Multimedia Information Systems

1. Multimedia Information Systems Shahram GhandeharizadehComputer Science Department University of Southern California

2. Reading • First 11 (until Section 3.2) pages of: S. Ghandeharizadeh and R. Muntz, “Design and Implementation of Scalable Continuous Media Servers,” Parallel Computing, Elsevier 1998.

3. MULTIMEDIA • Multimedia now: • Multimedia in a few years from now: • Remaining:

4. Continuous Media: Audio & Video • Display of a clip as a function of time. Bytes Bytes Time Time Constant Bit Rate Variable Bit Rate

5. Continuous Media: Audio & Video • A clip has a fixed display time. Bytes Bytes Time Time Constant Bit Rate Variable Bit Rate Clip display time

6. Continuous Media: Audio & Video • A clip has a fixed size. Bytes Bytes Time Time Constant Bit Rate Variable Bit Rate Clip size

7. Continuous Media: Audio & Video • Average bandwidth for continuous display is clip size divided by the clip display time. Bytes Bytes BW = Line slope Time Time Constant Bit Rate Variable Bit Rate Display bandwidth requirements

8. Time and space • One may manipulate the bandwidth required to display a clip by prefetching a portion of the clip. Bytes Prefetch portion Time Startup latency Constant Bit Rate Media

9. Continuous display from magnetic disk • Target architecture Memory CPU System Bus Display

10. Continuous display • Once display is initiated, it should not starve for data. Otherwise, display will suffer from frequent disruptions and delays, termed hiccups. Memory CPU System Bus Display

11. Continuous display: using memory • Given the low latency between memory and display, stage the entire clip from disk onto memory and then initiate its display. Memory CPU System Bus Display

12. Continuous display: using memory • Limitations: • Forces the user to wait un-necessarily. • Requires a large memory module in the order of Gigabytes for 2 hour movies. Memory CPU System Bus Display

13. Continuous display: pipelining • Partition a clip X into n fixed size blocks: X1, X2, X3, …, Xn • Stage Xi in memory and initiate its display. • Stage Xi+1 in memory prior to completion of the display of Xi Disk X1 X2 Display Display X1 Display X2 Time Period

14. Pipelining: multiple displays • With multiple displays, disk is multiplexed between multiple requests, resulting in disk seeks. Seek + Rotational delay Disk Xi Wj Zk Xi+1 Wj+1 Zk+1 Display Display Xi Display Xi+1 Time Period

15. How to manage disk seeks? • Live with it: • Assume the worst seek time in order to guarantee hiccup-free display • Assume average seek time if hiccups are acceptable. • Use the elevator algorithm by delaying display of a block to the end of a time period, termed Group Sweeping Scheme (GSS): Zk+1 X2 Disk X1 Wj Zk Wj+1 Zk+2 Display Display X1 Time Period

16. Impact of block size • Disk service time with transfer-rate tfr and block size B is: • Tdisk = Tseek + TRotLatency + (B / tfr) • Number of simultaneous displays supported by a single disk is: N = Tp/Tdisk • Simple pipelining requires (N+1)B memory, GSS requires 2NB. • The observed transfer rate of a disk drive is a function of B and its physical characteristics: tfrobs = tfr ( B / [B + (Tseek + Tlatency) tfr] ) • Percentage of wasted disk bandwidth: 100 * (tfr – tfrobs) / tfr

17. Impact of block size • MPEG-1 clips with 1.5 Mbps bandwidth requirements • Target disk characteristics: Seek: max = 17 msec, min = 2 msec Rotational latency: Max = 8.3 msec, min = 4.17 msec Disk tfr = 68.6 Mbps • Throughput and startup latency as a function of block size:

18. Modern disks are multi-zoned • Each zone provides a different storage capacity (number of tracks and sectors per track) and transfer rate. • Outermost zone is typically twice faster than the innermost zone.

19. Seagate ST31200W zones

20. Seagate ST31200W • Consists of 2697 cylinders. One may model its seek characteristics as follows:

21. Seagate ST31200W

22. IBM’s Logical Track • Let Zmin denote the zone with fewest track, Tmin • A disk with Z zones is collapsed into a logical disk consisting of one zone with Tmin tracks. Size of each track is Z * Tavg • The size of a block must be a multiple of the logical track size Logical Track 1 Logical Track 2 Logical Track 3 • Disadvantage: Z+1 seeks to retrieve a logical track

23. HP’s Track Pairing • Let Zmin denote the zone with fewest track, Tmin • Pair outermost track with the innermost one and continue inward. • A disk with Z zones is collapsed into a logical disk consisting of one zone with (Z*Tmin)/2 tracks. • The size of a block must be a multiple of a track pair Logical Track 1 Logical Track 2 Logical Track 3 ... Logical Track 8 • Disadvantage: 2 seeks to retrieve a logical track

24. USC’s region-based approach • Partition the Z zones into R regions. A region may consist of 1 or more consecutive zones. The slowest participating zone dictates transfer rate of its assigned region. • Assign blocks of a clip to regions in a round-robin manner. • Display of clips requires visiting regions one at a time, multiplexing their bandwidth between N active requests. Both fix sized blocks and variable length blocks are supported. Region 1 Region 2

25. Multi-zone disk drives • With all 3 techniques, one may selectively drop zones: sacrifice storage for bandwidth! • Example: USC’s region-based approach Region 1 Region 2

26. FIXB • Partition a clip into fix sized blocks and assign them to the regions in a round-robin manner. • During a time period, retrieve blocks from one region at a time. • Display starts when sufficient data is in main memory.

27. FIXB • Amount of data produced during (1 maximum seek + TScan) is identical to the amount of data displayed during TScan.

28. FIXB

29. VARB • Variable size blocks dictated by the transfer rate of each zone. • Amount of data produced during one TMUX is identical to the amount of data displayed during TMUX. • Limitation: complex to implement due to variable block sizes.

30. Comparison • FIXB and VARB waste space due to: • Round-robin assignment of blocks to zones • Different zones offer different storage capacities.

31. Comparison