Apache Spark: What's under the hood

© 2015 IBM Corporation
DSK-3576
Apache Spark:
What's Under the Hood?
Adarsh Pannu
Senior Technical Staff Member
IBM Analytics Platform
adarshrp@us.ibm.com

Abstract
This session covers Spark's
architectural components through
the life of simple Spark jobs. Those
already familiar with the basic
Spark API will gain deeper
knowledge of how Spark works with
the potential of becoming an
advanced Spark user, administrator
or contributor.
1

Outline
Why Understand Spark Internals? To Write Better Applications.
●  Simple Spark Application
●  Resilient Distributed Datasets
●  Cluster architecture
●  Job Execution through Spark components
●  Tasks & Scheduling
●  Shuffle
●  Memory management
●  Writing better Spark applications: Tips and Tricks

First Spark Application
On-Time Arrival Performance Dataset
Record of every US airline flight since
1980s.
Fields:
Year, Month, DayofMonth
UniqueCarrier,FlightNum
DepTime, ArrTime, ActualElapsedTime
ArrDelay, DepDelay
Origin, Dest, Distance
Cancelled, ...
“Where, When, How Long? ...”
3

First Spark Application (contd.)
4
Year,Month,DayofMonth,DayOfWeek,DepTime,CRSDepTime,ArrTime,CRS
ArrTime,UniqueCarrier,FlightNum,TailNum,ActualElapsedTime,CRSElapse
dTime,AirTime,ArrDelay,DepDelay,Origin,Dest,Distance,TaxiIn,TaxiOut,Can
celled,CancellationCode,Diverted,CarrierDelay,WeatherDelay,NASDelay,S
ecurityDelay,LateAircraftDelay
2004,2,5,4,1521,1530,1837,1838,CO,65,N67158,376,368,326,-1,-9,EWR,LAX,2454,...
UniqueCarrier
Origin
Dest
Year,Month,DayofMonth
FlightNum
ActualElapsedTime
Distance
DepTime

First Spark Application (contd.)
Which airports handles the most airline carriers?
!  Small airports (E.g. Ithaca) only served by a few airlines
!  Larger airports (E.g. Newark) handle dozens of carriers
In SQL, this translates to:
SELECT Origin, count(distinct UniqueCarrier)
FROM flights
GROUP BY Origin
5

// Read data rows
sc.textFile("hdfs:///.../flights").
2004,2,5...,AA,..,EWR,... 2004,3,22,...,UA,...SFO,... 2004,3,22,...,AA,...EWR,..
// Extract Airport (key) and Airline (value)
(EWR, AA) (SFO, UA) (EWR, UA)
(EWR, [AA, UA]) (SFO, UA)
// Discard duplicate pairs, and compute group size
mapValues(values => values.toSet.size).
// Return results to client
map(row => (row.split(",")(16), row.split(",")(8))).
// Group by Airport
groupByKey.
collect
(EWR, 2) (SFO, 1)
[ (EWR, 2), (SFO, 1) ]

sc.textFile("hdfs:///.../flights").
groupByKey.
collect
Base RDD
Transformed
RDDs
Action

What’s an RDD?
8
CO780, IAH, MCI
CO683, IAH, MSY
CO1707, TPA, IAH
...
UA620, SJC, ORD
UA675, ORD, SMF
UA676, ORD, LGA
...
DL282, ATL, CVG
DL2128, CHS, ATL
DL2592, PBI, LGA
DL417, FLL, ATL
...
Resilient Distributed Datasets
•  Key abstraction in Spark
Immutable collection of objects
•  Distributed across machines
Can be operated on in parallel
Can hold any kind of data
Hadoop datasets
Parallelized Scala collections
RDBMS or No-SQL, ...
Can recover from failures, be cached, ...

Resilient Distributed Datasets (RDD)
1.  Set of partitions (“splits” in Hadoop)
2.  List of dependencies on parent RDDs
3.  Function to compute a partition given its parent(s)
4.  (Optional) Partitioner (hash, range)
5.  (Optional) Preferred location(s)
9
Lineage
Optimized
Execution

We’ve written the code, what next?
Spark supports four different cluster managers:
●  Local: Useful only for development
●  Standalone: Bundled with Spark, doesn’t play well with other
applications
●  YARN
●  Mesos
Each mode has a similar “logical” architecture although physical
details differ in terms of which/where processes and threads are
launched.
10

Spark Cluster Architecture: Logical View
11
Driver represents the application. It runs the main() function.
SparkContext is the main entry point for Spark functionality.
Represents the connection to a Spark cluster.
Executor runs tasks and keeps data in memory or disk storage
across them. Each application has its own executors.
Task
Driver Program
SparkContext
Cluster Manager
Executor
Cache
Task

What’s Inside an Executor?
12
Task
Task
Task
Internal Threads
Single JVM
Running
Tasks
RDD1-1
RDD2-3
Cached RDD
partitions
Shuffle, Transport,
GC, ...
Free
Task
Slots
Broadcast-1
Broadcast-2
Other
global
memory

Spark Cluster Architecture: Physical View
13
Task
Task
Task
RDD P1
RDD P2
RDD P3
Internal
Threads
Node 1
Executor 1
Task
Task
Task
RDD P5
RDD P6
RDD P1
Internal
Threads
Node 2
Executor 2
Task
Task
Task
RDD P3
RDD P4
RDD P2
Internal
Threads
Node 3
Executor 3
Driver

Ok, can we get back to the code?
14

Spark Execution Model
15
sc.textFile(”...").
map(row =>...).
groupByKey.
mapValues(...).
collect
Application
Code
RDD
DAG
DAG and Task
Scheduler
Executor(s)
Task
Task
Task
Task

Spark builds DAGs
16
Directed (arrows)
Acyclic (no loops)
Graph
•  Spark applications are
written in a functional
style.
•  Internally, Spark turns a
functional pipeline into
a graph of RDD
objects.
sc.textFile("hdfs:///.../flightdata").
groupByKey.
collect

Spark builds DAGs (contd.)
17
HadoopRDD
MapPartitionsRDD
MapPartitionsRDD
ShuffleRDD
MapPartitionsRDD
Data-set level View Partition level View
•  Spark has a rich
collection of operations
that generally map to
RDD classes
•  HadoopRDD,
FilteredRDD, JoinRDD,
etc.

DAG Scheduler
18
textFile
mapValues
map
groupByKey
Stage 1
Stage 2
collect
•  Split the DAG into
stages
•  A stage marks the
boundaries of a
pipeline
•  One stage is
completed before
starting the next
stage

DAG Scheduler: Pipelining
2004,2,5...,AA,..,EWR,... 2004,3,22,...,UA,...SFO,... 2004,3,22,...,AA,...EWR,..
(EWR, AA) (SFO, UA) (EWR, UA)
(EWR, [AA, UA]) (SFO, UA)

Task Scheduler (contd.)
20
textFile
map
Stage 1
•  Turn the Stages into Tasks
•  Task = Data + Computation
•  Ship tasks across the cluster
•  All RDDs in the stage have the same number of partitions
•  One task computes each pipeline

21
textFile
map
Stage 1
hdfs://.../flights
(partition 1)
hdfs://.../flights
(partition 2)
hdfs://.../flights
(partition 3)
hdfs://.../flights
(partition 4)
Computation
Data
Task 1 Task 2 Task 3 Task 4
1 2 3 4

22
HDFS
Partitions
Time
•  3 cores
•  4 partitions1
2
2
3
3
4
MapPartitionsRDD
HadoopRDD
1
MapPartitionsRDD
HadoopRDD
2
MapPartitionsRDD
HadoopRDD
3
MapPartitionsRDD
HadoopRDD
4

Task Scheduler: The Shuffle
23
textFile
mapValues
map
groupByKey
Stage 1
Stage 2
collect
•  All-to-all data
movement

Task Scheduler: The Shuffle (contd.)
•  Redistributes data among partitions
•  Typically hash-partitioned but can have user-defined partitioner
•  Avoided when possible, if data is already properly partitioned
•  Partial aggregation reduces data movement
!  Similar to Map-Side Combine in MapReduce
24
1 2 3
1 2
4Stage 1
Stage 2

Task Scheduler: The Shuffle (contd.)
25
1
1 2
1 2
2
1 2
3
1 2
4
1 2
•  Shuffle writes intermediate files to disk
•  These files are pulled by the next stage
•  Two algorithms: sort- based (new/default) and hash-based (older)

groupByKey
26
mapValues
groupByKey
Stage 2
collect
•  After shuffle, each “reduce-side”
partition contains all groups of
the same key
•  Within each partition,
groupByKey builds an in-
memory hash map
•  EWR -> [UA, AA, ...]
•  SFO -> [UA, ...]
•  JFK -> [DL, UA, AA, DL, ...]
•  Single key-values pair must fit in
memory. Can be spilled to disk in
its entirety.
Stage 1

Task Execution
27
Stage 2
Stage 1
Stage N
•  Spark jobs can have any number
of stages (1, 2, ... N)
•  There’s a shuffle between stages
•  The last stage ends with an
“action” – sending results back to
client, writing files to disk, etc.

Writing better Spark applications
How can we optimize our application? Key considerations:
•  Partitioning – How many tasks in each stage?
•  Shuffle – How much data moved across nodes?
•  Memory pressure – Usually a byproduct of partitioning and
shuffling
•  Code path length – Is your code optimal? Are you using the best
available Spark API for the job?
31

Writing better Spark applications: Partitioning
•  Too few partitions?
!  Less concurrency
!  More susceptible to data skew
!  Increased memory pressure
•  Too many partitions
!  Over-partitioning leads to very short-running tasks
!  Administrative inefficiencies outweigh benefits of parallelism
•  Need a reasonable number of partitions
!  Usually a function of number of cores in cluster ( ~2 times is a good
rule of thumb)
!  Ensure tasks task execution time > serialization time
32

Writing better Spark applications: Memory
•  Symptoms
!  Bad performance
!  Executor failures (OutOfMemory errors)
•  Resolution
!  Use GC and other traces to track memory usage
!  Give Spark more memory
!  Tweak memory distribution between User memory vs Spark
memory
!  Increase number of partitions
!  Look at your code!
33

Rewriting our application
sc.textFile("hdfs://localhost:9000/user/Adarsh/flights").
groupByKey.
collect
34
Partitioning
Shuffle size
Memory pressure
Code path length

Rewriting our application (contd.)
repartition(16).
groupByKey.
collect
35
Partitioning
Shuffle size
Memory pressure
Code path length
How many stages do you see?
Answer: 3

repartition(16).
distinct.
groupByKey.
collect
37
Partitioning
Shuffle size
Memory pressure
Code path length
How many stages do you see now?
Answer: 4

distinct(numPartitions = 16).
groupByKey.
mapValues(values => values.size).
collect
39
Partitioning
Shuffle size
Memory pressure
Code path length

map(row => {
val cols = row.split(",")
(cols(16), cols(8))
}).
groupByKey.
mapValues(values => values.size).
collect
41
Partitioning
Shuffle size
Memory pressure
Code path length

map(row => {
(cols(16), cols(8))
}).
map(e => (e._1, 1)).
reduceByKey(_ + _).
collect
42
Partitioning
Shuffle size
Memory pressure
Code path length

Original
groupByKey.
collect
Revised
map(row => {
(cols(16), cols(8))
}).
map(e => (e._1, 1)).
reduceByKey(_ + _).
collect
43
OutOfMemory Error
after running for
several minutes on
my laptop
Completed in
seconds

Writing better Spark applications: Configuration
How many Executors per Node?
--num-executors OR spark.executor.instances
How many tasks can each Executor run simultaneously?
--executor-cores OR spark.executor.cores
How much memory does an Executor have?
--executor-memory OR spark.executor.memory
How is the memory divided inside an Executor?
spark.storage.memoryFraction and spark.shuffle.memoryFraction
How is the data stored? Partitioned? Compressed?
44

As you can see, writing Spark jobs is easy. However, doing so in
an efficient manner takes some know-how.
Want to learn more about Spark?
______________________________________________
Reference slides are at the end of this slide deck.
45

46
Notices and Disclaimers
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Any statements regarding IBM's future direction, intent or product plans are subject to change or withdrawal without notice.
Performance data contained herein was generally obtained in a controlled, isolated environments. Customer examples are presented as
illustrations of how those customers have used IBM products and the results they may have achieved. Actual performance, cost, savings or other
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47
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Acknowledgements
Some of the material in this session was informed and inspired
by presentations done by:
Matei Zaharia (Creator of Apache Spark)
Reynold Xin
Aaron Davidson
Patrick Wendell
... and dozens of other Spark contributors.
50

Intro
What is Spark? How does it relate
to Hadoop? When would you use
it?
1-2 hours
Basic
Understand basic technology and
write simple programs
1-2 days
Intermediate
Start writing complex Spark
programs even as you understand
operational aspects
5-15 days, to
weeks and
months
Expert
Become a Spark Black Belt! Know
Spark inside out.
Months to
years
How deep do you want to go?

Intro Spark
Go through these additional presentations to understand the value of Spark. These
speakers also attempt to differentiate Spark from Hadoop, and enumerate its comparative
strengths. (Not much code here)
"  Turning Data into Value, Ion Stoica, Spark Summit 2013 Video & Slides 25 mins
https://spark-summit.org/2013/talk/turning-data-into-value
"  Spark: What’s in it your your business? Adarsh Pannu 60 mins
IBM Insight Conference 2015
"  How Companies are Using Spark, and Where the Edge in Big Data Will Be, Matei
Zaharia, Video & Slides 12 mins
http://conferences.oreilly.com/strata/strata2014/public/schedule/detail/33057
"  Spark Fundamentals I (Lesson 1 only), Big Data University <20 mins
https://bigdatauniversity.com/bdu-wp/bdu-course/spark-fundamentals/

Basic Spark
" Pick up some Scala through this article co-authored
by Scala’s creator, Martin Odersky. Link
http://www.artima.com/scalazine/articles/steps.html
Estimated time: 2 hours

Basic Spark (contd.)
"  Do these two courses. They cover Spark basics and include a
certification. You can use the supplied Docker images for all other
labs.
7 hours

Basic Spark (contd.)
"  Go to spark.apache.org and study the Overview and the
Spark Programming Guide. Many online courses borrow
liberally from this material. Information on this site is
updated with every new Spark release.
Estimated 7-8 hours.

Intermediate Spark
"  Stay at spark.apache.org. Go through the component specific
Programming Guides as well as the sections on Deploying and More.
Browse the Spark API as needed.
Estimated time 3-5 days and more.

Intermediate Spark (contd.)
•  Learn about the operational aspects of Spark:
"  Advanced Apache Spark (DevOps) 6 hours # EXCELLENT!
Video https://www.youtube.com/watch?v=7ooZ4S7Ay6Y
Slides https://www.youtube.com/watch?v=7ooZ4S7Ay6Y
"  Tuning and Debugging Spark Slides 48 mins
Video https://www.youtube.com/watch?v=kkOG_aJ9KjQ
•  Gain a high-level understanding of Spark architecture:
"  Introduction to AmpLab Spark Internals, Matei Zaharia, 1 hr 15 mins
Video https://www.youtube.com/watch?v=49Hr5xZyTEA
"  A Deeper Understanding of Spark Internals, Aaron Davidson, 44 mins
Video https://www.youtube.com/watch?v=dmL0N3qfSc8
PDF https://spark-summit.org/2014/wp-content/uploads/2014/07/A-Deeper-
Understanding-of-Spark-Internals-Aaron-Davidson.pdf

Intermediate Spark (contd.)
•  Experiment, experiment, experiment ...
"  Setup your personal 3-4 node cluster
"  Download some “open” data. E.g. “airline” data on
stat-computing.org/dataexpo/2009/
"  Write some code, make it run, see how it performs, tune it, trouble-shoot it
"  Experiment with different deployment modes (Standalone + YARN)
"  Play with different configuration knobs, check out dashboards, etc.
"  Explore all subcomponents (especially Core, SQL, MLLib)

Read the original academic papers
"  Resilient Distributed Datasets: A Fault-
Tolerant Abstraction for In-Memory Cluster
Computing, Matei Zaharia, et. al.
"  Discretized Streams: An Efficient and Fault-
Tolerant Model for Stream Processing on
Large Clusters, Matei Zaharia, et. al.
"  GraphX: A Resilient Distributed Graph
System on Spark, Reynold S. Xin, et. al.
"  Spark SQL: Relational Data Processing in
Spark, Michael Armbrust, et. al.
Advanced Spark: Original Papers

Advanced Spark: Enhance your Scala skills
This book by
Odersky is excellent
but it isn’t meant to
give you a quick
start. It’s deep stuff.
"  Use this as your
primary Scala text
"  Excellent MooC by Odersky. Some of
the material is meant for CS majors.
Highly recommended for STC
developers.
35+ hours

Advanced Spark: Browse Conference Proceedings
Spark Summits cover technology and use cases. Technology is also covered in
various other places so you could consider skipping those tracks. Don’t forget to
check out the customer stories. That is how we learn about enablement
opportunities and challenges, and in some cases, we can see through the Spark
hype ☺
100+ hours of FREE videos and associated PDFs available on spark-
summit.org. You don’t even have to pay the conference fee! Go back in time and
“attend” these conferences!

Advanced Spark: Browse YouTube Videos
YouTube is full of training videos, some good, other not so
much. These are the only channels you need to watch though.
There is a lot of repetition in the material, and some of the
videos are from the conferences mentioned earlier.

Advanced Spark: Check out these books
Provides a good overview of
Spark much of material is also
available through other sources
previously mentioned.
Covers concrete statistical analysis /
machine learning use cases. Covers
Spark APIs and MLLib. Highly
recommended for data scientists.

Advanced Spark: Yes ... read the code
Even if you don’t intend to contribute to Spark, there are a ton of valuable
comments in the code that provide insights into Spark’s design and these will
help you write better Spark applications. Don’t be shy! Go to github.com/
apache/spark and check it to out.

Apache Spark: What's under the hood

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