Parallel Extensions was the development name for amanagedconcurrencylibrary developed by a collaboration betweenMicrosoft Research and theCLR team atMicrosoft. The library was released in version 4.0 of the.NET Framework.[1] It is composed of two parts:Parallel LINQ (PLINQ) andTask Parallel Library (TPL).[2][3] It also consists of a set ofcoordination data structures (CDS) – sets ofdata structures used to synchronize and co-ordinate the execution of concurrent tasks.[4]
PLINQ, orParallelLINQ, parallelizing the execution of queries on objects (LINQ to Objects) and XML data (LINQ to XML). PLINQ is intended for exposingdata parallelism by use of queries.[2] Any computation on objects that has been implemented as queries can be parallelized by PLINQ. However, the objects need to implement theIParallelEnumerable interface, which is defined by PLINQ itself. Internally it usesTPL for execution.[4][5]
TheTask Parallel Library (TPL) is thetask parallelism component of the Parallel Extensions to .NET.[6] It exposes parallel constructs like parallelFor andForEach loops, using regular method calls anddelegates, thus the constructs can be used from anyCLI languages. The job of spawning and terminatingthreads, as well as scaling the number of threads according to the number of available processors, is done by the library itself,[3] using awork stealing scheduler.[7]
TPL also includes other constructs likeTask andFuture. ATask is an action that can be executed independent of the rest of the program. In that sense, it is semantically equivalent to a thread, except that it is a more light-weight object and comes without the overhead of creating an OS thread. Tasks are queued by aTask Manager object and are scheduled to run on multiple OS threads in athread pool when their turn comes.
Future is a task that returns a result. The result is computed in a background thread encapsulated by theFuture object, and the result is buffered until it is retrieved.[3] If an attempt is made to retrieve the result before it has been computed then the requesting thread will block until the result is available.[6]
The other construct of TPL isParallel class.TPL provides a basic form of structured parallelism via three static methods in the Parallel class:
The main concept in the Parallel Extensions to .NET is aTask, which is a small unit of code, usually represented as alambda function, that can be executed independently. Both PLINQ and the TPL API provides methods to create the Tasks – PLINQ divides a query into smaller Tasks, and theParallel.For,Parallel.ForEach andParallel.Invoke methods divide a loop into Tasks.
PFX includes aTask Manager object which schedules the Tasks for execution. A Task Manager contains a globalqueue of Tasks, which are then executed. It also encapsulates multiplethreads onto which the Tasks are executed. By default, as many threads as there are processors (or processor cores) on the system are created, though this number may be manually modified. Each thread is associated with a thread-specific queue of Tasks. When idle, each thread picks up a batch of Tasks and puts them on its local queue, where they are then executed, one by one. If the global queue is empty, a thread will look for Tasks in the queues of its peers, and will take the Tasks which have been in the queue the longest (task stealing). When in execution, the Tasks will be executed independently, with the change in state of one Task independent of others. As a result, if they use a shared resource, they still need to be synchronized manually using locks or other constructs.
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