process scheduling iii. processes - uni konstanz · process scheduling 3. assignment 2. assignment...

Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009

III.Process Scheduling

1 Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009

Intended Schedule

Date Lecture Hand out Submission0 20.04. Introduction to Operating Systems Course reggistration1 27.04. Systems Programming using C (File Subsystem) 1. Assignment2 04.05. Systems Programming using C (Process Control) 2. Assignment 1. Assignment3 11.05. Process Scheduling 3. Assignment 2. Assignment4 18.05. Process Synchronization 4. Assignment 3. Assignment5 25.05. Inter Process Communication 5. Assignment 4. Assignment6 01.06. Pfingstmontag 6. Assignment 5. Assignment7 08.06. Input / Output 7. Assignment 6. Assignment8 15.06. Memory Management 8. Assignment 7. Assignment9 22.06.

Filesystems9. Assignment 8. Assignment

10 29.06.Filesystems

10. Assignment 9. Assignment11 06.07. Special subject: Transactional Memory 10. Assignment12 13.07. Special subject: XQuery your Filesystem13 20.07. Wrap up session

27.07. First examination daate12.10. Second examination date

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Processes

3 Silberschatz, Galvin and Gagne ©2005Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009

An operating system executes a variety of programs:

Batch system – jobs

Time-shared systems – user programs or tasks

Textbooks use the terms job and process almost interchangeably

Process – a program in execution; process execution must progress in sequential fashion

For the OS, processes are the unit of all resource allocation

A process includes:

program counter

stack

data section

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Process in memory

Silberschatz, Galvin and Gagne ©2005Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009

Information associated with each process

Process state

Program counter

CPU registers

CPU scheduling information

Memory-management information

Accounting information

I/O status information


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 process

terminated: The process has finished execution

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At any time, processes can be described as either:

I/O-bound – spends more time doing I/O than computations, many short CPU bursts

CPU-bound – spends more time doing computations; few very long CPU bursts

typical processes follow a CPU–I/O burst cycle

Process execution consists of a cycle of CPU execution and I/O wait

Operating system tries to maximize CPU utilization: multiprogramming

run multiple processes and try to interleave their CPU-I/O burst cycles

Scheduling

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Single Process


Process Switching

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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

Context-switch time is overhead; the system does no „useful work“ while switching

Time dependent on hardware support


Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue

controls the degree of multiprogramming

invoked very infrequently (sec, min) may be slow

Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU

invoked very frequently (msec) must be fast

Medium-term scheduler – swaps running processes out

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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

11 Silberschatz, Galvin and Gagne ©2005Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009 12


Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them

CPU scheduling decisions may take place 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


Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves:

switching context

switching to user mode

jumping to the proper location in the user program to restart that program

Dispatch latency – time it takes for the dispatcher to stop one process and start another running

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SchedulingCPU 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

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)

Deadlines – esp. for real-time scheduling

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OptimizationMax CPU utilization

Max throughput

Min turnaround time

Min waiting time

Min response time


Scheduling Strategies

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Process Burst Time (duration)

P1 24

P2 3

P3 3

Suppose that the processes arrive in the order: P1 , P2 , P3

The Gantt Chart for the schedule is:

Waiting time for P1 = 0; P2 = 24; P3 = 27

Average waiting time: (0 + 24 + 27)/3 = 17

P1 P2 P3

24 27 300


Suppose that the processes arrive in the order

P2 , P3 , P1

The Gantt chart for the schedule is:

Waiting time for P1 = 6; P2 = 0; P3 = 3

Average waiting time: (6 + 0 + 3)/3 = 3

Much better than previous case

Convoy effect (or phenomenon) short processes behind long process

P1P3P2

63 300

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Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time

Two schemes:

nonpreemptive – once CPU given to the process it cannot be preempted until completes its CPU burst

preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF)

SJF is optimal – gives minimum average waiting time for a given set of processes


Process Arrival Time Burst Time

P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4

SJF (non-preemptive)

Average waiting time = (0 + 6 + 3 + 7)/4 = 4

P1 P3 P2

73 160

P4

8 12

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Process Arrival Time Burst Time

P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4

SJF (preemptive)

Average waiting time = (9 + 1 + 0 +2)/4 = 3

P1 P3P2

42 110

P4

5 7

P2 P1

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21Silberschatz, Galvin and Gagne ©2005Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009

Can only estimate the length

Can be done by using the length of previous CPU bursts, using exponential averaging

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= 0

n+1 = n

Recent history does not count

= 1

n+1 = tn

Only the actual last CPU burst counts

If we expand the formula, we get:

n+1 = tn+(1 - ) tn -1 + …

+(1 - )j tn -j + …

+(1 - )n +1 0

Since both and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor

23 Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009 24

= 1/2

0 = 10


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 a priority scheduling where priority is the predicted next CPU burst time

Problem Starvation – low priority processes may never execute

Solution Aging – as time progresses increase the priority of the process


Each process gets a small unit of CPU time (time quantum), usually 10-100 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.

Performance

q large FIFO

q small q must be large with respect to context switch, otherwise overhead is too high

rule of thumb: 80% of CPU bursts should be smaller than time quantum

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Process Burst Time

P1 53

P2 17

P3 68

P4 24

The Gantt chart is:

Typically, higher average turnaround than SJF, but better response

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162


Silberschatz, Galvin and Gagne ©2005Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009 29 Silberschatz, Galvin and Gagne ©2005Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009

Ready queue is partitioned into separate queues, e.g.,

foreground (interactive)

background (batch)

Each queue has its own scheduling algorithm

foreground – RR

background – FCFS

Scheduling must be done between the queues

Fixed priority scheduling; (i.e., serve all from foreground then from background). Possibility of starvation.

Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; i.e., 80% to foreground in RR

20% to background in FCFS

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Silberschatz, Galvin and Gagne ©2005Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009 31 Silberschatz, Galvin and Gagne ©2005Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009

A process can move between the various queues; aging can be implemented this way

Multilevel-feedback-queue scheduler defined by the following parameters:

number of queues

scheduling algorithms for each queue

method used to determine when to upgrade a process

method used to determine when to demote a process

method used to determine which queue a process will enter when that process needs service

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Three queues:

Q0 – RR with time quantum 8 milliseconds

Q1 – RR time quantum 16 milliseconds

Q2 – FCFS

Scheduling

A new job enters queue Q0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1.

At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2.



Multiple ProcessorsCPU scheduling more complex when multiple CPUs are available

Homogeneous processors within a multiprocessor

Load sharing

Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing

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Real-Time SchedulingHard real-time systems – required to complete a critical task within a guaranteed amount of time

Soft real-time computing – requires that critical processes receive priority over less fortunate ones


Threads

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Threads...?

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• Multiple streams of program execution within a single process

• „Light-weight“ processes

• Switching from one thread to another requires less effort

• Threads share (some) context

• ... can be implemented „on top“, e.g., in a library (user-level threads)orinside the kernel (kernel-level threads)

Silberschatz, Galvin and Gagne ©2005Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009 38


Responsiveness

Resource Sharing

Economy

Utilization of MP Architectures


Thread management done by user-level threads library

Three primary thread libraries:

POSIX Pthreads

Win32 threads

Java threads

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Supported by the Kernel

Examples

Windows XP/2000

Solaris

Linux

Tru64 UNIX

Mac OS X


Many-to-One: Many user-level threads mapped to single kernel thread

Solaris Green Threads

GNU Portable Threads

One-to-One: Each user-level thread maps to kernel thread

Windows NT/XP/2000

Linux

Solaris 9 and later

Many-to-Many

Allows many user level threads to be mapped to many kernel threads

Allows the operating system to create a sufficient number of kernel threads

Solaris prior to version 9

Windows NT/2000 with the ThreadFiber package


Silberschatz, Galvin and Gagne ©2005Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009 43 Silberschatz, Galvin and Gagne ©2005Operating Systems • Prof. Dr. Marc H. Scholl • DBIS • U KN • Summer Term 2009 44


Similar to M:M, except that it allows a user thread to be bound to kernel thread

Examples

IRIX

HP-UX

Tru64 UNIX

Solaris 8 and earlier


Semantics of fork() and exec() system calls

Does fork() duplicate only the calling thread or all threads?

Thread cancellation

Terminating a thread before it has finished

Two general approaches:

Asynchronous cancellation terminates the target thread immediately

Deferred cancellation allows the target thread to periodically check if it should be cancelled

Signal handling

Thread pools

Thread specific data

Allows each thread to have its own copy of data

Useful when you do not have control over the thread creation process (i.e., when using a thread pool)

Scheduler activations

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Both M:M and Two-level models require communication to maintain the appropriate number of kernel threads allocated to the application

Scheduler activations provide upcalls - a communication mechanism from the kernel to the thread library

This communication allows an application to maintain the correct number kernel threads


Examples

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A POSIX standard (IEEE 1003.1c) API for thread creation and synchronization

API specifies behavior of the thread library, implementation is up to development of the library

Common in UNIX operating systems (Solaris, Linux, Mac OS X)


Implements the one-to-one mapping

Each thread contains

A thread id

Register set

Separate user and kernel stacks

Private data storage area

The register set, stacks, and private storage area are known as the context of the threads

The primary data structures of a thread include:

ETHREAD (executive thread block)

KTHREAD (kernel thread block)

TEB (thread environment block)

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Linux refers to them as tasks rather than threads

Thread creation is done through clone() system call

clone() allows a child task to share the address space of the parent task (process)


Java threads are managed by the JVM

Java threads may be created by:

Extending Thread class

Implementing the Runnable interface

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Intended Schedule

Date Lecture Hand out Submission0 20.04. Introduction to Operating Systems Course reggistration1 27.04. Systems Programming using C (File Subsystem) 1. Assignment2 04.05. Systems Programming using C (Process Control) 2. Assignment 1. Assignment3 11.05. Process Scheduling 3. Assignment 2. Assignment4 18.05. Process Synchronization 4. Assignment 3. Assignment5 25.05. Inter Process Communication 5. Assignment 4. Assignment6 01.06. Pfingstmontag 6. Assignment 5. Assignment7 08.06. Input / Output 7. Assignment 6. Assignment8 15.06. Memory Management 8. Assignment 7. Assignment9 22.06.

Filesystems9. Assignment 8. Assignment

10 29.06.Filesystems

10. Assignment 9. Assignment11 06.07. Special subject: Transactional Memory 10. Assignment12 13.07. Special subject: XQuery your Filesystem13 20.07. Wrap up session

27.07. First examination daate12.10. Second examination date


IV.Process

Synchronization

54

process scheduling iii. processes - uni konstanz · process scheduling 3. assignment 2. assignment...

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