A look at asynchronous and parallel programming in F# given at the F# User Group on 8/3/2009

1. Async and Parallel Programming in F# Matthew Podwysocki Senior Consultant http://codebetter.com/ @mattpodwysocki

2. Agenda F# in 15 Minutes Why is concurrent programming so hard? What can F# do to help?

3. ...a functional, object-oriented, imperative and explorative programming language for .NET with influences from OCaml, C#, Haskell and Erlang Hi! I’m F#

5. F# in 15 Minutes – The Facts F# is a general purpose .NETlanguage F# is a multi-paradigm language F# is a statically-typed language

6. F# in 15 Minutes - The Syntax binding names to values let lets = "Hello World" let (x, y) = (45, 54) let answer = x + y let numbers = [1 .. 10] let odds = [1; 3; 5; 7; 9] let square x = x * x let squareOf4 = square 4

7. F# in 15 Minutes - The Syntax functions as values fun let square x = x * x let squares = List.map (fun x -> x * x) [1..10] let squares = List.map square [1..10] Operators are functions too!

8. F# in 15 Minutes – The Syntax |> bringing order to chaos let (|>) x f = f x letsumOfSquares = List.sum (List.map square [1..10]) letsumOfSquares = [1..10] |> List.map square |> List.sum <| >> << See also:

9. F# in 15 Minutes – The Syntax discriminated unions type type Suit = | Spade | Heart | Club | Diamond type Rank = | Ace | King | Queen | Jack | Value ofint type Card = Card of Suit * Rank Microsoft Confidential

10. F# in 15 Minutes – The Syntax pattern matching match letcardValue (Card(s,r)) = match r with | Ace -> 11 | King | Queen | Jack -> 10 | Value(x) -> x let (x, y) = ("x", "y") let [a; b] = [1 ; 2] Microsoft Confidential

11. Concurrent programming with shared state… Is really hard! Microsoft Confidential

12. Shared State Gives Us… Race conditions! Obscure error messages! Late night debugging! Locks, mutexes and semaphores, oh my! Microsoft Confidential

13. How Can F# Help Us? Granularity Purity Immutability Libraries

14. In Praise of Immutability Immutable objects ... can be relied upon ... can transfer between threads ... can be aliased safely ... lead to (different) optimization opportunities

15. There is no silver bullet

16. Concurrency Styles Asynchronous Programming Parallel Programming Data Task Actor Model Concurrency

17. Asynchronous Programming

18. publicstaticvoidProcessImagesInBulk() { Console.WriteLine("Processing images... "); long t0 = Environment.TickCount; NumImagesToFinish = numImages; AsyncCallbackreadImageCallback = newAsyncCallback(ReadInImageCallback); for (inti = 0; i < numImages; i++) { ImageStateObject state = newImageStateObject(); state.pixels = newbyte[numPixels]; state.imageNum = i; FileStreamfs = newFileStream(ImageBaseName + i + ".tmp", FileMode.Open, FileAccess.Read, FileShare.Read, 1, true); state.fs = fs; fs.BeginRead(state.pixels, 0, numPixels, readImageCallback, state); } boolmustBlock = false; lock (NumImagesMutex) { if (NumImagesToFinish > 0) mustBlock = true; } if (mustBlock) { Console.WriteLine("All worker threads are queued. " + " Blocking until they complete. numLeft: {0}", NumImagesToFinish); Monitor.Enter(WaitObject); Monitor.Wait(WaitObject); Monitor.Exit(WaitObject); } long t1 = Environment.TickCount; Console.WriteLine("Total time processing images: {0}ms", (t1 - t0)); } publicstaticvoidReadInImageCallback(IAsyncResultasyncResult) { ImageStateObject state = (ImageStateObject)asyncResult.AsyncState; Streamstream = state.fs; intbytesRead = stream.EndRead(asyncResult); if (bytesRead != numPixels) thrownewException(String.Format ("In ReadInImageCallback, got the wrong number of " + "bytes from the image: {0}.", bytesRead)); ProcessImage(state.pixels, state.imageNum); stream.Close(); FileStreamfs = newFileStream(ImageBaseName + state.imageNum + ".done", FileMode.Create, FileAccess.Write, FileShare.None, 4096, false); fs.Write(state.pixels, 0, numPixels); fs.Close(); state.pixels = null; fs = null; lock (NumImagesMutex) { NumImagesToFinish--; if (NumImagesToFinish == 0) { Monitor.Enter(WaitObject); Monitor.Pulse(WaitObject); Monitor.Exit(WaitObject); } } } using System; using System.IO; usingSystem.Threading; usingSystem.Runtime.InteropServices; usingSystem.Runtime.Remoting.Messaging; usingSystem.Security.Permissions; publicclassBulkImageProcAsync { publicconstStringImageBaseName = "tmpImage-"; publicconstintnumImages = 200; publicconstintnumPixels = 512 * 512; publicstaticintprocessImageRepeats = 20; publicstaticintNumImagesToFinish = numImages; publicstaticObject[] NumImagesMutex = newObject[0]; publicstaticObject[] WaitObject = newObject[0]; publicclassImageStateObject { publicbyte[] pixels; publicintimageNum; publicFileStreamfs; } “Asynchronous File I/O”http://msdn.microsoft.com/en-us/library/kztecsys.aspx State of Asynchronous I/O

19. letProcessImageAsynci = async { useinStream = File.OpenRead(sprintf"Image%d.tmp"i) let! pixels = inStream.AsyncRead(numPixels) let pixels' = ProcessImage(pixels, i) useoutStream = File.OpenWrite(sprintf"Image%d.done"i) do!outStream.AsyncWrite(pixels') } letProcessImagesAsync() = let tasks = [ foriin 1..numImages ->ProcessImageAsync(i) ] letparallelTasks = Async.Parallel tasks Async.RunSynchronouslyparallelTasks Why Isn’t it This Easy?

20. Read stream asynchronously letProcessImageAsynci = async { useinStream = File.OpenRead(sprintf"Image%d.tmp"i) let! pixels = inStream.AsyncRead(numPixels) let pixels' = ProcessImage(pixels, i) useoutStream = File.OpenWrite(sprintf"Image%d.done"i) do!outStream.AsyncWrite(pixels') } letProcessImagesAsync() = let tasks = [ for i in 1..numImages ->ProcessImageAsynci ] letparallelTasks = Async.Parallel tasks Async.RunSynnchronouslyparallelTasks This object coordinates Write stream asynchronously Generate the tasks and queue them in parallel “!” = “asynchronous” Digging Deeper…

21. letgetHtml (url:string) = async { let request = WebRequest.Createurl use! response = request.AsyncGetResponse() use stream = response.GetResponseStream() use reader = newStreamReader(stream) return! reader.AsyncReadToEnd() } What we’re really writing letgetHtml (url:string) = async.Delay(fun () -> let request = WebRequest.Createurl async.Bind(request.AsyncGetResponse(), funresponse -> async.Using(response, fun response -> async.Using(response.GetResponseStream(), fun stream -> async.Using(new StreamReader(stream), fun reader -> reader.AsyncReadToEnd())))))

22. How does it work? Success Continuation Async<T> Execution Request Exception Continuation Cancellation Continuation

23. Anatomy of an Async Operation Async operations Begin/End typeSystem.Net.WebRequestwith memberthis.AsyncGetRequestStream() = Async.BuildPrimitive( this.BeginGetRequestStream, this.EndGetRequestStream)

24. What Do We Get For Free? Code that makes sense Exception propagation Cancellation checking Simple resource lifetime management

25. Twitter Example

26. Parallel Programming Task Parallel Data Parallel

27. Data Parallel ProgrammingPLINQ Enable F# developers to leverage parallel hardware Abstracts away parallelism details Partitions and merges data intelligently Works on any seq<'a>/IEnumerable<T> let q = ratings |> PSeq.adapt |> PSeq.filter(funp ->p.Name = movieName && p.Rating >= 3.0 && p.Rating <= 5.0) |> PSeq.sortBy(fun p ->p.Rating) |> PSeq.map(funp ->p.CustomerId)

28. Task Parallel Programming Enable F# developers to create tasks in parallel letcomputeHash path algorithm = async { use stream = File.OpenRead path let! bytes = stream.AsyncRead(intstream.Length) let crypt = HashAlgorithm.Create algorithm returncrypt.ComputeHash bytes } let algorithms = ["MD5";"SHA1";"SHA256";"SHA384";"SHA512"] let [|md5;sha1;sha256;sha384;sha512|] = Async.RunSynchronously( Async.Parallel [for algorithm in algorithms -> computeHash path algorithm])

29. Task Parallel + Async Workflows TPL Tasks integrated in .NET 4.0 Async Workflows Integrate with pre-defined tasks Calculate Futures letgetStreamData (uri:string) = async { let request = WebRequest.Createuri use! response = request.AsyncGetResponse() return [use stream = response.GetResponseStream() use reader = new StreamReader(stream) while not reader.EndOfStream doyieldreader.ReadLine()] } let result = Async.CreateAsTask <| getStreamDatamyUri do result.Start() letresultValue = result.Value

30. Bing Translator Example

31. Actor Model Concurrency Lots of little tasks Each process does one task Ant Colony Ants are processes sending messages to each other

32. Actor Model in Action… let (<--) (m:'aMailboxProcessor) msg = m.Post(msg) typePinger = Pong typePonger = Ping ofMailboxProcessor<Pinger> | Stop letponger = newMailboxProcessor<Ponger>(funinbox -> let rec loop pongCount = async { let! msg = inbox.Receive() matchmsgwith | Ping outbox -> outbox <-- Pong return! loop(pongCount + 1) | Stop -> return () } loop 0)

33. Actor Model in Action… letpinger count pong = newMailboxProcessor<Pinger>(funinbox -> let rec sendPing() = async { pong <-- Ping(inbox) return! loop (count - 1) } and loop pingsLeft = async { let! msg = inbox.Receive() matchmsgwith | Pong -> ifpingsLeft > 0 then pong <-- Ping(inbox) return! loop(pingsLeft - 1) else pong <-- Stop return () } sendPing())

34. Web Crawling Example

35. Ant Colony Example

36. High Performance Computing MPI.NET Dryad/DryadLINQ

37. What’s in Store for 2010?

38. Ways to Learn F# http://www.fsharp.net F# Interactive (FSI) Language Specification F# Team Blogs F# (FSharp) Discussion DL http://cs.hubfs.net CodePlex F# Samples .NET Reflector Go to Definition

39. Books about F#

40. Resources - Blogs Don Symehttp://blogs.msdn.com/dsyme/ Luke Hobanhttp://blogs.msdn.com/lukeh/ Brian McNamarahttp://lorgonblog.spaces.live.com/ Chris Smithhttp://blogs.msdn.com/chrsmith/ Jomo Fisherhttp://blogs.msdn.com/jomo_fisher/ Planet F#http://feeds.feedburner.com/planet_fsharp