II. Operating Systems and Processes!

Transcription

1 II. Operating Systems and Processes! Computer Architecture & Operating Systems: 725G84 Ahmed Rezine

2 What is an Operating System? A program that acts as an intermediary between a user of a computer and the computer hardware Operating system goals: Execute user programs and make solving user problems easier Make the computer system convenient to use Use the computer hardware in an efficient manner

3 What is an Operating System?

4 What Operating Systems Do Depends on the point of view Users want convenience, ease of use and good performance Don t care about resource utilization But shared computers such as mainframes must keep all users happy Users of dedicated systems such as workstations have dedicated resources but frequently use shared resources from servers Handheld computers are resource poor, optimized for usability and battery life Some computers have little or no user interface, such as embedded computers in devices and automobiles

5 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

6 Operating System Definition (Cont.) No universally accepted definition Everything a vendor ships when you order an operating system is a good approximation But varies wildly The one program running at all times on the computer is the kernel. Everything else is either a system program (ships with the OS), or an application program.

7 Computer System Organization One or more CPUs, device controllers connect through common bus providing access to shared memory Concurrent execution of CPUs and devices competing for memory cycles

8 CPU I/O Device Interaction (1) I/O devices and the CPU can execute concurrently. Each device controller has a local buffer. CPU moves data from/to main memory to/from local buffers I/O is from the device to local buffer of controller. Device controller informs CPU that it has finished its operation by causing! an interrupt. Remark: Alternative to interrupt usage: Polling / Busywaiting, see [SGG7] Ch

9 Background: Interrupt Program execution (von-neumann cycle) by a processor Instruction Fetch Instruction Decode Instruction Execute Update Program Counter No way to react to events not explicitly anticipated in the (user) program code TDDB68 by Prof. C. Kessler

10 Background: Interrupt Program execution (von-neumann cycle) by a processor Instruction Fetch Save processor state Instruction Decode Instruction Execute Execute interrupt service routine (ISR) Check for Interrupt No Update Program Counter Yes Restore processor state TDDB68 by Prof. C. Kessler

11 Common Functions of Interrupts Interrupt transfers control to the interrupt service 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 for an OS service

12 Transition from User to Kernel Mode Timer to prevent infinite loop / process hogging resources Timer is set to interrupt the computer after some time period Keep a counter that is decremented by the physical clock. Operating system set the counter (privileged instruction) When counter zero generate an interrupt Set up before scheduling process to regain control or terminate program that exceeds allotted time

13 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 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) jobs are switched 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

14 Operating System Operations Dual mode, system calls CPU management Uniprogramming, Multiprogramming, Multitasking Process management Memory management File system and mass storage management Protection and security

15 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 Typically a 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

16 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

17 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, datatransfer 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

18 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

19 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

20 Process Concept Program is passive entity stored on disk (executable file), process is active. Consider multiple users executing the same program Process a program in execution; process execution must progress in sequential fashion Multiple parts The program code, also called text section Current activity including program counter, processor registers Stack containing temporary data: function parameters, return addresses, local variables Data section containing global variables Heap containing memory dynamically allocated during run time

21 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

22 Process Control Block (PCB) Information associated with each process (also called task control block) Process state: running, waiting, etc Program counter: location of next instruction CPU registers: contents of processor s immediate memory 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

23 CPU Switch From Process to Process

24 Process Scheduling Maximize CPU use, quickly switch processes onto CPU for time sharing Process scheduler selects among available processes for next execution on CPU Maintains scheduling queues of processes Job queue set of all processes in the system Ready queue set of all processes residing in main memory, ready and waiting to execute Device queues set of processes waiting for an I/O device Processes migrate among the various queues Queuing diagram

25 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 è the longer the context switch

26 Concurrency vs. Parallelism Concurrent execution on single-core system: single core T 1 T 2 T 3 T 4 T 1 T 2 T 3 T 4 T 1 time Parallelism on a multi-core system: core 1 T 1 T 3 T 1 T 3 T 1 core 2 T 2 T 4 T 2 T 4 T 2 time

27 Scheduling: Basic Concepts Maximum CPU utilization obtained with multiprogramming CPU I/O Burst Cycle Process execution consists of a cycle of CPU execution and I/O wait CPU burst followed by I/O burst CPU burst distribution is of main concern load store add store read from file wait for I/O store increment index write to file wait for I/O load store add store read from file wait for I/O CPU burst I/O burst CPU burst I/O burst CPU burst I/O burst

28 Histogram of CPU-burst Times

29 CPU Scheduler from Ready Queue CPU scheduling decisions may happen when a process: 1. Switches from running to waiting state 2. Switches from running to ready state 3. Switches from waiting to ready 4. Terminates Scheduling under 1 and 4 is nonpreemptive All other scheduling is preemptive. shared data, preemption during crucial OS activities

30 Scheduling Criteria CPU utilization keep the CPU as busy as possible Throughput # of processes that complete their execution per time unit Turnaround time amount of time to execute a particular process: from submission to completion, including waiting to get to memory, ready queue, executing on CPU, I/O, Waiting time amount of time a process has been waiting in the ready queue Response time amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)

31 First- Come, First-Served (FCFS) Scheduling Process Burst Time P 1 24 P 2 3 P 3 3 Suppose that the processes arrive in the order: P 1, P 2, P 3 The Gantt Chart for the schedule is:!!!! P P P Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 Average waiting time: ( )/3 = 17

32 FCFS Scheduling (Cont.) Process Burst Time P 1 24 P 2 3 P 3 3 Suppose that the processes arrive in the order: P 2, P 3, P 1 The Gantt chart for the schedule is:! P 2 P Waiting time for P 1 = 6; P 2 = 0 ; P 3 = 3 P 1 Average waiting time: ( )/3 = 3 Much better than previous case Convoy effect - short process behind long process Consider one CPU-bound and many I/O-bound processes

33 Shortest-Job-First (SJF) Scheduling Associate with each process the length of its next CPU burst Use these lengths to schedule the process with the shortest time SJF is optimal gives minimum average waiting time for a given set of processes The difficulty is knowing the length of the next CPU request

34 Example of SJF Process Burst Time P 1 6 P 2 8 P 3 7 P 4 3 SJF scheduling chart P 4 P 1 P 3 P Average waiting time = ( ) / 4 = 7

35 Example of Shortest-remaining-time-first Process Arrival Time Burst Time P P P P Now we add the concepts of varying arrival times and preemption to the analysis Preemptive SJF Gantt Chart P 1 P 2 P 4 P 1 P Average waiting time = [(10-1)+(1-1)+(17-2)+(5-3)]/ 4 = 26/4 = 6.5 msec

36 Priority Scheduling A priority number (integer) is associated with each process The CPU is allocated to the process with the highest priority (smallest integer highest priority) Preemptive Nonpreemptive SJF is priority scheduling where priority is the inverse of predicted next CPU burst time Problem Starvation low priority processes may never execute Solution Aging as time progresses increase the priority of the process

37 Example of Priority Scheduling Process Burst Time Priority P P P P P Priority scheduling Gantt Chart P P P P P Average waiting time = 8.2 time units

38 Round Robin (RR) Each process gets a small unit of CPU time (time quantum q), usually milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue. If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units. Timer interrupts every quantum to schedule next process Performance q large FIFO q small q must be large with respect to context switch, otherwise overhead is too high

39 Example of RR with Time Quantum = 4 Process Burst Time The Gantt chart is: P 1 24 P 2 3 P 3 3 P P 2 P 3 P 1 P 1 P 1 P P Typically, higher average turnaround than SJF, but better response q should be large compared to context switch time q usually 10ms to 100ms, context switch < 10 usec

40 Time Quantum and Context Switch Time