Transcript Join & Turn on 2010

Parallel Programming in .NET 4.0
Tasks and Threading
Ingo Rammer, thinktecture
weblogs.thinktecture.com/ingo
@ingorammer
Ingo Rammer and thinktecture
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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
Declarative Parallelism: PLINQ
List<DemoData> lst = ...;
var lst = from p in lst.AsParallel()
where p.Index > 3123 && p.Index < 5892
select p;
int sum2 = lst
.Where(p=>p.Index>3123 && p.Index<5892)
.Sum(p => p.Process());
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>
static BlockingCollection<int> _workItems;
void EnqueueProc()
{
_workItems.Add(123);
_workItems.CompleteAdding();
}
void ThreadProc()
{
while (!_workitems.IsCompleted)
{
var itm = _workitems.Take();
Process(itm);
}
}
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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