then all its children must also be terminated. • cascading termination.All children, grandchildren, etc. are terminated. • The termination is initiated by the operating system. • The parent process may wait for termination of a child process by using the wait()system call. The call returns status information and the pid of the terminated process pid = wait(&status); • If no parent waiting (did not invoke wait()) process is a zombie • If parent terminated without invoking wait , process is an orphan

40. Review Process Management • What is the difference between a program and a process? • A program is a passive entity and a process is an active entity. • Both have code, but a process has a state, a stack, a heap, a program counter, registers, paging information, possibly file descriptors and other resources, possibly child processes or threads. • What are the possible process states? • New, Ready, Running, Waiting (multiple forms), Terminated • What is the term for switching out one process and replacing it with another process? • Context switch – can take 1-2 thousand machine instructions. • On Linux/Unix systems, how do we create a new process? • fork() – made more efficient by using “Copy-on-Write”, used to be vfork() • exec() – overwrites a process with program specified in arguments.

41. Review Process Control Block • How do we terminate a process? • exit – called from within a process to terminate • abort – called from outside to terminate (usually by the parent). • What information is contained in the process control block? • Process id and state – running, waiting, etc • Program counter – location of instruction to next execute • CPU registers – contents of all process-centric registers • CPU scheduling information- priorities, scheduling queue pointers • Memory-management information – memory allocated to the process • Accounting information – CPU used, clock time elapsed since start, time limits • I/O status information – I/O devices allocated to process, list of open files

42. Interprocess Communication • Processes within a system may be independentor cooperating • Cooperating process can affect or be affected by other processes, including sharing data • Reasons for cooperating processes: • Information sharing • Computation speedup to increase throughput • Modularity for creating separable services that can cooperate • Convenience such as piping output of one command into another • Cooperating processes need interprocess communication (IPC) • Two models of IPC • Shared memory • Message passing

43. Communications Models (a) Message passing. (b) shared memory.

44. Producer-Consumer Problem • Paradigm for cooperating processes, producer process produces information that is consumed by a consumer process • unbounded-buffer places no practical limit on the size of the buffer • bounded-buffer assumes that there is a fixed buffer size Any problems? • An unbounded-buffer dynamically expands to the need of the data passed. Subject to hardware or system limits. • Consumer should be as fast or faster than producer • A bounded-buffer fixed buffer size may be a multiple of the message size or may be a function of the page size.

45. Bounded-Buffer – Producer item next_produced; while (true) { /* produce an item in next produced */ while (((in + 1) % BUFFER_SIZE) == out) ; /* do nothing */ buffer[in] = next_produced; in = (in + 1) % BUFFER_SIZE; } // Busy wait loop. The process keeps the CPU busy checking. // Put a sleep or wait in the loop, let’s another process run.

46. Bounded Buffer – Consumer item next_consumed; while (true) { while (in == out) ; /* do nothing */next_consumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; /* consume the item in next consumed */ } // Busy wait loop. The process keeps the CPU busy checking. // Put a sleep or wait in the loop, let’s another process run.

47. Interprocess Communication – Shared Memory • An area of memory shared among the processes that wish to communicate • The communication is under the control of the user processes not the operating system. • Synchronization as well. • Processes must reside on the same system. • Major issues is to provide mechanism that will allow the user processes to synchronize their actions when they access shared memory. • Synchronization is discussed in great details in Chapter 5.

48. Interprocess Communication – Message Passing • Mechanism for processes to communicate and to synchronize their actions • Message system – processes communicate with each other without resorting to shared variables • IPC facility provides two operations: • send(message) • receive(message) • The message size is either fixed or variable • Fixed message size is agreed on in advance. • Variable message size has to send information about the size in the message.

49. Message Passing (Cont.) • Implementation of communication link • Physical: • Shared memory • Hardware bus • Network • Logical: • Direct or indirect (via an intermediary) • Synchronous or asynchronous • Automatic or explicit buffering (message queuing)

50. Direct Communication • Processes must name each other explicitly: • send (P, message) – send a message to process P • receive(Q, message) – receive a message from process Q • Properties of communication link • Links are established automatically • A link is associated with exactly one pair of communicating processes • Between each pair there exists exactly one link • The link may be unidirectional, but is usually bi-directional