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  • 1. MapReduce: Model, Internals, and Its Improvements June 29, 2012 Kyong-Ha Lee bart7449@gmail.com Copyright © KAIST Database Lab. All Rights Reserved.
  • 2. OutlineThree topics that I will discuss today :♦ MapReduce programming model♦ Anatomy of the MapReduce framework – Basic principles about the MapReduce framework – Hadoop internal architecture – Not much discussion on implementation details, but will be happy to discuss them if there are any questions.♦ A brief survey on the study of improving the conventional MapReduce framework Copyright © KAIST Database Lab. All Rights Reserved. 2
  • 3. Big Data♦ “A loosely-defined term used to describe data sets so large and complex that they become awkward to work with using an on-hand database in a single node.” - Wikipedia♦ Data growth challenges are defined by:* – Increasing volume (amount of data) » e.g. UniProtKB, a protein knowledgebase now hits more than 108GB a file. – Velocity (speed of data in/out) » 6TB of new log data is collected at Facebook everyday *source: A comparison of join algorithms for log processing in MapReduce, SIGMOD’10 – Variety (range of data types, sources) » Multimedia formats, unstructured and semi-structured data. * Doug Laney, “3D Data Management: Controlling Data Volume, Velocity and Variety”, 2001 Copyright © KAIST Database Lab. All Rights Reserved. 3
  • 4. Commodity Clusters♦ It is hard to store and process big data on a single machine in a timely manner♦ Standard architecture emerging: – Cluster of commodity Linux nodes – Gigabit Ethernet interconnects – Low-price commodity PCs♦ How to organize computations on this architecture? – Practical issues such as SW/HW failures Copyright © KAIST Database Lab. All Rights Reserved. 4
  • 5. Fault Tolerance ♦ Cheap nodes fail frequently, if you have many – An average of 1.7 percent of drives fail within a year and more than 8.6 percent of drives fail after three years*. » MTBF for 1 node = 3 years » MTBF for 1000 nodes = 1 day in average – An average of 1.29 percent of nodes suffer from uncorrectable memory errors within a year*. ♦ Putting fault-tolerance into system is necessary ♦ Job posting from data center team at Google * – One of requirements: » “ able to lift/move 20-30 lbs equipment on a daily basis”* Failure trends in a large disk population-FAST’07* DRAM Errors in the Wild: A Large-Scale Field Study, SIGMETRICS, 2009* http://www.xing.com/net/datacenter/possible-positions-130745/google-datacenter-roles-berlin-frankfurt-munich-9051768 Copyright © KAIST Database Lab. All Rights Reserved. 5
  • 6. A Simple Cluster Architecture 8 Gbps backbone between racks 1 Gbps between Switch any pair of nodes in a rack Switch Switch CPU CPU CPU CPU Mem … Mem Mem … Mem Disk Disk Disk Disk Information about Yahoo! cluster used for TeraSort: • Approximately 3,800 nodes; each rack contains 40 nodes • 2 quad core Xeons @ 2.5GHz per node, 8GB RAM, 4 SATA HDDs • Redhat Enterprise Linux server R5.1 (Kernel 2.6.18) • Sun JDK 1.6.0 05 and 13 Copyright © KAIST Database Lab. All Rights Reserved. 6
  • 7. The Need of Stable Storages♦ Distributed File System – Provides global file namespace – Typical usage patterns – Huge files (100s of GB to TB) » Data is rarely updated in place » Reads and appends are common I/O patterns♦ Google GFS; Hadoop HDFS – Master manages metadata – Data transfers happen directly between clients/chunk servers – Files broken into blocks(typically 64 MB) – Data replication (typically 3 replicas, asynchronous) – Immutable data blocks Copyright © KAIST Database Lab. All Rights Reserved. 7
  • 8. GFS Design Copyright © KAIST Database Lab. All Rights Reserved. 8
  • 9. Principles of Parallel Processing♦ Large-scale data processing – Googlers want to use 1,000s of CPUs – But don’t want hassle of managing things♦ They also want their system to provide: – Automatic parallelization & distribution – Fault-tolerance during processing – Efficient I/O scheduling – Monitoring & status updates Copyright © KAIST Database Lab. All Rights Reserved. 9
  • 10. MapReduce♦ A useful solution for big data processing♦ Both a programming model and a framework for massive parallel processing of large datasets with many commodity machines – Popularized and controversially patented by Google Inc. – Analogous to Group-By-Aggregation in databases♦ Easy to distribute a job across nodes – Support of data parallelism♦ No hassle of managing jobs across nodes – By hiding details of parallel execution and allow users to focus only on data processing strategies♦ Nice retry/failure semantics♦ Runtime scheduling with speculative execution Copyright © KAIST Database Lab. All Rights Reserved. 10
  • 11. Its Importance and Impact♦ “Data center is the computer. If MapReduce is the first instruction of the data center computer, I can’t wait to see the rest of the instruction set, as well as the data center programming language, the data center operating system, the data center storage systems, and more.” - David A. Patterson. Technical perspective: the data center is the computer. CACM, 51(1):105, 2008.♦ A list of institutions that are using Hadoop, an open-source Java implementation of MapReduce♦ Its scholastic impact!as of Dec 31, 2011 as of June 28, 2012 Copyright © KAIST Database Lab. All Rights Reserved. 11
  • 12. Usage Statistics at Google Aug ‘04 Mar ‘06 Sep ‘07 Sep ‘09The number of jobs 29K 171K 2,217K 3,467KAverage completion 634 874 395 475time (secs)Machine years used 217 2,002 11,081 25,562Input data read(TB) 3,288 52,254 403,152 544,130Intermediate data(TB) 758 6,743 3,4774 90,120Output data 193 2,970 14,018 57,520written(TB) Average worker 157 268 394 488* machines Design, Lessons, Advices from Building Large Distributed System, Keynote , LADIS 2009. source: J. Dean, Copyright © KAIST Database Lab. All Rights Reserved. 12
  • 13. MapReduce Programming Model♦ Input: a set of key/value pairs♦ A user implements two functions: – map(key1, value1)  (key2, value2) – reduce(key2, a list of value2)  (key3, value3)♦ (key2, value2) is an intermediate key/value pair♦ Output is the set of (k3,v3) pairs♦ Many problems can be phrased in this way – But not for all! Copyright © KAIST Database Lab. All Rights Reserved. 13
  • 14. Example : Building Inverted Indexmap(key, value):// key1: document Id; value1: text of document for each word w in value1: emit(w, docId )reduce(key, values):// key2: a word w ; value2 : docId initialize a list L for each docId d in values: put d into L emit(w, L) output(word, a sorted list of docIds) Copyright © KAIST Database Lab. All Rights Reserved. 14
  • 15. Map stage in a M/R JobInput Intermediatekey-value pairs key-value pairs k v mapk v k v mapk v k v … … mapk v k v Copyright © KAIST Database Lab. All Rights Reserved. 15
  • 16. Reduce Stage in a M/R Job OutputIntermediate Key-value groups key-value pairskey-value pairs reduce k v k v v v k v reduce k v k v v k v k v grouping … … … k v k v reduce k v Copyright © KAIST Database Lab. All Rights Reserved. 16
  • 17. Data in a M/R Job♦ Input and final output are stored on DFS – Scheduler tries to schedule map tasks “close” to physical storage location of input data – Fault tolerance guaranteed by replicas.♦ Intermediate results are stored on local disks of map and reduce workers for fault tolerance. – Until the next tasks are fully completed.♦ Outputs of a M/R job often become inputs of another MR job – Consecutive or iterative M/R jobs for a single work » e.g. binary joins and PageRank Copyright © KAIST Database Lab. All Rights Reserved. 17
  • 18. Parallel Execution across Nodes1. Partition input key/value pairs into blocks and then run map() tasks in parallel2. After all map()s are complete, consolidate all emitted values for each unique emitted key3. Now partition space of output map keys, and run reduce() in parallel4. In reduce(), values for each key are grouped together then aggregated, reduced output are stored on DFS Copyright © KAIST Database Lab. All Rights Reserved. 18
  • 19. Combiner♦ Often a map task will produce many pairs of the form (k,v1), (k,v2), … for the same key k – e.g., popular words in Word Count♦ It can save many I/Os by pre-aggregating in mappers – combine(k1, list(v1))  v2 – Usually same as reduce function; in-mapper aggregation♦ Combiners works correctly only if reduce function is commutative and associative Copyright © KAIST Database Lab. All Rights Reserved. 19
  • 20. System Architecture Input Block 1 Block 2 Block 3 ... Block n Map Local sort Mapper Mapper Mapper Combiner Intermediate result Barrier pull Shuffle/Copy Reduce Merge Reducer Reduce Output Copyright © KAIST Database Lab. All Rights Reserved. 20
  • 21. Sort-Merge in MapReduce Inputk v Reduce() k v v k v vk v k v v v Local sort Merge k v k vk v Shufflek v k v v k vk v Local sort k v k v v k v v vk v Merge Copyright © KAIST Database Lab. All Rights Reserved. 21
  • 22. Data Partitioning♦ Inputs to map tasks are created by contiguous splits of input file♦ For reduce, we need to ensure that records with the same intermediate key end up at the same worker♦ System uses a default partition function e.g., hash(key) mod R♦ Sometimes useful to override – e.g., hash(hostname(URL)) mod R ensures URLs from a host end up in the same output file Copyright © KAIST Database Lab. All Rights Reserved. 22
  • 23. Fault Tolerance♦ If tasks fail, the tasks are executed again in another node – Detect failure via periodic heartbeats – Re-execute in-progress map tasks – Re-execute in-progress reduce tasks♦ If a node crashes: – Re-launch its current tasks on other nodes – Re-run any maps the node previously ran » Necessary because their output files were lost along with the crashed node♦ If a task is going slowly (straggler): – Launch second copy of task on another node (“speculative execution”) – Take the output of whichever copy finishes first, and kill the other Copyright © KAIST Database Lab. All Rights Reserved. 23
  • 24. ♦ Hadoop Distributed File System – An open-source clone of GFS – Write-one and read-many – Data partitioned into 64MB or 128MB blocks, each block replicated 3 times. – NameNode holds filesystem metadata – Files are broken up and spread over the DataNode♦ Hadoop MapReduce – Java implementation of MapReduce – JobTracker schedules and manages jobs – TaskTracker executes individual map() and reduce() tasks on each node. Copyright © KAIST Database Lab. All Rights Reserved. 24
  • 25. A Brief History of Apache HadoopBranches & Releases*source: http://www.cloudera.com/blog/2012/01/an-update-on-apache-hadoop-1-0/ Copyright © KAIST Database Lab. All Rights Reserved. 25
  • 26. Hadoop 2 alpha :HDFS FederationMapReduce V2 aka. YARN Copyright © KAIST Database Lab. All Rights Reserved. 26
  • 27. Hadoop Ecosystem Copyright © KAIST Database Lab. All Rights Reserved. 27
  • 28. System Behavior on a Single Node *Source: A comparison of join Algorithms for log processing in MR, SIGMOD’10 Copyright © KAIST Database Lab. All Rights Reserved. 28
  • 29. Overall Execution Time*Source: A patform for scalable one-pass analytics using MapReduce, SIGMOD’11 Copyright © KAIST Database Lab. All Rights Reserved. 29
  • 30. Criticism♦ D. DeWitt and M. Stonebraker badly criticized that “MapReduce is a major step backwards”[5]. – He first regarded it as a simple Extract-Transform-Load tool.♦ A technical comparison was done by Pavlo et al.[6] – Compared with a commercial row-wise DBMS and Vertica – After that, technical debates btw. researchers vs. practitioners are triggered♦ CACM welcomed this technical debate, inviting both sides in The Communications of ACM, Jan 2010 [7,8] Copyright © KAIST Database Lab. All Rights Reserved. 30
  • 31. The Technical Debate Copyright © KAIST Database Lab. All Rights Reserved. 31
  • 32. *Source: A Comparison of Approaches to Large-Scale Data Analysis, SIGMOD’09Copyright © KAIST Database Lab. All Rights Reserved. 32
  • 33. Advantages♦ Simple and easy to use – Users code only Map() and Reduce() – BashReduce implements MapReduce for standard Unix commands such as sort, awk, grep, join etc. – Users need not to consider how to distribute their job♦ Flexible – No data model, no schema – Users can treat any irregular or unstructured data with MapReduce♦ Independent of the underlying storage♦ Fault tolerance – Users need not to worry about faults during running – Reported that MapReduce can continue to work in spite of an avg. of 1.2 failures per job at Google.♦ High scalability – Yahoo! reported that Hadoop gear could scale out more than 4,000 nodes in 2008. Copyright © KAIST Database Lab. All Rights Reserved. 33
  • 34. Pitfalls♦ No high-level language – M/R itself does not support any high-level language li and also query optimization♦ Schema-free and index-free – Impromptu data processing throws away the benefits of data modeling – Requires data parsing and full scan♦ A single fixed dataflow – Ease of use with a simple abstraction – Complex algorithms are hard to be phrased in a single M/R jobs.♦ Low efficiency – Sacrifice of disk I/O for fault-tolerance » Result materialization on local disks in each step -> no pipelining » Three replicas on DFS – Block level restarts; a simple heuristic runtime scheduling with speculative execution♦ Blocking operators – Caused by merge-sort for grouping values – Reduce()begins after all map tasks end♦ Very young compared to over 40 years of database – Not mature yet and third-party tools are still relatively few Copyright © KAIST Database Lab. All Rights Reserved. 34
  • 35. A Short List of Improvements♦ Sacrifice of disk I/O for fault-tolerance ♦ A simple heuristic scheduling /load – Main difference against DBMS balancing – Compression techniques like LZO and – LATE, Leen, SkewTune, … snappy, … ♦ Relatively poor performance – Column-wise: RCFile, CIF, CoHadoop, – Adaptive and automatic performance tuning. DREMEL, . . . – Work sharing/Multiple jobs♦ A single fixed dataflow » MRShare, ReStore – Dryad, SCOPE, Nephele/PACT, .. » Hive, Pig Latin – Map-Reduce-Merge for binary operators, » fair/capacity sharing, ParaTimer Map-Join-Reduce and some join techniques ♦ Cowork with other tools – Twister & HaLoop for iterative workload – SQL/MapReduce, HadoopDB, Teradata – Map-Join-Reduce & Join algorithms in EDW’s Hadoop integration, Oracle in- MapReduce database Hadoop, …♦ No schema ♦ DW based on MR – Protocol buffer, JSON, Avro, …. – Cheetah, Osprey, Tenzing, RICARDO,…♦ No indexing ♦ Optimization – HadoopDB, Hadoop++, Trojan, … – Self-tuning, multi query processing, query♦ No high-level language optimization – Hive, Sawzall, SCOPE, Pig Latin, … , Jaql, ♦ Other complements Dryad/LINQ♦ Blocking operators – MapReduce Online, Mortar, MR-Hash, … 35
  • 36. Data Skewness in M/R♦ When skew arises, some partitions of an operation take longer to process, slowing down the entire computation.♦ Type – (1) skew caused by an uneven distribution of input data to operator partitions (or tasks) and – (2) skew caused by some portions of the input data taking longer to process *A timing chart of a M/R job running the than others. PageRank algorithm♦ Map-skew/reduce-skew from Cloud 9 *A Study of Skew in MapReduce Applications, Copyright © KAIST Database Lab. All Rights Reserved. OpenCirrus’11 36
  • 37. Simple Solution for reduce-skew♦ Assigning ReduceIds to mapped outputs as keys according to statistics acquired by sampling or scanning input data – For skewed data, assign more ReduceIds for repartition♦ Pros and cons – Simple, no modification of M/R internals – But, impose additional overhead, i.e. the 1st M/R job SimpleMap Reduce delivery Map Reduce Mapped (assign reduceId) output Shuffle Statistics by ReduceId Copyright © KAIST Database Lab. All Rights Reserved. 37
  • 38. SkewTune♦ Key challenges – Requires no extra input from the user yet work for all M/R implementation – Impose minimal overhead if there is no skew.♦ Detecting and mitigating skew at runtime – When a node becomes idle, it identifies the task with the greatest expected remaining processing time. – Unprocessed input data of the straggling task is then proactively repartitioned. Copyright © KAIST Database Lab. All Rights Reserved. 38
  • 39. Schema Support♦ MapReduce parses each data record at reading time – Causing performance degradation♦ Serialization formats are rather used. – Google’s protocol buffers, JSON, Apache’s thrift and avro.♦ Compression techniques are sometimes used together to reduce I/O costs. – LZO, snappy, … – Fast compression/decompression is sometimes important than compression ratio. – Data processing even with compressed data, e.g. run-length encoding. Copyright © KAIST Database Lab. All Rights Reserved. 39
  • 40. HadoopDB♦ A hybrid system that connects multiple single site DBMS with M/R♦ Combining M/R-style scalability and the performance of DBMS♦ It utilizes M/R as a distributing system – Queries are written in SQL and distributed via M/R across nodes. – Data processing is boosted by the features of DBMS engines as much as possible. Copyright © KAIST Database Lab. All Rights Reserved. 40
  • 41. Twister and HaLoop♦ Copyright © KAIST Database Lab. All Rights Reserved. 41
  • 42. Dryad♦ Allows to design and execute a dataflow graph as users’ wish♦ A form of DAG – Vertex: a program that can run when all inputs are ready – Channel: either of file, TCP pipe, or shared memory, ..♦ A logical dataflow graph is mapped onto physical resources by a job scheduler at runtime. Copyright © KAIST Database Lab. All Rights Reserved. 42
  • 43. MapReduce Online ♦ To address an issue that pull-based communication and checkpoints of mapped outputs limit pipelined processing. ♦ Support online aggregation and continuous queries in MapReduce ♦ Mappers push their data temporarily stored into reducers periodically in the same M/R job. ♦ Enables approximate answers and stream processing – Can also reduce the response times of jobsReadInput File map reduce Block 1 HDFS HDFS Block 2 map reduce Write Snapshot Copyright © KAIST Database Lab. All Rights Reserved. Answer 43
  • 44. Hash-based Partitioning♦ Merge sort affects both the performance and the nature of M/R♦ As soon as each map task outputs its intermediate results, the results are hashed and pushed to hash tables held by reducers.♦ Reducers perform aggregation on the value in each bucket.♦ No grouping is required♦ On-the-fly aggregation even when all mappers are not completed yet. Copyright © KAIST Database Lab. All Rights Reserved. 44
  • 45. Column-wise Storages ♦ Column-oriented storage formats – read-optimized by avoiding unnecessary column reads during table scans – It outperforms the other structures in most cases. – RCFile uses column-wise compression and thus provides efficient storage space utilization. ♦ Systems – Google’s BigTable, Dremel – Facebook’s Record Columnar File(RCFIle), CIF, Llama, . . . CIF Copyright © KAIST Database Lab. All Rights Reserved.RCFile 45
  • 46. Block Collocation Copyright © KAIST Database Lab. All Rights Reserved. 46
  • 47. Sharing Multiple Query Executionsin M/R♦ Inspired by multi query optimization techniques in database♦ three sharing opportunities across multiple MR jobs: – scan sharing, mapped outputs sharing, and Map function sharing♦ Intermediate result sharing improves the execution time significantly.♦ Sharing all scans may yield poor performance because of the cost of merge-sort. – No scan sharing, |D| * n for n M/R jobs with |D|-size input – With scan sharing, n*|D|* log(n*|D|), compared to |D|log|D| * n Copyright © KAIST Database Lab. All Rights Reserved. 47
  • 48. Reusing Results of M/R jobs♦ The current practice is to delete intermediate results from DFS♦ more useful if these intermediate results can be stored and reused in future M/R jobs.♦ ReStore matches input workflows of M/R jobs with previously executed jobs and rewrites the workflows to reuse the stored results of the matched jobs. Copyright © KAIST Database Lab. All Rights Reserved. 48
  • 49. Research Challenges and Issues♦ Parallelizing conventional algorithms – Finding algorithms easy to be phrased in M/R jobs. – Or, develop algorithms for that support. – Not good for ad-hoc queries♦ Performance Improvements – Not so well utilize the modern HW features » Multi-core, GPGPU, SSD, etc – Some caveats still exist in the model » e.g. iterative and incremental processing – Self-tuning » 150+ tuning knobs in Hadoop » Long-running analysis and batch processing – Real-time MapReduce Copyright © KAIST Database Lab. All Rights Reserved. 49
  • 50. Summary♦ M/R is simple, but provides good scalability and fault- tolerance for massive data processing♦ M/R does not substitute DBMS♦ M/R complements DBMS with scalable and flexible parallel processing for various data analysis♦ I/O efficiency of MapReduce still needs to be addressed for more successful implications – sort-merge based grouping and frequent checkpoints♦ Many application domains and room for improvements Copyright © KAIST Database Lab. All Rights Reserved. 50
  • 51. Thank you!Questions or comments? Copyright © KAIST Database Lab. All Rights Reserved. 51
  • 52. References1. David A. Patterson. Technical perspective: the data center is the computer. Communications of ACM, 51(1):105, 2008.2. Hadoop. users List; http://wiki.apache.org/hadoop/PoweredBy3. Jeffrey Dean and Sanjay Ghemawat, MapReduce: Simplified Data, Processing on Large Clusters, In Proceedings of OSDI 2004 and CACM Vol. 51, No. 1 pp. 107-113, 20084. S. Ghemawat and et al. The Google File System, ACM SIGOPS Operating Systems Review, Vol. 37, No. 5 pp. 29- 43, 20035. David J. DeWitt and Michael Stonebraker, MapReduce: a major step backwards, Database column blog, 20086. Andrew Pavlo and et al. A Comparison of Approaches to Large-Scale Data Analysis, In Proceedings of SIGMOD 20097. Michael Stonebraker and et al. MapReduce and Parallel DBMSs: Friends or Foes?, Communications of ACM, Vol 53, No. 1 pp. 64-71, Jan 20108. Jeffrey Dean and Sanjay Ghemawat, MapReduce: A Flexible Data Processing Tool, Communications of the ACM, Vol. 53, No. 1 pp. 72-72 Jan 20109. M. Stonebraker, The case for shared-nothing. Data Engineering Bulletine, 9(1):4-9, 198610. D. DeWitt and J. Gray, Parallel database systems: the future of high performance database systems, Communications of the ACM 35(6):85-98, 199211. B. Schroeder and et a. Disk failures in the real world: What does an MTTF of 1,000,000 hours mean to you. In Proceedings of the 5th USENIX Conference on File and Storage Technologies (FAST), pages 1–16, 2007.12. B. Schroeder and et al. DRAM errors in the wild: a large-scale field study. In Proceedings of the eleventh international joint conference on Measurement and modeling of computer systems, pages 193–204. ACM New York, NY, USA, 2009 Copyright © KAIST Database Lab. All Rights Reserved. 52
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