Overview of the Concurrency Runtime


For the latest documentation on Visual Studio 2017 RC, see Visual Studio 2017 RC Documentation.

This document provides an overview of the Concurrency Runtime. It describes the benefits of the Concurrency Runtime, when to use it, and how its components interact with each other and with the operating system and applications.

System_CAPS_ICON_important.jpg Important

In Visual Studio 2015 and later, the Concurrency Runtime Task Scheduler is no longer the scheduler for the task class and related types in ppltasks.h. Those types now use the Windows ThreadPool for better performance and interoperability with Windows synchronization primitives. Parallel algorithms such as parallel_for continue to use the Concurrency Runtime Task Scheduler.

This document contains the following sections:

A runtime for concurrency provides uniformity and predictability to applications and application components that run simultaneously. Two examples of the benefits of the Concurrency Runtime are cooperative task scheduling and cooperative blocking.

The Concurrency Runtime uses a cooperative task scheduler that implements a work-stealing algorithm to efficiently distribute work among computing resources. For example, consider an application that has two threads that are both managed by the same runtime. If one thread finishes its scheduled task, it can offload work from the other thread. This mechanism balances the overall workload of the application.

The Concurrency Runtime also provides synchronization primitives that use cooperative blocking to synchronize access to resources. For example, consider a task that must have exclusive access to a shared resource. By blocking cooperatively, the runtime can use the remaining quantum to perform another task as the first task waits for the resource. This mechanism promotes maximum usage of computing resources.


The Concurrency Runtime is divided into four components: the Parallel Patterns Library (PPL), the Asynchronous Agents Library, the Task Scheduler, and the Resource Manager. These components reside between the operating system and applications. The following illustration shows how the Concurrency Runtime components interact among the operating system and applications:

Concurrency Runtime Architecture

The Concurrency Runtime Architecture

System_CAPS_ICON_important.jpg Important

The Task Scheduler and Resource Manager components are not available from a Windows 8.x Store app or when you use the task class or other types in ppltasks.h.

The Concurrency Runtime is highly composable, that is, you can combine existing functionality to do more. The Concurrency Runtime composes many features, such as parallel algorithms, from lower-level components.

The Concurrency Runtime also provides synchronization primitives that use cooperative blocking to synchronize access to resources. For more information about these synchronization primitives, see Synchronization Data Structures.

The following sections provide a brief overview of what each component provides and when to use it.

Parallel Patterns Library

The Parallel Patterns Library (PPL) provides general-purpose containers and algorithms for performing fine-grained parallelism. The PPL enables imperative data parallelism by providing parallel algorithms that distribute computations on collections or on sets of data across computing resources. It also enables task parallelism by providing task objects that distribute multiple independent operations across computing resources.

Use the Parallel Patterns Library when you have a local computation that can benefit from parallel execution. For example, you can use the concurrency::parallel_for algorithm to transform an existing for loop to act in parallel.

For more information about the Parallel Patterns Library, see Parallel Patterns Library (PPL).

Asynchronous Agents Library

The Asynchronous Agents Library (or just Agents Library) provides both an actor-based programming model and message passing interfaces for coarse-grained dataflow and pipelining tasks. Asynchronous agents enable you to make productive use of latency by performing work as other components wait for data.

Use the Agents Library when you have multiple entities that communicate with each other asynchronously. For example, you can create an agent that reads data from a file or network connection and then uses the message passing interfaces to send that data to another agent.

For more information about the Agents Library, see Asynchronous Agents Library.

Task Scheduler

The Task Scheduler schedules and coordinates tasks at run time. The Task Scheduler is cooperative and uses a work-stealing algorithm to achieve maximum usage of processing resources.

The Concurrency Runtime provides a default scheduler so that you do not have to manage infrastructure details. However, to meet the quality needs of your application, you can also provide your own scheduling policy or associate specific schedulers with specific tasks.

For more information about the Task Scheduler, see Task Scheduler.

Resource Manager

The role of the Resource Manager is to manage computing resources, such as processors and memory. The Resource Manager responds to workloads as they change at run time by assigning resources to where they can be most effective.

The Resource Manager serves as an abstraction over computing resources and primarily interacts with the Task Scheduler. Although you can use the Resource Manager to fine-tune the performance of your libraries and applications, you typically use the functionality that is provided by the Parallel Patterns Library, the Agents Library, and the Task Scheduler. These libraries use the Resource Manager to dynamically rebalance resources as workloads change.


Many of the types and algorithms that are defined by the Concurrency Runtime are implemented as C++ templates. Some of these types and algorithms take as a parameter a routine that performs work. This parameter can be a lambda function, a function object, or a function pointer. These entities are also referred to as work functions or work routines.

Lambda expressions are an important new Visual C++ language feature because they provide a succinct way to define work functions for parallel processing. Function objects and function pointers enable you to use the Concurrency Runtime with your existing code. However, we recommend that you use lambda expressions when you write new code because of the safety and productivity benefits that they provide.

The following example compares the syntax of lambda functions, function objects, and function pointers in multiple calls to the concurrency::parallel_for_each algorithm. Each call to parallel_for_each uses a different technique to compute the square of each element in a std::array object.

// comparing-work-functions.cpp
// compile with: /EHsc
#include <ppl.h>
#include <array>
#include <iostream>

using namespace concurrency;
using namespace std;

// Function object (functor) class that computes the square of its input.
template<class Ty>
class SquareFunctor
   void operator()(Ty& n) const
      n *= n;

// Function that computes the square of its input.
template<class Ty>
void square_function(Ty& n)
   n *= n;

int wmain()
   // Create an array object that contains 5 values.
   array<int, 5> values = { 1, 2, 3, 4, 5 };

   // Use a lambda function, a function object, and a function pointer to 
   // compute the square of each element of the array in parallel.

   // Use a lambda function to square each element.
   parallel_for_each(begin(values), end(values), [](int& n){n *= n;});

   // Use a function object (functor) to square each element.
   parallel_for_each(begin(values), end(values), SquareFunctor<int>());

   // Use a function pointer to square each element.
   parallel_for_each(begin(values), end(values), &square_function<int>);

   // Print each element of the array to the console.
   for_each(begin(values), end(values), [](int& n) { 
      wcout << n << endl;



For more information about lambda functions in C++, see Lambda Expressions.


The following table shows the header files that are associated with each component of the Concurrency Runtime:

ComponentHeader Files
Parallel Patterns Library (PPL)ppl.h


Asynchronous Agents Libraryagents.h
Task Schedulerconcrt.h
Resource Managerconcrtrm.h

The Concurrency Runtime is declared in the Concurrency namespace. (You can also use concurrency, which is an alias for this namespace.) The concurrency::details namespace supports the Concurrency Runtime framework, and is not intended to be used directly from your code.

The Concurrency Runtime is provided as part of the C Runtime Library (CRT). For more information about how to build an application that uses the CRT, see CRT Library Features.