map-reduce: win -or- epic win

MAP-REDUCE:WIN-OR-

EPIC WIN

CSC313: Advanced Programming Topics

Brief History of Google

BackRub: 19964 disk drives

24 GB total storage


BackRub: 19964 disk drives

24 GB total storage

=


Google: 199844 disk drives

366 GB total storage


Google: 199844 disk drives

366 GB total storage

=

Traditional Design Principles If big enough, supercomputer processes

work Use desktop CPUs, just a lot more of them But it also provides huge bandwidth to

memory Equivalent to many machines bandwidth at

once But supercomputers are VERY, VERY

expensive Maintenance also expensive once machine

bought But do get something: high-quality == low

downtime Safe, expensive solution to very large

problems

Why Trade Money for Safety?

How Was Search Performed?

http://www.yahoo.com/search?p=pager

DNS


http://www.yahoo.com/search?p=pager

DNS

http://209.191.122.70


DNS

http://209.191.122.70/search?p=pagerhttp://www.yahoo.com/search?p=pager

Google’s Big Insight

Performing search is “embarrassingly parallel” No need for supercomputer and all that

expense Can instead do this using lots & lots of

desktops Identical effective bandwidth &

performance

Google’s Big Insight

Performing search is “embarrassingly parallel” No need for supercomputer and all that

expense Can instead do this using lots & lots of

desktops Identical effective bandwidth &

performance But problem is desktop machines

unreliable Budget for 2 replacements, since

machines cheap Just expect failure; software provides

quality

A brief history of Google

Google: 2012?0,000 total

servers ??? PB total storage

How Is Search Performed Now?

http://209.85.148.100/search?q=android

How Is Search Performed Now?

http://209.85.148.100/search?q=androidSpell Checker

Ad Server

Document Servers (TB)Index Servers (TB)

Google’s Processing Model

Buy cheap machines & prepare for worst Machines going to fail, but still cheaper

approach Important steps keep whole system

reliable Replicate data so that information losses

limited Move data freely so can always

rebalance loads These decisions lead to many other

benefits Scalability helped by focus on balancing Search speed improved; performance

much better Utilize resources fully, since search

demand varies

Heterogeneous processing

By buying cheapest computers, variances are high Programs must handle homo- & hetero-

systems Centralized workqueue helps with different

speeds

Heterogeneous processing

By buying cheapest computers, variances are high Programs must handle homo- & hetero-

systems Centralized workqueue helps with different

speeds This process also leads to a few small

downsides Space Power consumption Cooling costs

Complexity at Google

Complexity at Google

Avoid this nightmare using abstractions

Google Abstractions

Google File System Handles replication to provide scalability &

durability BigTable

Manages large relational data sets Chubby

Gonna skip past that joke; distributed locking service

MapReduce If job fits, easy parallelism possible without much

work

Remember Google’s Problem

MapReduce Overview

Programming model makes details simple Automatic parallelization & load

balancing Network and disk I/O optimization Robust performance even if machines

fail

MapReduce Overview

Programming model provides good Façade Automatic parallelization & load balancing Network and disk I/O optimization Robust performance even if machines fail

MapReduce Overview


Idea came from 2 Lisp (functional) primitives Map Reduce

MapReduce Overview


Idea came from 2 Lisp (functional) primitives Map: process each entry in list using

some function Reduce: recombines data using given

function

Typical MapReduce problem

1. Read lots and lots of data (e.g., TBs)2. Map

Extract important data from each entry in input

3. Combine Maps and sort entries by key

4. Reduce Process each key’s entries to get result for

that key5. Output final result & watch money roll

in







in

Outline always same;Just map & reduce functions change







in

Algorithm always same;Just map & reduce functions change







in

Template method always same;Just the hook methods change

Pictorial View of MapReduce

Ex: Count Word Frequencies Processes files separately

MapKey=URLValue=text on page

Ex: Count Word Frequencies Processes files separately & count word

freq. in each


Key’=wordValue’=countKey’=word

Value’=countKey’=wordValue’=countKey’=word

Value’=count

Ex: Count Word Frequencies

Reduce

Key’=“to”Value’=“1”

Key’=“be”Value’=“1”

Key’=“or”Value’=“1”

Key’=“not”Value’=“1”



In shuffle step, Maps combined & entries sorted by key

Reduce

Ex: Count Word Frequencies In shuffle step, Maps combined & entries

sorted by key Reduce combines key’s results to compute

final output

Reduce



Key’=“or”Value’=“1”

Key’=“not”Value’=“1”



Key’’=“to”Value’’=“2”

Key’’=“be”Value’’=“2”

Key’’=“or”Value’’=“1”

Key’’=“not”Value’’=“1”

Word Frequency Pseudo-codeMap(String input_key, String input_values) {

String[] words = input_values.split(“ ”);foreach w in words { EmitIntermediate(w, "1");}

}

Reduce(String key, Iterator intermediate_values){int result = 0;foreach v in intermediate_values { result += ParseInt(v);}Emit(result);

}

Ex: Build Search Index

Processes files separately & record words found on each




Value’=URL

Ex: Build Search Index

Processes files separately & record words found on each

To get search Map, combine key’s results in Reduce




Value’=URL

ReduceKey’=wordValue’=countKey’=word


Value’=URL

Key=wordValue=URLs with word

Search Index Pseudo-codeMap(String input_key, String input_values) {

String[] words = input_values.split(“ ”);foreach w in words { EmitIntermediate(w, input_key);}

}

Reduce(String key, Iterator intermediate_values){List result = new ArrayList();foreach v in intermediate_values { result.addLast(v);}Emit(result);

}

Ex: Page Rank Computation

Google’s algorithm ranking pages’ relevance


MapKey=<URL, rank>Value=links on page


Value’=countKey’=wordValue’=countKey’=link on page

Value’=<URL, rank/N>

Reduce Key=<URL, rank>Value=links on page

Key=<URL, rank>Value=links on page


Value’=countKey’=wordValue’=countKey’=link to URL

Value’=<src, rank/N>


+

+


MapKey=<URL, rank>Value=links on page


Value’=countKey’=wordValue’=countKey’=link on page

Value’=<URL, rank/N>

Reduce Key=<URL, rank>Value=links on page



Value’=countKey’=wordValue’=countKey’=link to URL

Value’=<src, rank/N>


+

+

Repeat entire process

(e.g., input Reduce results back into Map)

until page ranks stabilize

(sum of changes to the ranksdrops below some threshold)


Google’s algorithm ranking pages’ relevance

Advanced MapReduce Ideas

How to implement? One master, many workers Split input data into tasks where each task

size fixed Will also be partitioning reduce phase into

tasks Dynamically assign tasks to workers during

each step Tasks assigned as needed & placed in in-

process list Once worker completes task, save result &

retire task Assume that a worker crashed, if not

complete in time Move incomplete tasks back into pool for

reassignment

Advanced MapReduce Ideas

How to implement? One invoker, many commands Split input data into tasks where each task

size fixed Will also be partitioning reduce phase into

tasks Dynamically assign tasks to workers during

each step Tasks assigned as needed & placed in in-

process list Once worker completes task, save result &

retire task Assume that a worker crashed, if not

complete in time Move incomplete tasks back into pool for

reassignment

map-reduce: win -or- epic win

Documents

total servers

machines bandwidth

pager http

machines cheapjust

disk drives

huge bandwidth

brief history of google

failure software