1 Symmetrical Pair Scheme: a Load Balancing Strategy to Solve Intra- movie Skewness for Parallel Video Servers Song Wu and Hai Jin Huazhong University of Science & Technology, Wuhan, China

2 Agenda  Introduction  Observations  Symmetrical Pair Scheme (SPS)  Performance Evaluation  Conclusion

3 Introduction  Avoids the load imbalance problem caused by video popularity  Use Coarse Grained Striping  Parallel video servers divide video objects into small segments  Each server node store one of the segment.

4 Introduction Server Nodes S[0]S[1]S[2] Segment1 of Movie 1 Segment2 of Movie 1 Segment3 of Movie 1 Segment1 of Movie 2 Segment2 of Movie 2 Segment3 of Movie 2

5 Introduction  The viewing time of users are different  The access numbers of movie segments are different  Some segments are more popular than others  Intra-movie skewness

6 Introduction  Solution 1: Fine Grained Striping Server Nodes S[0]S[1]S[2] …… … …… …

7 Introduction  Problem of Fine Grained Striping : –Stripe unit size is very small –Reduce the system throughput –Reduce the amount of concurrent session  Goal of the paper: –Study the characteristic of intra-movie skewness –Propose a data placement strategy with load- balancing performance based on the analysis results J. Gafsi, E. Biersack, “ Data Striping and reliability aspects in distributed video servers, ” Cluster Computing 2(1), pp , 1999

8 Observations  Data Analysis: –Source: log files from the video server located in the CCRNC (Center China Regional Network Center) –Information in the log files: the viewing time of all viewer –Number of movie analyzed: 48 –Length of log files analyzed: 3 months

9 Observations Different User Viewing Time (minutes)

10 Observations Movie Segment Segment Access Number

11 Observations  Classification of segments –Buffering segments  Segment within the buffering time, T b  User seldom stop watching during that period  Period: 0 to T b –Leaking segments  After buffering time, segment access number decreases sharply and almost linearly until a particular time, leaking time T l  Period: T b to T l

12 Observations –Normal segments  Users seldom stop watching during that period  Period: T l to T p (Duration of the movie)

13 Symmetrical Pair Scheme  Notations: –N: the number of server node in the parallel video server –S[i]: the i-th server node in the parallel video server –M: the number of movie in the parallel video server –m i [j]: j-th movie segment in i-th movie

14 Symmetrical Pair Scheme  Buffering segments: –Distributes the buffering segments uniformly –Segment length is T b /N –The buffering segment m i [j] located on S[j], j=0,1,…,N-1

15 Symmetrical Pair Scheme  Leaking Segments –Segment Length: (T l -T b )/N –If i mod 2 == 0  M i [N+j] located on S[((i div 2)mod N +j) mod N] –Else  M i [N+j] located on S[((i div 2)mod N + N -1 – j) mod N]

16 Symmetrical Pair Scheme  Normal Segments –Segment Length: (T p -T l )/(x*N) where x is the granularity factor and its value depends on the system configuration –Distributed uniformly on each server

17 Symmetrical Pair Scheme

18 Performance Evaluation  Compare SPS with the traditional round robin method Layout of leaking segments using traditional round robin manner Layout of leaking segments using SPS

19 Performance Evaluation  Let L[i] be the load of the i-th server node  L[i] = sum of access numbers of segments located on the node  Let L b [i], L l [i], and L n [i] respectively represents the sum of access numbers of buffering, leaking and normal segments located on the i-th node  L[i] = L b [i] + L l [i] + L n [i]

20 Performance Evaluation  L b [0] ≈ L b [1] ≈ … ≈ L b [N-1]  L n [0] ≈ L n [1] ≈ … ≈ L n [N-1]  Focus on L l [i] only  Metric: –UD (Unbalance Degree) –UD = max{| L l [i] - L l [j]|} (i, j = 0,1,…,N-1)

21 Performance Evaluation  Assumption: –The decrease of access number in the leaking segments is linearly and the slope is k –All movie has the same slope k  Let A be the access number of the most popular leaking segment.  UD change periodically

22 Performance Evaluation  For round robin: –UD RB = (N*M-M 2 )*A*k, M=0,1,…,N-1 –UD RB reach its maximum value of N 2 Ak/2 when M=N/2 or (N 2 -1)AK/4 when M=(N+1)/2 or M=(N- 1)/2 Unbalance Degree Number of Movie

23 Performance Evaluation  For SPS: Unbalance Degree Number of Movie

24 Performance Evaluation Number of Server Node Maximal Unbalance Degree

25 Performance Evaluation  Mirror SPS

26 Conclusion  Analyze the characteristic of intra-movie skewness  Symmetrical Pair Scheme has better balancing performance compared to traditional round robin placement  Future work: –The impact of intra-movie skewness on the caching policy –The relationship between intra- and inter- movie skewness