Efficient Support for Interactive Browsing Operations in Clustered CBR Video Servers

1. Efficient Support for Interactive Browsing Operations in Clustered CBR Video Servers IEEE Transactions on Multimedia, Vol. 4, No.1, March 2002 Min-You Wu and Wei Shu

2. Outline • Introduction • Video server architectures • The prefetching approach • The grouping approach • Simulation results

3. Implementing fast-forward • Increasing network bandwidth • Displaying frames at a higher rate • Without increasing network bandwidth • Reserve a separate encoded video file for each ratio • Block-skipping • Frame-skipping

4. A clustered video server Delivery node

5. Server architecture • Storage node • Storing video data • Dealing its own disk scheduling algorithm • Delivery node • Receiving requests from clients • Buffering and resequencing video blocks from storage nodes • Sending video blocks to clients

6. Distribution of video blocks • A video file is partitioned into many video blocks (CBR) • Video blocks are evenly distributed over N storage nodes in a round-robin fashion. • Block i is stored in the disk (i mod N)

7. Scheduling of video blocks for normal play Time cycle: time to play a block Time slot: storage node can service multiple requests in a time cycle

8. The prefetching approach • Block-skipping • When performing an fast-forward operation of ratio f, a block is retrieved after skipping f-1 blocks. • To avoid a hot spot, the number of storage nodes (N) and fast-forward ratio (f) must be relatively prime. • Otherwise, a video file can be distributed to a subset of storage nodes. • For example, if N=4 and f=2, then only storage node 0 and 2 will be accessed.

9. Layout and access pattern of fast-forward for block-skipping N = 5, F = 3

10. Prefetching approach for block-skipping retrieve Play

11. Delay • The delivery node retrieves blocks in the sequence of 0, 6, 12, 3, 9, 15, …and it delivers blocks in the sequence of 0, 3, 6, 9, 12, 15, … • The block 0 needs to be delayed to time t2, so that at time t3 block 3 can be delivered. • Consider an fast-forward operation of ration f starts at tk. The maximum delay

12. Example of maximum delay for different ratios f

13. Frame-skipping • Skipping frames within a video block. • Base substream – contains the frames for fast-forward • Enhancement substream – contains the frames only for normal play.

14. Prefetching approach for frame-skipping

15. The grouping approach • The pace is defined as the speed of play. • The grouping approach divides requests into different groups based on their paces.

16. Groups of request with different paces

17. Operations • Change the membership of a request • Adding • Removing • Change the number of time slots allocated to a group • Expanding • Shrinking

18. Grouping approach for block-skiiping

19. Frame-skipping Access f sub-blocks in a time slot

20. Comparison • Prefetching approach • Fully utilize the system bandwidth • Initial delay • Require some buffer space • Grouping approach • Does not require extra buffer space • The shrinking operation may lead to reallocation and delay of some requests.

21. Maximum (total) buffer requirement (prefetching approach)

22. Maximum (total) buffer requirement (prefetching approach)

23. Average delay and relocation (grouping approach)

24. Average delay and relocation (grouping approach)

25. Discuss • Interactive 20% ~ 50% • Pace = 1: 50% ~ 80% (5 ~ 6 time slot) • Pace = 5: 10% ~ 25% (2 time slot) • Pace = 11: 10% ~ 25% (2 time slot) • Expanding and shrinking operations are rarely invoked.