The document discusses how Scala aims to make parallel and concurrent programming easier by avoiding mutable state and enabling a functional programming style, and it presents Scala as a language that unifies object-oriented and functional programming in a way that allows both sequential and parallel applications to be written safely and efficiently. Examples are given of how parallel collections and actors can be used in Scala for parallelism and concurrency, and how domain-specific languages embedded in Scala can help program parallel applications for different domains like physics simulation.

1. Working Hard to Keep it Simple Martin Odersky Typesafe

2. The Challenge The world of mainstream software is changing: Moore’s law now achieved by increasing # of cores not clock cycles Huge volume workloads that require horizontal scaling “ PPP” Grand Challenge Data from Kunle Olukotun, Lance Hammond, Herb Sutter, Burton Smith, Chris Batten, and Krste Asanovic

3. Concurrency and Parallelism Parallel programming Execute programs faster on parallel hardware. Concurrent programming Manage concurrent execution threads explicitly. Both are too hard!

4. The Root of The Problem Non-determinism caused by concurrent threads accessing shared mutable state. It helps to encapsulate state in actors or transactions, but the fundamental problem stays the same. So, non-determinism = parallel processing + mutable state To get deterministic processing, avoid the mutable state! Avoiding mutable state means programming functionally . var x = 0 async { x = x + 1 } async { x = x * 2 } // can give 0, 1, 2

5. Space vs Time Time (imperative/concurrent) Space (functional/parallel)

6. Scala is a Unifier Agile, with lightweight syntax Object-Oriented Scala Functional Safe and performant, with strong static tpying

7. Scala is a Unifier Agile, with lightweight syntax Parallel Object-Oriented Scala Functional Sequential Safe and performant, with strong static tpying

9. Some adoption vectors: Web platforms Trading platforms Financial modeling Simulation Fast to first product, scalable afterwards

10. Scala’s Toolbox

11. Different Tools for Different Purposes Parallelism : Parallel Collections Collections Distributed Collections Parallel DSLs Concurrency : Actors Software transactional memory Akka Futures

12. Let’s see an example:

13. A class ... public class Person { public final String name ; public final int age ; Person(String name, int age) { this . name = name; this . age = age; } } class Person( val name: String, val age: Int ) ... in Java: ... in Scala:

14. ... and its usage import java.util.ArrayList; ... Person[] people ; Person[] minors ; Person[] adults ; { ArrayList<Person> minorsList = new ArrayList<Person>(); ArrayList<Person> adultsList = new ArrayList<Person>(); for ( int i = 0; i < people . length ; i++) ( people [i]. age < 18 ? minorsList : adultsList) .add( people [i]); minors = minorsList.toArray( people ); adults = adultsList.toArray( people ); } ... in Java: ... in Scala: val people: Array [Person] val (minors, adults) = people partition (_.age < 18) A simple pattern match An infix method call A function value

15. Going Parallel ? ... in Java: ... in Scala: val people: Array [Person] val (minors, adults) = people .par partition (_.age < 18)

16. Actors for Concurrent Programming Simple message-oriented programming model for multi-threading Serializes access to shared resources using queues and function passing. Easier for programmers to create reliable concurrent processing Many sources of contention, races, locking and dead-locks removed

17. Going further: Parallel DSLs But how do we keep a bunch of Fermi’s happy? How to find and deal with 10000+ threads in an application? Parallel collections and actors are necessary but not sufficient for this. Our bet for the mid term future: parallel embedded DSLs. Find parallelism in domains: physics simulation, machine learning, statistics, ... Joint work with Kunle Olukuton, Pat Hanrahan @ Stanford. EPFL side funded by ERC.

18. EPFL / Stanford Research Applications Domain Specific Languages Heterogeneous Hardware DSL Infrastructure OOO Cores SIMD Cores Threaded Cores Specialized Cores Programmable Hierarchies Scalable Coherence Isolation & Atomicity On-chip Networks Pervasive Monitoring Domain Embedding Language ( Scala ) Virtual Worlds Personal Robotics Data informatics Scientific Engineering Physics ( Liszt ) Scripting Probabilistic (RandomT) Machine Learning ( OptiML ) Rendering Parallel Runtime ( Delite, Sequoia, GRAMPS ) Dynamic Domain Spec. Opt. Locality Aware Scheduling Staging Polymorphic Embedding Task & Data Parallelism Hardware Architecture Static Domain Specific Opt.

19. Example: Liszt - A DSL for Physics Simulation Mesh-based Numeric Simulation Huge domains millions of cells Example: Unstructured Reynolds-averaged Navier Stokes (RANS) solver Fuel injection Transition Thermal Turbulence Turbulence Combustion

20. Liszt as Virtualized Scala val // calculating scalar convection (Liszt) val Flux = new Field[Cell,Float] val Phi = new Field[Cell,Float] val cell_volume = new Field[Cell,Float] val deltat = .001 ... untilconverged { for(f <- interior_faces) { val flux = calc_flux(f) Flux(inside(f)) -= flux Flux(outside(f)) += flux } for(f <- inlet_faces) { Flux(outside(f)) += calc_boundary_flux(f) } for(c <- cells(mesh)) { Phi(c) += deltat * Flux(c) /cell_volume(c) } for(f <- faces(mesh)) Flux(f) = 0.f } AST Hardware DSL Library Optimisers Generators … … Schedulers GPU, Multi-Core, etc

21. Follow us on twitter: @typesafe scala-lang.org typesafe.com