Multimedia Systems (Part 2) CS-502 Fall 20061 Multimedia Systems (Part 2) CS-502 Operating Systems...

Multimedia Systems (Part 2)

CS-502 Fall 2006 1


CS-502 Operating SystemsFall 2006

(Slides include materials from Operating System Concepts, 7th ed., by Silbershatz, Galvin, & Gagne and from Modern Operating Systems, 2nd ed., by Tanenbaum)


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Review

• What is multi-media?• Audio, video, etc., in digital computing environment

• Requirements and challenges for audio and video in computer systems

• CD devices and formats

• Systems for multimedia

• Compression and bandwidth• JPEG and MPEG


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This week

• Processor and disk scheduling• Real-time scheduling

• QoS: quality of service

• Network streaming and management• HTTP vs. RTSP

• Video Server organization


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Operating System Challenge

• How to get the contents of a movie file from disk or DVD drive to video screen and speakers.– Fixed frame rate (25 or 30 fps)– Steady audio rate– Bounded jitter

• Such requirements are known as Quality-of-Service (QoS) guarantees.

• Classical problem in real-time scheduling– Obscure niche become mainstream!– See Silbershatz, §19.1–19.5


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QoS Challenge

• Building a system to guarantee QoS effects the following:–– CPU processing– Scheduling and interrupt handling– File systems– Network protocols


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Three levels of QoS

• Best-effort service: the system makes a best effort with no attempt to guarantee

• Soft QoS: allows different traffic streams to be prioritized, but no QoS guarantees are made– Prioritization

• Hard QoS: system ensures QoS requirements are always met– Prioritization

– Admission control

– Bounded latency on interrupt handling, processing, etc.


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Parameters Defining QoS

• Throughput: – total amount of work completed during a specified time

interval

• Delay: – elapsed time from when request is first submitted to

desired result

• Jitter: – (uneven) variation in playback rate of a stream.

• Reliability: – how errors are handled during transmission and

processing of continuous media


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Further QoS Issues

• QoS may be negotiated between the client and server.

• Operating systems may use an admission control algorithm– Admits a request for service only if the server

has sufficient resources to satisfy the request


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Processor scheduling – two common methods

• Rate Monotonic Scheduling

• Earliest Deadline First

• Many variations

• Many analytic methods for proving QoS (Quality of Service)


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Rate Monotonic Scheduling (RMS)

• Assume m periodic processes– Process i requires ti msec of processing time

every pi msec.

– Equal processing every interval — like clockwork!


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Example• Periodic process i requires the CPU at specified intervals

(periods)• pi is the duration of the period• ti is the processing time• di is the deadline by when the process must be serviced

– Often same as end of period


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Rate Monotonic Scheduling (RMS)

• Assume m periodic processes– Process i requires ti msec of processing time every pi

msec.

– Equal processing every interval — like clockwork!

• Assume

• Assign priority of process i to be– Statically assigned

• Let priority of non-real-time processes be 0

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Rate Monotonic Scheduling (continued)

• Scheduler simply runs highest priority process that is ready– May pre-empt other real-time processes– Real-time processes become ready in time for

each frame or sound interval– Non-real-time processes run only when no real-

time process needs CPU


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Example

• p1 = 50 msec; t1 = 20 msec

• p2 = 100 msec; t2 = 35 msec

• Priority(p1) > Priority(p2)

• Total compute load is 75 msec per every 100 msec.

• Both tasks complete within every period• 25 msec per 100 msec to spare


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Example 2

• p1 = 50 msec; t1 = 25 msec

• p2 = 80 msec; t2 = 35 msec

• Priority(p1) > Priority(p2)

• Total compute load is ~ 94% of CPU.

• Cannot complete both tasks within some periods• Even though there is still CPU capacity to spare!


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Rate Monotonic Theorems (without proof)

• Theorem 1: using these priorities, scheduler can guarantee the needed Quality of Service (QoS), provided that

– Asymptotically approaches ln 2 as m ln 2 = 0.6931…

• Theorem 2: If a set of processes can be scheduled by any method of static priorities, then it can be scheduled by Rate Monotonic method.

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Example 2 again

• Note that p1 pre-empts p2 in second interval, even

though p2 has the earlier deadline!

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More on Rate Monotonic Scheduling

• Rate Monotonic assumes periodic processes

• MPEG-2 playback is not a periodic process!


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Earliest Deadline First (EDF)

• When each process i become ready, it announces deadline Di for its next task.

• Scheduler always assigns processor to process with earliest deadline.

• May pre-empt other real-time processes


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Earliest Deadline First Scheduling (continued)

• No assumption of periodicity• No assumption of uniform processing times

• Theorem: If any scheduling policy can satisfy QoS requirement for a sequence of real time tasks, then EDF can also satisfy it.– Proof: If i scheduled before i+1, but Di+1 < Di , then i

and i+1 can be interchanged without affecting QoS guarantee to either one.


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Earliest Deadline First Scheduling (continued)

• EDF is more complex scheduling algorithm• Priorities are dynamically calculated• Processes must know deadlines for tasks

• EDF can make higher use of processor than RMS

• Up to 100%

• There is a large body of knowledge and theorems about EDF analysis


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Example 2 (again)

• Priorities are assigned according to deadlines:

– the earlier the deadline, the higher the priority;

– the later the deadline, the lower the priority.


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Questions?


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Network Management

• General methods for delivering content from a server to a client across a network:–– Broadcasting: server delivers content to all

clients, whether they want it or not.– Multicasting: server delivers content to group

of clients who indicate they wish to receive the content.

– Unicasting: server delivers content to a single client.


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Network Management (continued)

• Broadcasting• Like cable TV

• Fixed portion of network bandwidth dedicated to set of broadcast streams; inflexible

• Multicasting• Typical webcast

• Multicast tree set up through internet to reach customers

• Customers can come and go

• Unicast• Dedicated connection between server and client

• Streaming version of familiar transport protocols


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Streaming –Delivery of Multimedia Data over Network

• Two types of streaming:–– Progressive download: client begins playback

of multimedia file during delivery. • File is ultimately stored on client computer

• (Hopefully) download speed > playback speed

– Real-time streaming: multimedia file is delivered to, but not stored on, client computer

• Played back at same speed as delivery

• Limited amount of buffering to remove jitter


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Two Types of Real-time Streaming

• Live streaming: to deliver a live event while it is occurring.

• Broadcast or multicast

• On-demand streaming: to deliver archived media streams

• Movies, lectures, old TV shows, etc.• Events not delivered when they occur.• Playback with pause, fast forward, reverse, etc• Unicast (usually)


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Network Streaming

• Traditional HTTP• Stateless

• Server responds to each request independently

• Real-Time Streaming Protocol (RTSP)• Client initiates a “push” request for stream

• Server provides media stream at frame rate


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Pull (HTTP) vs. Push (RTSP) server


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RTSP States

• SETUP: server allocates resources for client session.

• PLAY: server delivers stream to a client session.

• PAUSE: server suspends delivery of a stream.

• TEARDOWN: server releases resources and breaks down connection.


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RTSP state machine


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Bandwidth Negotiation

• Client application provides feedback to server to adjust bandwidth

• E.g., – Windows Media Player– RealPlayer– Quicktime


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Video Server

• Multiple CPUs

• Disk farm– 1000s of disks

• Multiple high-bandwidth network links– Cable TV– Video on demand– Internet


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Multimedia File & Disk Management

• Single movie or multimedia file on disk– Interleave audio, video, etc.

• So temporally equivalent blocks are near each other

– Attempt contiguous allocation• Avoid seeks within a frame

TextFrame

AudioFrame


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File organization – Frame vs. Block

• Frame organization• Small disk blocks (4-16 Kbytes)• Frame index entries point to starting block for each frame• Frames vary in size (MPEG)• Advantage: very little storage fragmentation• Disadvantage: large frame table in RAM

• Block organization• Large disk block (256 Kbytes or more)• Block index entries point to first I-frame of a sequence• Multiple frames per block• Advantage: much smaller block table in RAM• Disadvantage: large storage fragmentation on disk


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Frame vs. Block organization

smaller larger


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File Placement on Server

• Random

• Striped

• “Organ pipe” allocation– Most popular video in center of disk– Next most popular on either side of it, etc.– Least popular at edges of disk– Minimizes seek distance


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Resources on a file server


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Conclusion

• Multimedia computing is challenging• Possible with modern computers

• Compression is essential, especially for video

• Real-time computing techniques move into mainstream

• Processor and disk scheduling

• There is much more to this subject than fits into one class


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Reading Assignment

• Silbershatz, §20.7– CineBlitz video server


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