cs 414 - spring 2012 cs 414 – multimedia systems design lecture 31 – media server (part 1) klara...

CS 414 - Spring 2012

CS 414 – Multimedia Systems Design Lecture 31 – Media Server (Part 1)

Klara Nahrstedt

Spring 2012

Administrative MP2 posted MP2 Deadline – April 7, Saturday, 5pm.


Covered Aspects of Multimedia

Image/VideoCapture

MediaServerStorage

Transmission

CompressionProcessing

Audio/VideoPresentationPlaybackAudio/Video

Perception/ Playback

Audio InformationRepresentation

Transmission

AudioCapture

A/V Playback

Image/Video InformationRepresentation


Outline

Media Server Requirements Media Server Layered Architecture


Client/Server Video-on-Demand System


Video-on-Demand Systems must be designed with goals: Avoid starvation Minimize buffer space requirement Minimize initiation latency (video startup

time) Optimize cost


Media Server Requirements Real-time storage and retrieval

Media quanta must be presented using the same timing sequence with which they were captured

High-Data Transfer Rate and Large Storage SpaceHDTV quality: 1280x720 pixels/frame; 24 bits/pixel -

> 81 Mbytes per secondNTCS quality: 640x480 pixels/frame; 24 bits/pixel -

>27MBytes per seconds


YouTube Video Server (2010)

http://tech.fortune.cnn.com/2010/05/17/youtube-at-5-years-old-2-billion-served-per-day/

May 2010, 2 Billion videos served per day More than 24 hours of video uploaded

every minute Videos usually less than 10 minutes long


YouTube Video Server (2011)

Over 4 B videos are viewed a day 60 hours of video are uploaded every minute Over 800 million unique users visit YouTube

each month 70% of YouTube traffic comes from outside US In 2011, YouTube had more than 1 trillion views

(140 views for every person on Earth)


Source: http://www.youtube.com/t/press_statistics

Video Playback Issues Single Stream Playback

Possible approach – buffer the whole stream Problem:??

Possible approach – prefetch just short video part Problem:

Prevent starvation Minimize buffer space requirement Minimize initiation latency

Multiple Streams Playback Possible approach – dedicate a disk to each stream

Problem: ??

Possible approach – multiple streams per disk Problems: ??


Media Server Architecture


Storage device

Disk control - scheduling

Storage management

File System

Memory Management

Content Directory/Distribution

Network Attachment

Incoming requestDelivered data

Storage Device- Disk Layout


Track

Sector

Zoned Disk (ZBR – ZoneBit Recording) Traditional Random Access

Disk Layout

Advantage: Easy mapping of location Information to head movement and disk rotation Problem: loss of storage space

Advantage: Sector size sameRotation speed constant; efficient Usage of space

Storage/Disk Management

Optimal placement of MM data blocks Single diskmultiple disks

Timely disk scheduling algorithms and sufficient buffers to avoid jitter

Possible Admission control


Storage Management Storage access time to read/write disk

block is determined by 3 componentsSeek Time

Time required for the movement of read/write head

Rotational Time (Latency Time) Time during which transfer cannot proceed until the right

block or sector rotates under read/write head

Data Transfer Time Time needed for data to copy from disk into main memory


Impact of Access Time Metrics

If data blocks are arbitrarily placed If requests are served on FCFC basis Then effort for locating data place may cost

time period in order of magnitude 10ms This performance degrades disk efficiency Need techniques to reduce seek,

rotation and transmission times !!!


MM Data Placement on Single Disk


Continuous Placement Non-contiguous Placement

Simple to implement, but subject to fragmentation

Avoids fragmentation

Enormous copying overhead during insert/delete to maintain continuity

Avoid copying overhead

When reading file, only one seek required to position the disk head at the start of data

When reading file, seek operation incurs for each block , hence intrafile seek

Intra-file Seek Time Intra-file seek – can be avoided in non-contiguous

layout if the amount read from a stream always evenly divides block

Solution: select sufficient large block and read one block in each round If more than one block is required to prevent starvation

prior to next read, deal with intra-file seek Solution: constrained placement or log-structure

placement


Non-continuous Placement


Constrained Placement Approach: separation between successive file blocks

is bounded Bound on separation – not enforced for each pair of

successive blocks, but only on average over finite sequence of blocks

Attractive for small block sizes Implementation – expensive

For constrained latency to yield full benefit, scheduling algorithm must retrieve immediately all blocks for a given stream before switching to another stream


Log-Structure Placement This approach writes modified blocks sequentially

in a large contiguous space, instead of requiring seek for each block in stream when writing (recording) Reduction of disk seeks Large performance improvements during recording,

editing video and audio

Problem: bad performance during playback Implementation: complex


MM Data Placement at Multiple Disks


Data InterleavingOn Multiple Disks(Disks are not Synchronized)Striping data across Multiple disks

Data InterleavingOn single disk(consecutive blocks are placed on The same cylinderBut in interleaved way

Disk Scheduling Policies Goal of Scheduling in Traditional Disk

Management Reduce cost of seek time Achieve high throughput Provide fair disk access

Goal of Scheduling in Multimedia Disk Management Meet deadline of all time-critical tasks Keep necessary buffer requirements low Serve many streams concurrently Find balance between time constraints and efficiency


Disk Scheduling Framework Must support multiple QoS levels and requests


Source: Reddy et al, “Disk Scheduling in a Multimedia I/O System”, ACM TOMCCAP 2005

EDF (Earliest Deadline First) Disk Scheduling

Each disk block request is tagged with deadlineVery good scheduling policy for periodic requests

Policy: Schedule disk block request with earliest deadlineExcessive seek time – high overheadPure EDF must be adapted or combined with file

system strategies


EDF Example


Note: Consider that block number Implicitly encapsulates the disk track number

SCAN-EDF Scheduling Algorithm Combination of SCAN and EDF algorithms Each disk block request tagged with

augmented deadline Add to each deadline perturbation

Policy: SCAN-EDF chooses the earliest deadline If requests with same deadline, then choose

request according to scan direction


Implementation of SCAN-EDF Notation:

Di be deadline of disk block request ‘i’

Ni be track (block) position on disk

Nmax be maximum number of disk tracks

Deadline Modification: Di + f(Ni)

f(Ni) converts track number of ‘i’ into a small perturbation of deadline

Perturbation small enough so that Di + f(Ni) ≤ Dj + f(Nj) for Di ≤ Dj

Possible f(Ni) = Ni/Nmax


SCAN EDF Example (Nmax = 100)


Enhanced SCAN-EDF (1) Use more accurate perturbation of deadline Consider

Actual track position of disk head ‘N’ Nmax – max number of disk tracks

Ni – next track to be considered


Head Moves Upwards

Enhanced SCAN-EDF (2) Algorithm:

If head moves upwards (towards Nmax), then

(a)

(b)


maxmax )(,;

N

NNNfNNNN iiii

max

max)(,1;N

NNNfNNN iiii

Enhanced SCAN-EDF (3) If head moves downwards (towards 1), then

(a)

(b)


maxmax )(:;

N

NNfNNNN iiii

max

)(:1;N

NNNfNNN iiii

Group Sweeping Algorithms

Policy: Each Request consists of (Deadline, Block Number

)Disk Block Requests served in cycles In one cycle, requests divided into groups

according to similar deadlinesWithin group use SCANAs we retrieve blocks, we may need smoothing

buffers to ensure continuity


Group Sweeping Example


Mixed Scheduling (uses SSTF – Shortest Seek Time First)


Example of SSTF

Mixed Scheduling


SSTF (Shortest Seek Time First) + Balanced Strategy

Conclusion

Media Server Architecture Storage Management

Data/file placement on single disk Data/file placement on multiple disks

Disk Scheduling – important component in the timely delivery of streams

Admission should be done if one cares not to over subscribe


cs 414 - spring 2012 cs 414 – multimedia systems design lecture 31 – media server (part 1) klara...

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