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Multimedia Information Systems
Shahram GhandeharizadehComputer Science Department
University of Southern California
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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.
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MULTIMEDIA
Multimedia now:
Multimedia in a few years from now:
Remaining:
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Continuous Media: Audio & Video
Display of a clip as a function of time.
Constant Bit Rate Variable Bit Rate
Time Time
Bytes Bytes
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Continuous Media: Audio & Video
A clip has a fixed display time.
Constant Bit Rate Variable Bit Rate
Time Time
Bytes Bytes
Clip display time
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Continuous Media: Audio & Video
A clip has a fixed size.
Constant Bit Rate Variable Bit Rate
Time Time
Bytes Bytes
Clip size
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Continuous Media: Audio & Video
Average bandwidth for continuous display is clip size divided by the clip display time.
Constant Bit Rate Variable Bit Rate
Time Time
Bytes Bytes
Display bandwidth requirements
BW = Line slope
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Time and space
One may manipulate the bandwidth required to display a clip by prefetching a portion of the clip.
Constant Bit Rate Media
Time
Bytes
Startup latency
Prefetch portion
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Continuous display from magnetic disk
Target architecture
Memory CPU
Display
System Bus
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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
Display
System Bus
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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
Display
System Bus
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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
Display
System Bus
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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
Display X1 Display X2Display
Disk X1 X2
Time Period
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Pipelining: multiple displays
With multiple displays, disk is multiplexed between multiple requests, resulting in disk seeks.
Display Xi Display Xi+1Display
Disk Xi Xi+1
Time Period
Wj
Seek + Rotational delay
Zk Wj+1 Zk+1
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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):
Display X1Display
Disk X1 Wj+1
Time Period
Wj Zk Zk+1 X2 Zk+2
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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
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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:
Block size N Memory Required Latency Sec (2 Tp) Wasted disk BW (%)
8 KB 5 80 KB 0.012 88.9
16 KB 10 320 KB 0.167 77.7
32 KB 16 1 MB 0.333 64.7
64 KB 24 3 MB 0.67 47.5
128 KB 32 8 MB 1.33 30
256 KB 37 18.5 MB 2.67 19.1
512 KB 41 41 MB 5.33 10.3
1 MB 43 96 MB 10.66 6
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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.
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Seagate ST31200W zones
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Seagate ST31200W
Consists of 2697 cylinders. One may model its seek characteristics as follows:
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Seagate ST31200W
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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
Disadvantage: Z+1 seeks to retrieve a logical track
Logical Track 1
Logical Track 2
Logical Track 3
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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
Disadvantage: 2 seeks to retrieve a logical track
Logical Track 1
Logical Track 2
Logical Track 3
Logical Track 8
...
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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
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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
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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.
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FIXB
Amount of data produced during (1 maximum seek + TScan) is identical to the amount of data displayed during TScan.
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FIXB
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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.
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Comparison
FIXB and VARB waste space due to:1. Round-robin assignment of blocks to zones
2. Different zones offer different storage capacities.
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Comparison