MUNI
FI
MapReduce:
Simplified Data Processing on Large Clusters
PA154 Language Modeling (7.3)
Pavel Rychlý
pary@fi.muni.cz March 30, 2023
Source: Jeff Dean, Sanjay Ghemawat Google, Inc. December, 2004
https://research.google/pubs/pub62/
Pavel Rychlý • MapReduce • March 30, 2023
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Motivation: Large Scale Data Processing
Many tasks: Process lots of data to produce other data Want to use hundreds or thousands of CPUs
■ ... but this needs to be easy
MapReduce provides:
■ Automatic parallelization and distribution
■ Fault-tolerance
■ I/O scheduling
■ Status and monitoring
Pavel Rychlý • MapReduce • March 30, 2023
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Programming model
Input & Output: each a set of key/value pairs Programmer specifies two functions:
map (in_key,  in_value) -> I i st (out-key , intermediate_value )
■ Processes input key/value pair
■ Produces set of intermediate pairs
reduce (out-key,  list (intermediate-value )) -> I ist ( out.value )
■ Combines all intermediate values for a particular key
■ Produces a set of merged output values (usually just one)
Inspired by similar primitives in LISP and other languages
Pavel Rychlý • MapReduce • March 30, 2023 3 / 32
Example: Count word occurrences
map(String input-key, String input-value):
// input_key: document name // input_value: document contents for each word w in input-value: Emitlntermediate (w, "1");
reduce( String output-key ,  Iterator interimediate_values):
// output-key: a word
// output-values : a list of counts
int result = 0;
for each v in intermediate-values :
result += Parselnt(v); Emit (As St ring (result ));
Pseudocode: See appendix in paper for real code
Pavel Rychly • MapReduce • March 30, 2023
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Model is Widely Applicable
MapReduce Programs In Google Source Tree
1000 -I
Mar May  Jul   Sep  Nov  Jan Mar  May  Jul Sep 2003 2004
Example uses:
distributed grep        distributed sort web link-graph reversal
term-vector per host   web access log stats   inverted index construction document clustering   machine learning       statistical machine translation
Pavel Rychly • MapReduce • March 30, 2023
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Implementation Overview
Typical cluster:
■ 100s/1000s of 2-CPU x86 machines, 2-4 GB of memory
■ Limited bisection bandwidth
■ Storage is on local IDE disks
■ GFS: distributed file system manages data (SOSP'03)
■ Job scheduling system: jobs made up of tasks, scheduler assigns tasks to machines
Implementation is a C++ library linked into user programs
Pavel Rychly • MapReduce • March 30, 2023
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Execution
Input
t 7
Intermediate
Group by Key
0
Grouped
kl :v,v,v,v	k2:v	k3:v,v	k4:v,v,v	k5:v
R)(R)   0 0
Output
M
kl:vkl:vk2:v		kl :v	k3:vk4:v	k4:vk5:v	k4:v	kl:vk3:v
Pavel Rychly • MapReduce • March 30, 2023
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Parallel Execution
Map Task 1
"i r
Sort and Group		
k2:v	k4:v,vlv	k5:v
<i>		
		
Reduce Task 1
Map Task 2
Sort and Group	
kl iv.v.v.v	k3 :v,v
	
	
Reduce Task 2
			1 1			' 1		
		(i)		(i)				(i)
k 1 :v kl :v k2:v		kl:v	I |	k3 :v k4 :v	k4 :v k5 :v	1 |	k4:v	kl:v k3:v
Partitioning	Function		1 1	Partitioning	^ Function	1 1	Partitioning Function	
Map Task 3
Pavel Rychly • MapReduce • March 30, 2023
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Task Granularity And Pipelining
Fine granularity tasks: many more map tasks than machines
■ Minimizes time for fault recovery
■ Can pipeline shuffling with map execution
■ Better dynamic load balancing
Often use 200,000 map/5000 reduce tasks/ 2000 machines
Process	Time----------		>									
User Program	MapRednce()				... wait...							
Master		Assign	tasks to worker machines...									
Worker 1		Map 1	Map 3									
Worker 2		Map 2										
Worker 3			Read 1.1		Read 1.3		Read 1.2			Reduce 1		
Worker 4			Read 2.1					Read 2.2	Read 2.3		Reduce 2	
Pavel Rychly • MapReduce • March 30, 2023
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MapReduce status: MR Indexer-beta6-large-2003 10 28 00 03
Started: Fn Nov7 09:51:07 2003 -- up 0 hr 00 rain 18 sec 323 workers; 0 deaths
Type	Shards	Done	Active	Input(MB)	Dene (ME)	Oiitput(MB)
Map	13853	0	323	878934.6	1314.4	717.0
Shuffle	500	0	323	717.0	0.0	0.0
Reduce	500	0	0	0.0	0.0	0.0
100
90
8"
"S 70
B
v
"a 60
E
O
Li
C
u u 1-
ti 0-
Fi.j
4')
30 2" 10
Counters
Reduce Shard
Variable	Mimite
Mapped (MB/s)	72,5
Shuffle (MB/s)	0.0
Output (MB/s)	0,0
doc-index-hits	145825686
decs-indexed	506631
dups-in-index-	0
raerge	
mr-operator-calis	508192
mr-operator-	506631
Pavel Rychly • MapReduce • March 30, 2023
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MapReduce status: MR_Indexer-beta6-large-2003_10_28_00_03
Started: FnNov7 09:51:07 2003 - up 0 hr 05 min 07 sec 1707 workers: 1 deaths
Type	Shards	Done	Active	Input(ME)	Done(MB)	Output(MB)
Map	13853	1857	1707	878934.6	191995.S	113936.6
Shuffle	500	0	500	113936.6	57113.7	57113.7
Reduce	500	0	0	57113.7	0.0	0.0
100 90 80
3 70
4)
^ SO
Counters
Reduce Shard
Variable	Minute
Mapped (MB/s)	699.1
Shuffle (MB/s)	349.5
Output (MB/s)	0.0
doc-index-hits	5004411944
docs-indexed	17290135
dups-in-mdex-merge	0
mr-operator-calls	17331371
mr-operator-outouts	17290135
Pavel Rychly • MapReduce • March 30, 2023
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MapReduce status: MR Indexer-beta6-laige-2003 10 28 00 03
Started: Fn Nov 7 09:51:07 2003 - up 0 hr 10 min 18 sec 1707 workers, 1 deaths
100 90 30
5 70
*>
v
"a 60
E
O
*J
C -ft u 40 Ü L
B -vi a. -'-
Type	Shards	Done	Active	InputfMB)	Done(MB)	Output(MB)
Map	13853	5354	1707	873934 6	406020.1	241058.2
Shuffle	500	0	500	241058.2	196362.5	196362.5
Reduce	500	0	0	196362.5	0.0	0.0
: j
■Z-■Z> .-■-I
■I-
-3"
Counters
■z
ur.
Reduce Shard
Variable	- Minute
Mapped (MB/s)	704.4
Shuffle (MB/3)	371.9
I Output (MB/s)	0.0
doc-lnd ex-hits	5000364228
docs-indexed	17300709
dups-in-md ex-	0
merge	
mi-operator-calls	■ 17342493
mr-operator-outputs	17300709
Pavel Rychly • MapReduce • March 30, 2023
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8989999623^1^^58588888
MapReduce status: MR_Indexer-beta6-laxge-2003_10_28_00_03
Started: FriNov7 09:51:07 2003 --up 0 hr 15 min 31 sec 1707 workers; 1 deaths
Type	Shards	Done	Active	InputCMB)	Done (MB)	Output(MB)
Map	13353	3841	1707	878934.6	621608.5	3694?9.8
Shuffle	500	0	500	369459.8	326986 8	326986.8
Reduce	500	0	i]	326986.8	0.0	0.0
	iw -
	u:
	80
U	70
	
u	
	60
c	
	
u	50
->	
=	40
'J	
-	
L 0-	30
	20
	10
	
	
o
Counters
LT
Reduce Shard
Variable	Minute
Mapped (MB/s)	706.5
Shuffle (MB/s)	419.2
Output (MB/s)	0.0
doc-index-hits	4982870667
docs-lndexed	17229926
dups-in-indeH-	0
merge	
mr-operator-calls	17272056
mr-operator-OUtDUtS	17229926
Pavel Rychly • MapReduce • March 30, 2023
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92^^8680
MapReduce status: MR_Indexer-beta6-large-2003_10_28_00_03
up 0 hr 29 nun 45 sec
Started: Fri Nov 7 09:51:07 2003 1707 workers; 1 deaths
Type	Shards	Done	Active	Input(MB) Done (ME)		Output(ME)
Map	13853	13853	0	878934,6	673934.6	523499.2
Shuffle	500	195	305	523499 2	523389.6	523389.6
Reduce	500	0	195	523389,6	2685.2	2742 6
Counters
i: j tn
Reduce Shard
Variable	Minute	
Mapped (MB/s)	0.3	
Shuffle (MB/s)	0,5	
Output (MB/s)	457	
doc-index-hits	2313178	
docs-indexed	7936	
dups-in-index-rnerge	0	
mr-rnerge-calls	1954105	
mr-merge-outputs	1954105	
Pavel Rychly • MapReduce • March 30, 2023
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MapReduce status: MR_Indexer-betaó4arge-2003_10_28_00_03
Started: FriNov7 09:51:07 2003 up 0 hr 31 min 34 sec 1707 workers: 1 deaths
Type	Shards	Done	Active	Input(MB)	D(mp(TVrR>	- Output(MB)
Map	13353	13353	0	878934.6	878934.6	523499 2
Shuffle	500	500	Ü	523499.2	523499.5	523499.5
Reduce	500	li	500	523499.5	133837.8	136929.6
Reduce Shard
Counters
Variable Minute
Mapped (MB/s)
Shuffle
(MB/s)
Output (MB/s)
doc-
index-hits
docs-indexed
dups-in-
index-
merge
inr-
merge-c alls
c mr-merge -outputs
0.0
0.1
1238.3
0
51738599
51738599
Pavel Rychlý • MapReduce • March 30, 2023
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Map Reduce status: MR_Indexer-beta6-large-2003_10_28_00_03
Started: Fn Nov 7 09:51:07 2003 -- up 0 hr 33 mm 22 sec 1707 Workers: 1 deaths
Type	Shards	Done	Active	LiputflVlB)	Dime (ME)	Output(MB)
Map	13353	13853	0	378934.6	373934 fi	523499 2
Shuffle	500	500	0	523499.2	523499 5	523499.5
Re duce	500	0	500	523499.5	263283.3	269351.2
D ■U
0
(J 5C r  ..
U 4C
u
L
1
(L    - ■
O
o
Reduce Shard
Counters
Variable
Mapped (MB/s)
Shuffle
(MB/s)
Output (MB/s)
doc-
index-hits
docs-indexed
dups-in-
index-
merge
rnr-
merge-
calls
^ merge-I outputs
IV'Uiiute
0 0
0.0
1225.1
0
51842100
51842100
Pavel Rychly • MapReduce • March 30, 2023
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4849^524
^^0^56568887
MapReduce status: MR_Indexer-beta6-large-2003_10_28_00_03
Started: Fn Nov 7 09:51:07 2003 - up 0 hr 35 min 08 sec 1707 workers; 1 deaths
1 Type	Shards	Done	Active	Input(ME)	Done(MB)	Output(MB)
Map	13853	13853	0	873934.6	878934.6	523499.2
Shuffle	500	500	0	523499.2	523499.5	523499.5
Reduce	500	0	500	523499.5	390447.6	399457.2
V
u
(J =
u 'J
-
o o o o
Reduce Shard
Counters
Variable
Mapped (MB/s)
Shuffle (MB/s)
Output (MB/s)
doc-
mdex-hits
döcs-indexed
dups-in-index-rnerge
mr-
merge-calls
merge-outouts
Minute
0.0
0 0
1222.0
0
51640600
51640600
Pavel Rychly • MapReduce • March 30, 2023
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^099999869^^45114
Map Reduce status: MR_Indexer-beta6-large-2003_10_28_00_03
Started: Fn Nov 7 09:51:07 2003 - up 0 hr 37 imn 01 sec 1707 workers; 1 deaths
Type	Shards	Done	Active	Input(MB)	Done (MB)	Output(MB)
Map	13S53	13853	0	878934.6	873934 6	523499.2
Shuttle	500	500	0	523499.2	520468.6	520468.6
Reduce	500	406	94	520468.6	512265.2	514373.3
Reduce Shard
Counters
Variable
Mapped (MB/s)
Shuffle
(MB/s)
Output (MB/s)
doc-
index-hits
docs-mdexed
dups-in-
index-
merge
mr-calls
rnr-
merge-
outouts
Minute
0.0
0.0
849.5
0
350S3350
35033350
Pavel Rychly • MapReduce • March 30, 2023
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MapReduce status: MR_Indexer-beta6-large-2003_10_28_00_03
Started: Fri Nov 7 09:51:07 2003 - up 0 hr 33 ruin 56 sec 1707 workers: 1 deaths
Type	Shards	Done	Active	Input(MB)	Done (MB)	Output(ME)
Map	1335 3	1335?	0	373934.6	373934.6	523499.2
Shuffle	500	500	0	523499.2	519731.3	519731.8
Reduce	500	498	2	519781.8	519394 7	5194407
'.■J
o
Counters
G L.
Reduce Shard
Variable	Minute	
Mapped (MB/s)	0.0	
Shuffle (MB/s)	0.0	
Output (MB/s)	9.4	
doc-index-hits	0	105<
docs-lndexed	0	■
dups-in-index-merge	0	
mr-rnerge-calls	394792	*
mr-rnerge-outputs	394792	-
Pavel Rychly • MapReduce • March 30, 2023
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MapReduce status: MR_Indexer-beta6-large-2003_10_28_00_03
Started: FnNov7 09:51:07 2003 - up 0 hr 40 rnin43 sec 1707 workers; 1 deaths
1 Type	Shards	Done	Active	Input(ME)	Done(MB)	Output(MB)
Map	13853	13853	0	878934.6	878934.6	523499.2
Shuffle	500	500	0	523499.2	519774.3	519774.3
Reduce	500	499	1	519774.3	519735.2	519764.0
100
Counters
Reduce Shard
Variable	Minute	
Mapped (MB/s)	0.0	
Shuffle (MB/s)	0.0	
Output (MB/s)	1.9	
doc-mdex-hits	0	1051
döcs-indexed	0	■
dups-in-index-rnerge	0	
mr-merge-calls	73442	
mr-merge-outouts	73442	-
Pavel Rychly • MapReduce • March 30, 2023
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Fault tolerance: Handled via re-execution
■ On worker failure:
■ Detect failure via periodic heartbeats
■ Re-execute completed and in-progress map tasks
■ Re-execute in progress reduce tasks
■ Task completion committed through master
■ Master failure:
■ Could handle, but don't yet (master failure unlikely)
Robust: lost 1600 of 1800 machines once, but finished fine Semantics in presence of failures: see paper
Pavel Rychlý • MapReduce • March 30, 2023
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Refinement: Redundant Execution
Slow workers significantly lengthen completion time
■ Other jobs consuming resources on machine
■ Bad disks with soft errors transfer data very slowly
■ Weird things: processor caches disabled (!!)
Solution: Near end of phase, spawn backup copies of tasks
■ Whichever one finishes first "wins"
Effect: Dramatically shortens job completion time
Pavel Rychly • MapReduce • March 30, 2023
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Refinement: Locality Optimization
Master scheduling policy:
■ Asks GFS for locations of replicas of input file blocks
■ Map tasks typically split into 64MB (== GFS block size)
■ Map tasks scheduled so GFS input block replica are on same machine or same rack
Effect: Thousands of machines read input at local disk speed
■ Without this, rack switches limit read rate
Pavel Rychly • MapReduce • March 30, 2023
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Refinement: Skipping Bad Records
Map/Reduce functions sometimes fail for particular inputs
■ Best solution is to debug & fix, but not always possible
■ On seg fault:
■ Send UDP packet to master from signal handler
■ Include sequence number of record being processed
■ If master sees two failures for same record:
■ Next worker is told to skip the record
Effect: Can work around bugs in third-party libraries
Pavel Rychly • MapReduce • March 30, 2023
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Other Refinements (see paper)
■ Sorting guarantees within each reduce partition
■ Compression of intermediate data
■ Combiner: useful for saving network bandwidth
■ Local execution for debugging/testing
■ User-defined counters
Pavel Rychly • MapReduce • March 30, 2023
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Performance
Tests run on cluster of 1800 machines:
■ 4 GB of memory
■ Dual-processor 2 GHz Xeons with Hyperthreading
■ Dual 160 GB IDE disks
■ Gigabit Ethernet per machine
■ Bisection bandwidth approximately 100 Gbps
Two benchmarks:
MFLGrep   Scan 1010 100-byte records to extract records matching
a rare pattern (92K matching records) MFLSort    Sort 1010 100-byte records (modeled after TeraSort benchmark)
Pavel Rychly • MapReduce • March 30, 2023
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MR Grep
Locality optimization helps:
■ 1800 machines read 1 TB of data at peak of « 31 GB/s
■ Without this, rack switches would limit to 10 GB/s
Startup overhead is significant for short jobs
Pavel Rychly • MapReduce • March 30, 2023
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MRSort
Backup tasks reduce job completion time significantly System deals well with failures
Normal
No backup tasks
200 processes killed
20000 -I
10000 -
c
Done:
839 s
■n—i—i
0   200 400 600
80)
— 20000 -i
(ft
10000 -
<4-
3
IJJ
'S.
GÜ
0
~ r
0   200 400 600 80
20000 -,
10000 -
ä o
~    I r
—i-r
10001200
~i r
) 10001200
T
0   200 400 600 800 10001200 Seconds
20000 -I
10000
Done: 1235 s
1-1—I-1-1-r
0   200 400 600 800 1000
12(0
20000 -,
10000 -
20000
t    i r
0   200 400 600 800 1000
12(0
10000 -
_1—r
0   200 400 600 800 10001200 Seconds
20000 -1
10000
Done: 886 s
4
im—i-r
0   200 400 600 800
20000 -,
10000 -
20000
i    i r
0   200 400 600 800
10000 -
—i-r
10001200
~~i r
10001200
i    I    i    I    i r
0   200 400 600 800 10001200 Seconds
Pavel Rychly • MapReduce • March 30, 2023
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Experience: Rewrite of Production Indexing System
Rewrote Google's production indexing system using MapReduce
■ Set of +9, 44, 47, 24, 24 MapReduce operations
■ New code is simpler, easier to understand
■ MapReduce takes care of failures, slow machines
■ Easy to make indexing faster by adding more machines
Pavel Rychly • MapReduce • March 30, 2023
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Usage: MapReduce jobs run in August 2004
Number of jobs	29,423	
Average job completion time	634	sees
Machine days used	79,186	days
Input data read	3,288	TB
Intermediate data produced	758	TB
Output data written	193	TB
Average worker machines per job	157	
Average worker deaths per job	1.2	
Average map tasks per job	3,351	
Average reduce tasks per job	55	
Unique map implementations	395	
Unique reduce implementations	269	
Unique map/reduce combinations	426	
Pavel Rychly • MapReduce • March 30, 2023
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Related Work
■ Programming model inspired by functional language primitives
■ Partitioning/shuffling similar to many large-scale sorting systems
■ NOW-Sort ['97]
■ Re-execution for fault tolerance
■ BAD-FS ['04] and TACC ['97]
■ Locality optimization has parallels with Active Disks/Diamond work
■ Active Disks ['01], Diamond ['04]
■ Backup tasks similar to Eager Scheduling in Charlotte system
■ Charlotte ['96]
■ Dynamic load balancing solves similar problem as River's distributed queues
■ River ['99]
Pavel Rychlý • MapReduce • March 30, 2023
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Conclusions
■ MapReduce has proven to be a useful abstraction
■ Greatly simplifies large-scale computations at Google
■ Fun to use: focus on problem, let library deal w/ messy details
Thanks to Josh Levenberg, who has made many significant improvements and to everyone else at Google who has used and helped to improve MapReduce.
Pavel Rychly • MapReduce • March 30, 2023
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