Async and Parallel F#

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

Agenda
F# in 15 Minutes
Why 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 Erlang
Hi! I’m F#

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

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

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!

Concurrent programming
with shared state…
Is really hard!

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

How Can F# Help Us?
Granularity
Purity
Immutability
Libraries

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

There is no silver bullet

Concurrency Styles
Asynchronous Programming
Parallel Programming
Data
Task
Actor Model Concurrency

Asynchronous Programming

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

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?

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…

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())))))

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

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

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

Twitter Example

Parallel Programming
Task Parallel
Data Parallel

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)

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])

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

Bing Translator Example

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

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)

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())

Web Crawling Example

Ant Colony Example

High Performance Computing
MPI.NET
Dryad/DryadLINQ

What’s in Store for 2010?

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

Books about F#

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

Async and Parallel F#

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