1 / 30

CS 414 – Multimedia Systems Design Lecture 29 – Media Server (Part 3)

Explore efficient disk scheduling policies for multimedia systems to meet deadlines, reduce seek time, and manage fair disk access.

freemanjohn
Télécharger la présentation

CS 414 – Multimedia Systems Design Lecture 29 – Media Server (Part 3)

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. CS 414 – Multimedia Systems DesignLecture 29 – Media Server (Part 3) Klara Nahrstedt Spring 2010 CS 414 - Spring 2010

  2. Administrative MP3 – deadline Monday, April 12, 5-7pm demonstrations in 216 Please, sign up for demonstration on Monday, April 12 in class. Note: The MP3 demonstration must happen among four computers!!! CS 414 - Spring 2010

  3. Some Interesting Facts • DBMS2.com Source (May 2009) • Facebook had 400 terabytes of disks managed by Hadoop/Hive with an approx. 6:1 compression ratio • Facebook’s Hadoop/Hive system ingests 15 terabytes of new data per day • Facebook had 610 Hadoop nodes (in May 2009) running in a single cluster and was heading for 1000 • Yahoo had 2000 nodes (in May 2009) and was heading for 4000 CS 414 - Spring 2010

  4. Outline • Disk Scheduling • SCAN-EDF • Group Sweeping • Mixed Scheduling • Admission Control • File System Metadata/Indexing • Block Size Issues CS 414 - Spring 2010

  5. 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 CS 414 - Spring 2010

  6. EDF (Earliest Deadline First) Disk Scheduling • Each disk block request is tagged with deadline • Policy: • Schedule disk block request with earliest deadline • Excessive seek time – high overhead • Pure EDF must be adapted or combined with file system strategies CS 414 - Spring 2010

  7. EDF Example Note: Consider that block number Implicitly encapsulates the disk track number CS 414 - Spring 2010

  8. 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 CS 414 - Spring 2010

  9. Implementation of SCAN-EDF • Notation: • Dibe deadline of disk block request ‘i’ • Nibe 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 CS 414 - Spring 2010

  10. SCAN EDF Example (Nmax = 100) CS 414 - Spring 2010

  11. Enhanced SCAN-EDF (1) Head Moves Upwards • 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 CS 414 - Spring 2010

  12. Enhanced SCAN-EDF (2) • Algorithm: • If head moves upwards (towards Nmax), then • (a) • (b) CS 414 - Spring 2010

  13. Enhanced SCAN-EDF (3) If head moves downwards (towards 1), then (a) (b) CS 414 - Spring 2010

  14. Group Sweeping Algorithms • Policy: • Each Request consists of (Deadline, Block Number ) • Disk Block Requests served in cycles • Requests served in Round-Robin manner • In one cycle, requests divided into groups • As we retrieve blocks, we may need smoothing buffers to ensure continuity CS 414 - Spring 2010

  15. Group Sweeping Example CS 414 - Spring 2010

  16. Mixed Scheduling (uses SSTF – Shortest Seek Time First) Example of SSTF CS 414 - Spring 2010

  17. Mixed Scheduling SSTF (Shortest Seek Time First) + Balanced Strategy CS 414 - Spring 2010

  18. Admission Control Client 1 retrieves K1 blocks in one round Client 2 retrieves K2 blocks Server Client 3 retrieves K3 blocks Client 4 retrieves K4 blocks CS 414 - Spring 2010

  19. Admission Control • Disk block requests are timed • Media server must determine • admit a stream • serve (schedule) a stream without having negative effect on other streams already serviced. • Deterministic Guarantees • Admission control considers worst case scenario when admitting new stream • Constrained Disk Placement Example: M - size of blocks, G – size of gabs, rdt– data transfer of disk CS 414 - Spring 2010

  20. Admission Control α – overhead switching from one round (‘j-1’) To another round (j), and the transmitting the First block of the ‘j’ round β – transmission time of (Ki-1) blocks in ‘j’ round, i=1,..4 Ki – number of blocks retrieved by client ‘i’ ηi – Block granularity retrieved for client ‘i’ Ri – playback rates of client ‘i’ CS 414 - Spring 2010

  21. Admission Control • Statistical Guarantees • Deadlines are guaranteed with certain probability • Admission control considers statistical behavior of the disk system while admitting new stream (average performance) • Best effort Service • No guarantees CS 414 - Spring 2010

  22. Multimedia File Systems • Real-time Characteristics • Read operation must be executed before well-defined deadline with small jitter • Additional buffers smooth data • File Size • Can be very large even those compressed • Files larger than 232 bytes • Multiple Correlated Data Streams • Retrieval of a movie requires processing and synch of audio and video streams CS 414 - Spring 2010

  23. Placement of Mapping Tables • Fundamental Issue: keep track of which disk blocks belong to each file (I-nodes in UNIX) • For continuous files/contiguous placement • don’t need maps • For scattered files • Need maps • Linked lists (inefficient for multimedia files) • File allocation tables (FAT) CS 414 - Spring 2010

  24. Indexing and FAT Higher Level Index Table Per File File Allocation Table Block I1 Location PTR I Frame Block I2 Location PTR P Frame Block I3 Location PTR B Frame Block P11 Location PTR P Frame Block P12 Location PTR ……….. Block B1 Location PTR Block P21 Location PTR Block P22 Location PTR ………….. CS 414 - Spring 2010

  25. Constant and Real-time Retrieval of MM Data • Retrieve index in real-time • Retrieve block information from FAT • Retrieve data from disk in real-time • Real-time playback • Implement linked list • Random seek (Fast Forward, Rewind) • Implement indexing • MM File Maps • include metadata about MM objects: creator of video, sync info CS 414 - Spring 2010

  26. Fast Forward and Rewind(Implementation) • Play back media at higher rate • Not practical solution • Continue playback at normal rate, but skip frames • Define skip steps, e.g. skip every 3rd, or 5th frame • Be careful about interdependencies within MPEG frames • Approaches for FF: • Create a separate and highly compressed file • Categorize each frame as relevant or irrelevant • Intelligent arrangement of blocks for FF CS 414 - Spring 2010

  27. Block Size Issues in File Organization Movie Time line Frame index ……… A A A V V V V V V T T CS 414 - Spring 2010 • Small Block Sizes • Use smaller block sizes, smaller than average frame size • Organization Strategy: Constant Time Length • Need Metadata structure, called Frame Index • Frame means a time frame within a movie • Under the time frame read all blocks (audio, video, text) belonging to this time frame

  28. Block Size Issues Block Index A V A V A V A V V V • Large Block Size • Use large blocks (e.g., 256 KB) which include multiple audio/video/text frames • Organization Strategy: Constant Data Length • Need Metadata structure, called Block Index • Each block contains multiple movie frames CS 414 - Spring 2010

  29. Tradeoffs Frame index : needs large RAM usage while movie is playing, however little disk wastage Block index (if frames are not split across blocks): need low RAM usage, but major disk wastage – internal disk fragmentation Block index(if frames are split across blocks): need low Ram usage, no disk wastage, extra seek times CS 414 - Spring 2010

  30. Conclusion The data placement, scheduling, block size decisions are very important for any media server design and implementation. Still need to consider caching – next lecture CS 414 - Spring 2010

More Related