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Symmetrical Pair Scheme: a Load Symmetrical Pair Scheme: a Load Balancing Strategy to Solve Intra-Balancing Strategy to Solve Intra-movie Skewness for Parallel Video movie Skewness for Parallel Video ServersServers
Song Wu and Hai JinSong Wu and Hai Jin
Huazhong University of Science & Huazhong University of Science & Technology, Wuhan, ChinaTechnology, Wuhan, China
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AgendaAgenda
IntroductionIntroduction ObservationsObservations Symmetrical Pair Scheme (SPS)Symmetrical Pair Scheme (SPS) Performance EvaluationPerformance Evaluation ConclusionConclusion
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IntroductionIntroduction
Avoids the load imbalance problem Avoids the load imbalance problem caused by video popularitycaused by video popularity
Use Coarse Grained StripingUse Coarse Grained Striping Parallel video servers divide video Parallel video servers divide video
objects into small segmentsobjects into small segments Each server node store one of the Each server node store one of the
segment.segment.
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IntroductionIntroductionServer 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
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IntroductionIntroduction
The viewing time of users are The viewing time of users are differentdifferent
The access numbers of movie The access numbers of movie segments are differentsegments are different
Some segments are more popular Some segments are more popular than othersthan others
Intra-movie skewnessIntra-movie skewness
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IntroductionIntroduction
Solution 1: Fine Grained StripingSolution 1: Fine Grained StripingServer Nodes
S[0] S[1] S[2]
997 998 999
0 1 2
… … …
997 998 999
0 1 2
… … …
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IntroductionIntroduction
Problem of Fine Grained Striping :Problem of Fine Grained Striping :– Stripe unit size is very smallStripe unit size is very small– Reduce the system throughputReduce the system throughput– Reduce the amount of concurrent sessionReduce the amount of concurrent session
Goal of the paper:Goal of the paper:– Study the characteristic of intra-movie Study the characteristic of intra-movie
skewnessskewness– Propose a data placement strategy with load-Propose a data placement strategy with load-
balancing performance based on the analysis balancing performance based on the analysis resultsresults
J. Gafsi, E. Biersack, “Data Striping and reliability aspects in distributed video servers,” Cluster Computing 2(1), pp. 75-91, 1999
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ObservationsObservations
Data Analysis:Data Analysis:– Source: log files from the video server Source: log files from the video server
located in the CCRNC (Center China located in the CCRNC (Center China Regional Network Center)Regional Network Center)
– Information in the log files: the viewing Information in the log files: the viewing time of all viewertime of all viewer
– Number of movie analyzed: 48Number of movie analyzed: 48– Length of log files analyzed: 3 monthsLength of log files analyzed: 3 months
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ObservationsObservations
Different User
Vie
win
g T
ime (
min
ute
s)
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ObservationsObservations
Movie Segment
Segm
ent
Acc
ess
Num
ber
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ObservationsObservations
Classification of segmentsClassification of segments– Buffering segmentsBuffering segments
Segment within the buffering time, TSegment within the buffering time, Tbb
User seldom stop watching during that periodUser seldom stop watching during that period Period: 0 to TPeriod: 0 to Tbb
– Leaking segmentsLeaking segments After buffering time, segment access number After buffering time, segment access number
decreases sharply and almost linearly until a decreases sharply and almost linearly until a particular time, leaking time Tparticular time, leaking time Tll
Period: TPeriod: Tb b to Tto Tll
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ObservationsObservations
– Normal segmentsNormal segmentsUsers seldom stop watching during that Users seldom stop watching during that
period period Period: TPeriod: Tll to T to Tp p (Duration of the movie)(Duration of the movie)
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Symmetrical Pair SchemeSymmetrical Pair Scheme
Notations:Notations:– N: the number of server node in the N: the number of server node in the
parallel video serverparallel video server– S[i]: the i-th server node in the parallel S[i]: the i-th server node in the parallel
video servervideo server– M: the number of movie in the parallel M: the number of movie in the parallel
video servervideo server
– mmii[j]: j-th movie segment in i-th movie[j]: j-th movie segment in i-th movie
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Symmetrical Pair SchemeSymmetrical Pair Scheme
Buffering segments:Buffering segments:– Distributes the buffering segments Distributes the buffering segments
uniformlyuniformly
– Segment length is TSegment length is Tbb/N/N
– The buffering segment mThe buffering segment mii[j] located on [j] located on S[j], j=0,1,…,N-1S[j], j=0,1,…,N-1
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Symmetrical Pair SchemeSymmetrical Pair Scheme
Leaking SegmentsLeaking Segments– Segment Length: (TSegment Length: (Tll-T-Tbb)/N)/N
– If i mod 2 == 0If i mod 2 == 0MMii[N+j] located on S[((i div 2)mod N +j) mod [N+j] located on S[((i div 2)mod N +j) mod
N]N]
– ElseElseMMii[N+j] located on S[((i div 2)mod N + N -1 –[N+j] located on S[((i div 2)mod N + N -1 –
j) mod N]j) mod N]
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Symmetrical Pair SchemeSymmetrical Pair Scheme
Normal SegmentsNormal Segments– Segment Length: (TSegment Length: (Tpp-T-Tll)/(x*N) where x is )/(x*N) where x is
the granularity factor and its value the granularity factor and its value depends on the system configurationdepends on the system configuration
– Distributed uniformly on each serverDistributed uniformly on each server
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Symmetrical Pair SchemeSymmetrical Pair Scheme
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Performance EvaluationPerformance Evaluation Compare SPS with the traditional round robin methodCompare SPS with the traditional round robin method
Layout of leaking segments using traditional round robin manner
Layout of leaking segments using SPS
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Performance EvaluationPerformance Evaluation
Let L[i] be the load of the i-th server nodeLet L[i] be the load of the i-th server node L[i] = sum of access numbers of segments L[i] = sum of access numbers of segments
located on the nodelocated on the node Let LLet Lbb[i], L[i], Lll[i], and L[i], and Lnn[i] respectively [i] respectively
represents the sum of access numbers of represents the sum of access numbers of buffering , leaking and normal segments buffering , leaking and normal segments located on the i-th nodelocated on the i-th node
L[i] = LL[i] = Lbb[i] + L[i] + Lll[i] + L[i] + Lnn[i] [i]
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Performance EvaluationPerformance Evaluation
LLbb[0] ≈ L[0] ≈ Lbb[1] ≈ … ≈ L[1] ≈ … ≈ Lbb[N-1][N-1]
LLnn[0] ≈ L[0] ≈ Lnn[1] ≈ … ≈ L[1] ≈ … ≈ Lnn[N-1][N-1]
Focus on LFocus on Lll[i] only[i] only Metric:Metric:
– UD (Unbalance Degree)UD (Unbalance Degree)
– UD = max{| LUD = max{| Lll[i] - L[i] - Lll[j]|} (i, j = 0,1,…,N-[j]|} (i, j = 0,1,…,N-1)1)
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Performance EvaluationPerformance Evaluation
Assumption:Assumption:– The decrease of access number in the The decrease of access number in the
leaking segments is linearly and the leaking segments is linearly and the slope is kslope is k
– All movie has the same slope kAll movie has the same slope k Let A be the access number of the Let A be the access number of the
most popular leaking segment.most popular leaking segment. UD change periodicallyUD change periodically
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Performance EvaluationPerformance Evaluation For round robin:For round robin:
– UDUDRBRB = (N*M-M = (N*M-M22)*A*k, M=0,1,…,N-1)*A*k, M=0,1,…,N-1– UDUDRB RB reach its maximum value of Nreach its maximum value of N22Ak/2 when Ak/2 when
M=N/2 or (NM=N/2 or (N22-1)AK/4 when M=(N+1)/2 or M=(N-1)/2-1)AK/4 when M=(N+1)/2 or M=(N-1)/2
Unbala
nce
Degre
e
Number of Movie
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Performance EvaluationPerformance Evaluation
For SPS:For SPS:
Unbala
nce
D
egre
e
Number of Movie
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Performance EvaluationPerformance Evaluation
Number of Server Node
Maxim
al U
nbala
nce
Degre
e
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Performance EvaluationPerformance Evaluation
Mirror SPSMirror SPS
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ConclusionConclusion
Analyze the characteristic of intra-movie Analyze the characteristic of intra-movie skewness skewness
Symmetrical Pair Scheme has better balancing Symmetrical Pair Scheme has better balancing performance compared to traditional round performance compared to traditional round robin placementrobin placement
Future work: Future work: – The impact of intra-movie skewness on the The impact of intra-movie skewness on the
caching policycaching policy– The relationship between intra- and inter-The relationship between intra- and inter-
movie skewnessmovie skewness