Introduction to MapReduce

ECE7610

The Age of Big-Data

Big-data age Facebook collects 500 terabytes a day(2011) Google collects 20000PB a day (2011)

Data is an important asset to any organization Finance company; insurance company;

internet company We need new

Algorithms/data structures/programming model

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What to do ? (Word Count)

Consider a large data collection and count the occurrences of the different words

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Datacollectio

n

web 2

weed 1

green 2

sun 1

moon 1

land 1

part 1

ResultTable

Main

DataCollection

WordCounter

parse( )count( )

{web, weed, green, sun, moon, land, part, web, green,…}

What to do ?(Word Count)

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Datacollectio

n

web 2

weed 1

green 2

sun 1

moon 1

land 1

part 1

Thread

DataCollection ResultTable

WordCounter

parse( )count( )

Main

1..*1..*

Multi-threadLock on shared data

What to do?(Word Count)

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Datacollectio

n

Single machine cannot serve all the data: you need a distributed special (file) system

Large number of commodity hardware disks: say, 1000 disks 1TB each

Critical aspects: fault tolerance + replication + load balancing, monitoring

Exploit parallelism afforded by splitting parsing and counting

Provision and locate computing at data locations

What to do? (Word Count)

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WordList

Thread

Main

1..*

1..*

DataCollection

Parser1..*

Counter

1..*

ResultTable

KEY web weed green sun moon land part web green …….

VALUE

web 2

weed 1

green 2

sun 1

moon 1

land 1

part 1

Datacollectio

n

Separate countersSeparate data

Datacollectio

nData

collection

Datacollectio

n

Datacollectio

n

It is not easy to parallel….

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Fundamental issuesScheduling, data distribution, synchronization, inter-process communication, robustness, fault tolerance, …

Different programming modelsMessage Passing Shared Memory

Architectural issuesFlynn’s taxonomy (SIMD, MIMD, etc.), network topology, bisection bandwidth, cache coherence, …

Common problemsLivelock, deadlock, data starvation, priority inversion, …dining philosophers, sleeping barbers, cigarette smokers, …

Different programming constructs Mutexes, conditional variables, barriers, …masters/slaves, producers/consumers, work queues,. …

Actually, Programmer’s Nightmare….

MapReduce: Automate for you Important distributed parallel programming paradigm for large-scale

applications. Becomes one of the core technologies powering big IT companies, like

Google, IBM, Yahoo and Facebook. The framework runs on a cluster of machines and automatically

partitions jobs into number of small tasks and processes them in parallel.

Features: fairness, task data locality, fault-tolerance.

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MapReduce

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MAP: Input data <key, value> pair

DataCollection:

split1

web 1

weed 1

green 1

sun 1

moon 1

land 1

part 1

web 1

green 1

… 1

KEY VALUE

Split the data toSupply multipleprocessors

DataCollection:

split 2

DataCollection:

split n

Map

… …Map

…

MapReduce

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Reduce

Reduce

Reduce

MAP: Input data <key, value> pairREDUCE: <key, value> pair <result>

DataCollection:

split1Split the data toSupply multipleprocessors

DataCollection:

split 2

DataCollection:

split n Map

Map…

…

Map

…

C. Xu @ Wayne State 11

Cou

nt

Cou

nt

Cou

nt

Large scale data splits

Parse-hash

Parse-hash

Parse-hash

Parse-hash

Map <key, 1>Reducers (say, Count)

MapReduce

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How to store the data ?

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Compute Nodes

What’s the problem here?

Distributed File System

Don’t move data to workers… Move workers to the data! Store data on the local disks for nodes in the cluster Start up the workers on the node that has the data local

Why? Not enough RAM to hold all the data in memory Network is the bottleneck, disk throughput is good

A distributed file system is the answer GFS (Google File System) HDFS for Hadoop

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GFS/HDFS Design Commodity hardware over “exotic” hardware High component failure rates Files stored as chunks

Fixed size (64MB) Reliability through replication

Each chunk replicated across 3+ chunkservers Single master to coordinate access, keep metadata

Simple centralized management No data caching

Little benefit due to large data sets, streaming reads Simplify the API

Push some of the issues onto the client

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GFS/HDFS

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MapReduce Data Locality

Master scheduling policy Asks HDFS for locations of replicas of input file blocks Map tasks typically split into 64MB (== GFS block

size) Locality levels: node locality/rack locality/off-rack Map tasks scheduled as close to its input data as

possible

Effect Thousands of machines read input at local disk

speed. Without this, rack switches limit read rate and network bandwidth becomes the bottleneck.

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MapReduce Fault-tolerance

Reactive way Worker failure

• Heartbeat, Workers are periodically pinged by master

– NO response = failed worker

• If the processor of a worker fails, the tasks of that worker are reassigned to another worker.

Master failure• Master writes periodic checkpoints• Another master can be started from the last

checkpointed state• If eventually the master dies, the job will be

aborted

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MapReduce Fault-tolerance

Proactive way (Speculative Execution) The problem of “stragglers” (slow workers)

• Other jobs consuming resources on machine• Bad disks with soft errors transfer data very

slowly• Weird things: processor caches disabled (!!)

When computation almost done, reschedule in-progress tasks

Whenever either the primary or the backup executions finishes, mark it as completed

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MapReduce Scheduling Fair Sharing

conducts fair scheduling using greedy method to maintain data locality

Delay uses delay scheduling algorithm to achieve good data

locality by slightly compromising fairness restriction LATE(Longest Approximate Time to End)

improves MapReduce applications' performance in heterogenous environment, like virtualized environment, through accurate speculative execution

Capacity introduced by Yahoo, supports multiple queues for

shared users and guarantees each queue a fraction of the capacity of the cluster

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MapReduce Cloud Service• Providing MapReduce frameworks as a service in clouds

becomes an attractive usage model for enterprises. • A MapReduce cloud service allows users to cost-

effectively access a large amount of computing resources with creating own cluster.

• Users are able to adjust the scale of MapReduce clusters in response to the change of the resource demand of applications.

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Amazon Elastic MR

You

1. Scp data to cluster2. Move data into HDFS

3. Develop code locally

4. Submit MapReduce job4a. Go back to Step 3

5. Move data out of HDFS6. Scp data from cluster

0. Allocate Hadoop cluster

EC2

Your Hadoop Cluster

7. Clean up!

New Challenges

Interference between co-hosted VMs Slow down the job 1.5-7 times

Locality preserving policy no long effective Lose more than 20% locality (depends)

Need specifically designed scheduler for virtual MapReduce cluster Interference-aware Locality-aware

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MapReduce Programming Hadoop implementation of MR in Java (version 1.0.4) WordCount example:

hadoop-1.0.4/src/examples/org/apache/hadoop/examples/WordCount.java

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MapReduce Programming

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Map

Implement your own map class extending the Mapper class

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Reduce

Implement your own reducer class extending the reducer class

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Main()

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Demo

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