Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition, Chapter 19/20: Real-time and Multimedia Systems Inclusions from Tanenbaum, Boss, Modern Operating Systems, 2014

20.2 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Multimedia and Real-Time Systems Multimedia files Compression Requirements of Multimedia Kernels CPU Scheduling Disk Scheduling Network Management

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

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

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

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

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

20.8 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Multimedia Files Multimedia files are… more than one…

20.9 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Multimedia Files : Video For example, video must be displayed at frames per second. Multimedia video data must be delivered at a rate which guarantees 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

20.10 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Multimedia Files : Video

20.11 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Multimedia Files : Video For example, video must be displayed at frames per second. Multimedia video data must be delivered at a rate which guarantees 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)

20.12 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Multimedia Files : Audio Audio waves are converted to binary by quantizing the sample of the sine wave

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

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

20.15 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Multimedia Files Multimedia files are… more than one…

20.16 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Multimedia Files : Video 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

20.17 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Multimedia File Systems File placement Contiguous Interleave video, audio and text in a single contiguous file per movie Block model

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

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

20.20 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Disk Scheduling (Cont) The EDF scheduler uses a queue to order requests according to the time it must be completed (its deadline)

20.21 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Disk Scheduling (Cont) SCAN-EDF scheduling is similar to EDF except that requests with the same deadline are ordered according to a SCAN policy

20.22 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Deadline and cylinder requests for SCAN-EDF scheduling

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

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

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

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

20.27 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 3 periodic processes

20.28 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Multimedia Disk Scheduling

20.29 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Rate Montonic Scheduling A priority is assigned based on the inverse of its period Shorter periods = higher priority; Longer periods = lower priority P 1 is assigned a higher priority than P 2.

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

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

20.32 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition RealTime Streaming Protocol (RTSP) Standard HTTP is stateless whereby the server does not maintain the status of its connection with the client

20.33 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Streaming media from a conventional web server

20.34 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Realtime Streaming Protocol

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

20.36 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition RTSP state machine

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition, End of Chapter 20

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

20.39 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Exercise 20.10