Parallel Programming in .NET 4.0 Tasks and ThreadingIngo Rammer, thinktectureweblogs.thinktecture.com/ingo@ingorammer

Ingo Rammer and thinktecture

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• Slides/Samples: http://weblogs.thinktecture.com/ingo• ingo.rammer@thinktecture.com

The Problem

128 cores(Yes, this is a real screenshot)

The future brings ...

... more cores @ lower speed

Multithreading vs. Parallelism

What do we need?• Parallelism, not just multithreading

• Partitioning of work• Queue management• Synchronization

• Today• Threads• ThreadPool (==> limited API)

.NET <= 3.5• Manual management of parallelism• Important

• Threads: unit of scheduling, not unit of work!

• ThreadPool: limited API

.NET 4.0• Fine-Grained Parallelism: Task-API and

coordination structures (the foundation of it all)

• Structured Parallelism: Parallel• Declarative Parallelism: PLINQ

• And some underlying optimizations in the ThreadPool

Task API

Task t1 = Task.Factory.StartNew(DoSomething);Task t2 = Task.Factory.StartNew(delegate {DoSomething();});Task t3 = Task.Factory.StartNew(()=>DoSomething());

Something tmp = GetData(); // just dummy parameter

Task t4 = Task.Factory.StartNew(p=>((Something)p).DoIt(), tmp);Task t5 = Task.Factory.StartNew(()=>tmp.DoIt());

t1.Wait();Task.WaitAll(t2,t3,t4);Task.WaitAny(t3,t4,t6);

Tasks with Results

Task<int> t1 = Task.Factory.StartNew<int>(() => data.Process());

int val = t1.Result; // blocks until t1 is finished

Task Continuation

var firstTask = new Task<int>(() => First());var secondTask = firstTask.ContinueWith((t) => Second());

firstTask.Start();secondTask.Wait();

Debugging: Parallel Tasks

Debugging: Parallel Stacks

Structured Parallelism

• Parallel.Invoke

Parallel.Invoke( () => Foo(), () => Bar(), () => Baz());

Structured Parallelism• Parallel.ForEach

string[] foo = {"bar","baz","qux"};

Parallel.ForEach(foo, (p) =>{ DoIt(p);});

// OR ...to support stopping:

Parallel.ForEach(foo, (p,s) =>{ DoIt(p); if (p == "baz") s.Stop();});

// s is implicitly of type ParallelLoopState

int sum2 = lst .Where(p=>p.Index>3123 && p.Index<5892) .Sum(p => p.Process());

List<DemoData> lst = ...;var lst = from p in lst.AsParallel() where p.Index > 3123 && p.Index < 5892 select p;

Declarative Parallelism: PLINQ

PLINQ Options

lst.AsParallel() .WithDegreeOfParallelism(10) .AsOrdered() .WithMergeOptions(ParallelMergeOptions.FullyBuffered)

Cancellation Support• Unified cancellation with

CancellationSource and CancellationToken• Tasks can be manually cancelled, or

automatically when parent is cancelled• PLINQ-Queries can specify a

CancellationToken

CancellationSource src = new CancellationSource();

lst.AsParallel().WithCancellation(src.Token).Sum(...);

src.Cancel();

BlockingCollection<T>

void ThreadProc() { while (!_workitems.IsCompleted) { var itm = _workitems.Take(); Process(itm); }}

static BlockingCollection<int> _workItems;

void EnqueueProc(){ _workItems.Add(123); _workItems.CompleteAdding();}

ThreadPool Optimizations

• Local queues• Work stealing• Locality by LIFO

What you get with .NET 4.0

• Fine-Grained Parallelism: Task, Task<T>• Structured Parallelism: Parallel.Invoke,

Parallel.ForEach• Declarative Parallelism:

PLINQ // .AsParallel()

Next steps…Think of possible ways to add parallel capabilities to your existing applications. Otherwise you'll only ever use 1/128th of That Big Machine.

• Resources• Daniel Moth's weblog:

http://www.danielmoth.com/Blog/• PFX Team blog:

http://blogs.msdn.com/pfxteam/• Home: http://msdn.microsoft.com/concurrency/

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