1 symmetrical pair scheme: a load balancing strategy to solve intra- movie skewness for parallel...

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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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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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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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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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Different User

Vie

win

g T

ime (

min

ute

s)

1010


Movie Segment

Segm

ent

Acc

ess

Num

ber

1111


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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– 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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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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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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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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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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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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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

2323


For SPS:For SPS:

Unbala

nce

D

egre

e

Number of Movie

2424


Number of Server Node

Maxim

al U

nbala

nce

Degre

e

2525


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

1 symmetrical pair scheme: a load balancing strategy to solve intra- movie skewness for parallel...

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