Async and Parallel F#

Async and Parallel Programming in F#Matthew PodwysockiSenior Consultanthttp://codebetter.com/@mattpodwysocki

AgendaF# in 15 MinutesWhy is concurrent programming so hard?What can F# do to help?

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

F# in 15 Minutes – The FactsF# is a general purpose .NETlanguageF# is a multi-paradigm languageF# is a statically-typed language

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

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

Concurrent programmingwith shared state…Is really hard!Microsoft Confidential

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

How Can F# Help Us?GranularityPurityImmutabilityLibraries

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

Concurrency StylesAsynchronous ProgrammingParallel ProgrammingDataTaskActor Model Concurrency

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.aspxState of Asynchronous I/O

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 tasksAsync.RunSynchronouslyparallelTasksWhy Isn’t it This Easy?

Read streamasynchronouslyletProcessImageAsynci =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 tasksAsync.RunSynnchronouslyparallelTasksThis object coordinatesWrite stream asynchronouslyGenerate the tasks and queue them in parallel“!” = “asynchronous”Digging Deeper…

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

How does it work?Success ContinuationAsync<T>Execution RequestException ContinuationCancellation Continuation

Anatomy of an Async OperationAsync operationsBegin/EndtypeSystem.Net.WebRequestwithmemberthis.AsyncGetRequestStream() =Async.BuildPrimitive(this.BeginGetRequestStream, this.EndGetRequestStream)

What Do We Get For Free?Code that makes senseException propagationCancellation checkingSimple resource lifetime management

Parallel ProgrammingTask ParallelData Parallel

Data Parallel ProgrammingPLINQEnable F# developers to leverage parallel hardwareAbstracts away parallelism detailsPartitions and merges data intelligentlyWorks 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)

Task Parallel ProgrammingEnable F# developers to create tasks in parallelletcomputeHash path algorithm =async { use stream = File.OpenRead pathlet! bytes = stream.AsyncRead(intstream.Length)let crypt = HashAlgorithm.Create algorithmreturncrypt.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])

Task Parallel + Async WorkflowsTPL Tasks integrated in .NET 4.0 Async WorkflowsIntegrate with pre-defined tasksCalculate FuturesletgetStreamData (uri:string) =async { let request = WebRequest.Createuriuse! response = request.AsyncGetResponse()return [use stream = response.GetResponseStream()use reader = new StreamReader(stream)while not reader.EndOfStreamdoyieldreader.ReadLine()] }let result = Async.CreateAsTask <| getStreamDatamyUrido result.Start()letresultValue = result.Value

Actor Model ConcurrencyLots of little tasksEach process does one taskAnt ColonyAnts are processes sending messages to each other

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

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())

High Performance ComputingMPI.NETDryad/DryadLINQ

Ways to Learn F#http://www.fsharp.netF# Interactive (FSI)Language SpecificationF# Team BlogsF# (FSharp) Discussion DLhttp://cs.hubfs.netCodePlex F# Samples.NET ReflectorGo to Definition

Resources - BlogsDon 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

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