Chapter 19/20: Real-time and Multimedia Systems

Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition,

Chapter 19/20: Real-time and Multimedia Systems

Inclusions from Tanenbaum, Boss, Modern Operating Systems, 2014

20.2 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition

Multimedia and Real-Time Systems

Multimedia files

Compression

Requirements of Multimedia Kernels

CPU Scheduling

Disk Scheduling

Network Management


What is Multimedia?

Document or file containing two or more continuous media.

Media that must be accessed and played back at a specific rate or over a specific time interval.


What is Multimedia

Multimedia data is stored in the file system like other ordinary data

Key characteristics of multimedia:

Multimedia files can be large! 100 minute movie (mpeg) requires 1.125 GB

Multimedia uses high data transfer rates Comes from the nature of visual and acoustic information

– Eye and ear can process prodigious amounts of information and must be fed at a constant rate.

Multimedia requires real-time playback Once a file is delivered to a client, it must continue at a certain rate

of playback.


Definitions: Streaming

Streaming is delivering a multimedia file from a server to a client - typically the deliver occurs over a network connection

There are two different types of streaming:

1. Progressive download - the client begins playback of the multimedia file as it is delivered. The file is ultimately stored on the client computer

2. Real-time streaming - the multimedia file is delivered to - but not stored on - the client’s computer


Definitions: Real-time Streaming

There are two types of real-time streaming:

(1) Live streaming - used to deliver a live event while it is occurring

(2) On-demand streaming - used to deliver media streams such as movies, archived lectures, etc. The events are not delivered in real-time


Multimedia Operating System Issues

The operating system must guarantee the specific data rate and timing requirements of continuous media

QOS : Quality of Service

Guarantee specific rate and timing requirements of media

Guarantee clear picture/voice synchronization

Influences OS tasks such as

Compression and decoding may require significant CPU

Disk Scheduling: File systems must be efficient to meet rate requirements

CPU Scheduling: multimedia task must be scheduled with certain priorities to ensure meeting deadline requirements of continuous media.

Network protocols must support bandwidth requirements


Multimedia Files Multimedia files are… more than one…


Multimedia Files : Video

For example, video must be displayed at 24-30 frames per second. Multimedia video data must be delivered at a rate which guarantees 24-30 frames/second.

Experiments show that people notice a flicker less than 24 frames / second

SO, to increase frame rate

Progressive technique

- use more bandwidth to send more frames

Interlacing: (analog)

- show odd scan lines then show even ones





For example, video must be displayed at 24-30 frames per second. Multimedia video data must be delivered at a rate which guarantees 24-30 frames/second.

Experiments show that people notice a flicker less than 24 frames / second

SO, to increase frame rate

Progressive technique

- use more bandwidth to send more frames

Interlacing: (analog)

- show odd scan lines then show even ones

Repaint the screen (digital)


Multimedia Files : Audio

Audio waves are converted to binary by quantizing the sample of the sine wave


Multimedia Files: Compression

Multimedia files are big.

Because of the size and rate requirements of multimedia systems, multimedia files are often compressed into a smaller form

Compression requires 2 algorithms: encoding - decoding

JPEG – (Joint Photographic Experts Group)

Quantizes blocks of the picture so that each shows the amount by which it differs from the corresponding element in the previous block.

• Linearizes the elements to produces the bit string file.


Multimedia Files: Compression

MPEG – (Motion Picture Experts Group)

3 different kinds of frames

I (Intra-coded) frames:

Self-contained JPEG pictures

P (predictive) frames:

Block by block differences with last frame

B (Bidirectional) frames:

Differences from the last and next frame.


Multimedia Files Multimedia files are… more than one…



Definitions:

Flicker is determined by the number of times the screen is painted/sec.

A still image painted 20 frames/sec will not show jerky motion but it will flicker because the image will decay from the retina.

Smoothness of motion is determined by the number of different images /sec

A movie with 20 different frames/sec, each painted 4x in a row will not flicker, but the motion will appear jerky


Multimedia File Systems

File placement

Contiguous

Interleave video, audio and text in a single contiguous file per movie

Block model


Multimedia File Systems

Use a different paradigm than traditional file systems

Traditional: open > read (block) > read (unrelated block)

… doesn’t work

Pull Server Push Server

• Predictable next buffer• Timing deadlines• Rate requirements


Disk Scheduling

Disk scheduling algorithms must be optimized to meet the timing deadlines and rate requirements of continuous media

Earliest-Deadline-First (EDF) Scheduling

SCAN-EDF Scheduling


Disk Scheduling (Cont)

The EDF scheduler uses a queue to order requests according to the time it must be completed (its deadline)


Disk Scheduling (Cont)

SCAN-EDF scheduling is similar to EDF except that requests with the same deadline are ordered according to a SCAN policy


Deadline and cylinder requests for SCAN-EDF scheduling


CPU Scheduling

Multimedia systems require hard realtime scheduling to ensure critical tasks will be serviced within timing deadlines

Most hard realtime CPU scheduling algorithms assign realtime processes static priorities that do not change over time


Requirement of Multimedia Operating Systems

There are three levels of QoS

(1) Best-effort service - the system makes a best effort with no QoS guarantees

(2) Soft QoS - allows different traffic streams to be prioritized, however no QoS guarantees are made

(3) Hard QoS - the QoS rquirements are guaranteed


Further QoS Issues

Different processes must run at different frequencies with different amounts of work with different deadlines

Scheduling of multiple competing processes, some or all with different deadlines to be met real-time scheduling


Real-Time CPU Scheduling

Periodic processes require the CPU at specified intervals (periods)

p is the duration of the period

d is the deadline by when the process must be serviced

t is the processing time


3 periodic processes


Multimedia Disk Scheduling


Rate Montonic Scheduling

A priority is assigned based on the inverse of its period

Shorter periods = higher priority;

Longer periods = lower priority

P1 is assigned a higher priority than P2.


Earliest Deadline First Scheduling

Priorities are assigned according to deadlines:

the earlier the deadline, the higher the priority;

the later the deadline, the lower the priority


Network Management

Three general methods for delivering content from a server to a client across a network:

(1) Unicasting - the server delivers the content to a single client

(2) Broadcasting - the server delivers the content to all clients, regardless whether they want the content or not

(3) Multicasting - the server delivers the content to a group of receivers who indicate they wish to receive the content


RealTime Streaming Protocol (RTSP)

Standard HTTP is stateless whereby the server does not maintain the status of its connection with the client


Streaming media from a conventional web server


Realtime Streaming Protocol


RTSP States

SETUP - the server allocates resources for a client session

PLAY - the server delivers a stream to a client session

PAUSE - the server suspends delivery of a stream

TEARDOWN - the server breaks down the connection and releases the resources allocated for the session


RTSP state machine

Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition,

End of Chapter 20



Exercise 20.10

Documents

Chapter 19/20: Real-time and Multimedia Systems