Plinq Presentation Steen L. Knudsen

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Agenda Quick Linq Intro Plinq Intro Plinq Pitfalls Task Parallel Library

Quick Linq Intro Slides by Paul Litwin

PLinq Parrallel Linq Make it easy for the developer to take advantage of multiple cores. Declarative way of expressing parrallel computations

Thread Pool A lot of code needed to manage the plumbing invovled in this

Plinq thread Pool Split the enumerable list into partitions and let the threads run the tasks

AsParrallel.AsParrallel Extension to IEnumerable public static ParallelQuery AsParallel( this IEnumerable source ) The Ienumerable source should represent the tasks that should run in parrallel

ParallelQuery.WithDegreeOfParallelism() The number of threads to be used Example:

Partitioner The Plinq runtime choose how to partition the source The default for List source is the range partitioner

Partitioner Chunk Partitioning Striped Partitioning Hash Partitioning

Partitioner You can write a custom Partitioner that inherits from Partitioner This can tune the performance if the partitioning is a bottleneck Example

ParallelQuery WithMergeOptions() FullyBuffered each thread process the part, then merge Example

ParallelQuery WithCancellation(CancellationToken) Makes it possible to cancel the parrallel query AsOrdered() Wait for all threads to finish and then orders the result as a normal query

ParallelQuery ForAll() Do some work for all elements but returns nothing

Exceptions AggregateException Collects all the exceptions thrown by the threads InnerExceptions member returns the list of exceptions

When to use Plinq N elements - M threads - T time to process Overhead Split the enumerable into parts O(N) Start the threads O(M) Merge the part results into the complete result O(N) Gain O(N/M*T) O(N/M*T) > (2*O(N)+ O(M))

Plinq and db sources Beware if the source for the AsParrallel is a database source. Example

Task Parallel Library

Parallel.Invoke Parallel.Invoke( () => MethodA(), () => MethodB(), () => MethodC()); Parallel.ForEach Parallel.ForEach(list, e => { DoWork(e) }); Parallel.For Parallel.For(start, end, (i) => DoSomeWork(i));

Task Parallel Library Task ”Future” a proxy for an object not yet computed. Task task = Task.Factory.StartNew(() => DoAction()); Is the task done : If(task.IsCompleted) Wait to the job to be done task.Wait();