Web Server Load Balancing/Scheduling

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Web Server Load Balancing/Scheduling. Asima Silva Tim Sutherland. Outline. Web Server Introduction Information Management Basics Load Sharing Policies FLEX WARD EquiLoad AdaptLoad Summary Conclusions Future Work. Request enters a router
Web Server Load Balancing/SchedulingAsima SilvaTim SutherlandOutline
  • Web Server Introduction
  • Information Management Basics
  • Load Sharing Policies
  • FLEX
  • WARD
  • EquiLoad
  • AdaptLoad
  • Summary
  • Conclusions
  • Future Work
  • Request enters a router Load balancing server determines which web server should serve the requestSends the request to the appropriate web serverRequestResponseWeb ServersIntroduction to Web Server Load BalancingInternetRouterLoad-Balancing ServerTraditional Web ClusterContentServer FarmHow do we split up information??ReplicationInformation StrategiesPartitionLoad Balancing ApproachesIssues
  • Efficiently processing requests with optimizations for load balancing
  • Send and process requests to a web server that has files in cache
  • Send and process requests to a web server with the least amount of requests
  • Send and process requests to a web server determined by the size of the request
  • FLEX
  • Locality aware load-balancing strategy based on two factors:
  • Accessed files, memory requirements
  • Access rates (working set), load requirements
  • Partitions all servers into equally balanced groups
  • Each server transfers the response to the browser to reduce bottleneck through the router (TCP Handoff)
  • Flex DiagramS1S2RequestsForwardsRequestS3S4To Client BrowserS5S6W(S1) ≈ W(S2) ≈ W(S3) ≈ … ≈ W(S6)Ar(S1) ≈ Ar(S2) ≈ Ar(S3) ≈ … ≈ Ar(S6)FLEX Cont.
  • Advantages:
  • Highly scalable
  • Reduces bottleneck by the load balancer
  • No software is required
  • Reduces number of cache misses
  • FLEX Cont. II
  • Disadvantages:
  • Not dynamic, routing tale must be recreated
  • Only compared to RR
  • Number of access logs required on each server could be tremendous
  • Responsibility of load-balancing and transferring response is given to web servers – unorganized responsibility
  • How often to update access rates and working sets? Monitor?
  • WARD
  • Workload-Aware Request Distribution Strategy
  • Server core are essential files that represent majority of expected requests
  • Server core is replicated at every server
  • Ward-analysis computes the nearly optimal core size determined by workload access patterns
  • Number of nodes
  • Node RAM
  • TCP handoff overhead
  • Disk access overhead
  • DistributorServerServerDispatcherSwitchDistributorServerDispatcherFront EndFront EndDistributorServerServerLANLANWARD Cont.
  • Three components: dispatcher (load balancer), distributor (router), web server
  • Three progressive architectures:
  • WARDCARDLARDDispatcherDistributorServerSwitchFront EndDispatcherDistributorServerLANSingle front-end distributor, centralized dispatcherCo-located distributor and serverCo-located distributor, server, and dispatcherWARD DiagramS1S2S3Queue:Queue:RequestsQueue:S4Queue:S5S6
  • Each computer is a distributor
  • and a dispatcher
  • Queue:Queue:WARD Cont. II
  • Similar to FLEX, sends response directly to client
  • Minimizes forwarding overhead from handoffs for the most frequent files
  • Optimizes the overall cluster RAM usage
  • “by mapping a small set of most frequent files to be served by multiple number of nodes, we can improve both locality of accesses and the cluster performance significantly”
  • WARD Cont. III
  • Advantages:
  • No decision making, core files are replicated on every server
  • Minimizes transfer of requests and disk reads, both are “equally bad”
  • Outperforms Round Robin
  • Efficient use of RAM
  • Performance gain with increased number of nodes
  • WARD Cont. IV
  • Disadvantages:
  • Core files are created on past day’s data, could decrease performance up to 15%
  • Distributed dispatcher increases the number of TCP requests transfers
  • If core files not selected correctly, higher cache miss rate and increased disk accesses
  • WARD ResultsEquiLoad
  • Determines which server will process a request determined by the size of the requested file
  • Splits the content on each server by file size, forcing the queues sizes to be consistent.
  • EquiLoad Solves Queue Length Problems
  • This is bad
  • QueueQueue
  • This is better
  • QueueQueueEquiLoad DiagramS1S21k-2kRequests2k-3kForwardsRequestS3S43k-10k10k-20kTo Client BrowserS5S620k-100k>100kEquiLoad
  • Advantages
  • Dynamic repartitioning
  • Can be implemented at various levels
  • DNS
  • Dispatcher
  • Server
  • Minimum queue buildup
  • Performs well under variable workload and high system load
  • EquiLoad
  • Disadvantages
  • Cache affinity is neglected
  • Requires a front end dispatcher
  • Distributor must communicate with servers
  • Thresholds of parameter adjustment
  • EquiLoad  AdaptLoad
  • AdaptLoad improves upon EquiLoad using “fuzzy boundaries”
  • Allows for multiple servers to process a request
  • Behaves better in situations where server partitions are very close in size
  • AdaptLoad DiagramS1S21k-3kRequests2k-4kForwardsRequestS3S43k-10k8k-20kTo Client BrowserS5S615k-100k>80kAdaptLoad ResultsSummaryFLEXEquiLoad, AdaptLoadWARDConclusions
  • There is no “best” way to distribute content among servers.
  • There is no optimal policy for all website applications.
  • Certain strategies are geared towards a particular website application.
  • Future Work
  • Compare and contrast the three policies
  • Figure out how often nodes should be repartitioned
  • Compare each policy to a standard benchmark
  • Figure out which policy works in a particular environment
  • Questions?
  • Anyone have one?
  • Related Search
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