CS-3013 & CS-502, Summer 2006

Multimedia topics (continued) 1

Multimedia Topics(continued)

CS-3013 & CS-502Operating Systems

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Multimedia topics (continued) 2

Review

• CD-ROM & DVD devices• Reading assignment about CD-R and DVD

• General requirements for multimedia in a computer

• Jitter-free• Guaranteed scheduling• Multiple interleaved streams• Wide variety of bandwidths

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Multimedia topics (continued) 3

Review (continued)

• Compression• MP-3 – 10:1• JPEG – 20:1• MPEG-2 – 50:1- 80:1

• Asymmetric compression• Longer to compress than to decompress• Take advantage of temporal locality

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Multimedia topics (continued) 4

Outline

• Processor scheduling• File, disk, and network management

for multimedia in computers

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Multimedia topics (continued) 5

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 (24, 25 or 30 fps)– Steady audio rate– Bounded jitter

• Classical problem in real-time scheduling– Obscure niche becomes mainstream!– See Silbershatz, Chapter 19

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Multimedia topics (continued) 6

Two common methods

• Rate Monotonic Scheduling• Earliest Deadline First

• Many variations• Many analytic methods for proving

QoS (Quality of Service)

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Processor Scheduling for Real-Time

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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Multimedia topics (continued) 8

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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Multimedia topics (continued) 9

Processor Scheduling for Real-Time

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

11

m

i i

i

p

t

ip

1

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Multimedia topics (continued) 10

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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Multimedia topics (continued) 11

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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Multimedia topics (continued) 12

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

– Asymtotically 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.

)12(1

1

mm

i i

i mp

t

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Multimedia topics (continued) 14

Example 2 again

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

even though p2 has the earlier deadline!

828.01229375.080

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50

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Multimedia topics (continued) 15

More on Rate Monotonic Scheduling

• Rate Monotonic assumes periodic processes

• MPEG-2 playback is not a periodic process!

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Multimedia topics (continued) 16

Processor Scheduling for Real-Time

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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Multimedia topics (continued) 17

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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Multimedia topics (continued) 18

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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Multimedia File & Disk Management

• Single movie or multimedia file on PC 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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Multimedia topics (continued) 22

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)• 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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Multimedia topics (continued) 23

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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Multimedia topics (continued) 25

Placing Multiple files on Single Disk

Zipf’s Law• Rank items by “popularity”• Zif’s Law says: Probability that next

customer will choose item k is approximately C/k– Where C is a normalization constant such that

11

N

kk

C

N = 10 C = 0.341N = 100 C = 0.193N = 1000 C = 0.134N = 10000 C = 0.102

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Zipf’s Law applied to US cities

• Points represent populations 20 largest cities– sorted on rank order

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Multimedia topics (continued) 27

Application of Zipf’s Law tomovie file placement

• Organ-pipe distribution of files on server– most popular movie in middle of disk– next most popular either on either side, etc.

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Multimedia topics (continued) 28

Calculate …

• Probability that next movie is in the middle five cylinders

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Questions?

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Disk Scheduling (server)

• Scheduling disk activity is just as important as scheduling processor activity

• Advantage:– Predictability• Unlike disk activity of ordinary computing

• In server, there will be multiple disk requests in each frame interval

• One request per frame for each concurrent video stream

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Multimedia topics (continued) 31

Disk Scheduling (continued)

• SCAN (Elevator) algorithm for each frame interval

• Sort by cylinder #• Complete in time for start of next frame interval

• Variation – SCAN–EDF:• Sort requests by deadline• Group similar deadlines together, apply SCAN to

group• Particularly useful for non-uniform block sizes

and frame intervals

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Multimedia topics (continued) 32

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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Multimedia topics (continued) 33

Push vs. Pull server

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Bandwidth Negotiation

• Client (or application) provides feedback to server to adjust bandwidth

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

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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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Multimedia topics (continued) 36

Break