Multimedia Information Systems Shahram Ghandeharizadeh Computer Science Department University of Southern California

Reading n 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.

MULTIMEDIA n Multimedia now: n Multimedia in a few years from now: n Remaining:

Continuous Media: Audio & Video n Display of a clip as a function of time. Constant Bit RateVariable Bit Rate Time Bytes

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

Continuous Media: Audio & Video n A clip has a fixed size. Constant Bit RateVariable Bit Rate Time Bytes Clip size

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

Time and space n 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

Continuous display from magnetic disk n Target architecture MemoryCPU Display System Bus

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

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

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

Continuous display: pipelining n Partition a clip X into n fixed size blocks: X 1, X 2, X 3, …, X n n Stage X i in memory and initiate its display. n Stage X i+1 in memory prior to completion of the display of X i Display X1Display X2Display Disk X1X2 Time Period

Pipelining: multiple displays n With multiple displays, disk is multiplexed between multiple requests, resulting in disk seeks. Display X i Display X i+1 Display Disk XiXi X i+1 Time Period WjWj Seek + Rotational delay ZkZk W j+1 Z k+1

How to manage disk seeks? n Live with it: – Assume the worst seek time in order to guarantee hiccup-free display – Assume average seek time if hiccups are acceptable. n 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 X1X1 W j+1 Time Period WjWj ZkZk Z k+1 X2X2 Z k+2

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

Impact of block size n MPEG-1 clips with 1.5 Mbps bandwidth requirements n 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 n Throughput and startup latency as a function of block size: Block sizeNMemory RequiredLatency Sec (2 T p )Wasted disk BW (%) 8 KB580 KB KB10320 KB KB161 MB KB243 MB KB328 MB KB MB KB4141 MB MB4396 MB10.666

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

Seagate ST31200W zones

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

Seagate ST31200W

IBM’s Logical Track n Let Z min denote the zone with fewest track, T min n A disk with Z zones is collapsed into a logical disk consisting of one zone with T min tracks. Size of each track is Z * T avg n The size of a block must be a multiple of the logical track size n Disadvantage: Z+1 seeks to retrieve a logical track Logical Track 1 Logical Track 2 Logical Track 3

HP’s Track Pairing n Let Z min denote the zone with fewest track, T min n Pair outermost track with the innermost one and continue inward. n A disk with Z zones is collapsed into a logical disk consisting of one zone with (Z*T min )/2 tracks. n The size of a block must be a multiple of a track pair n Disadvantage: 2 seeks to retrieve a logical track Logical Track 1 Logical Track 2 Logical Track 3 Logical Track

USC’s region-based approach n 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. n Assign blocks of a clip to regions in a round-robin manner. n 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

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

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

FIXB n Amount of data produced during (1 maximum seek + T Scan ) is identical to the amount of data displayed during T Scan.

FIXB

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

Comparison n FIXB and VARB waste space due to: 1. Round-robin assignment of blocks to zones 2. Different zones offer different storage capacities.

Comparison