Schedule Groups

This document describes the role of schedule groups in the Concurrency Runtime. A schedule group affinitizes, or groups, related tasks together. Every scheduler has one or more schedule groups. Use schedule groups when you require a high degree of locality among tasks, for example, when a group of related tasks benefit from executing on the same processor node. Conversely, use scheduler instances when your application has specific quality requirements, for example, when you want to limit the amount of processing resources that are allocated to a set of tasks. For more information about scheduler instances, see Scheduler Instances.


The Concurrency Runtime provides a default scheduler, and therefore you are not required to create one in your application. Because the Task Scheduler helps you fine-tune the performance of your applications, we recommend that you start with the Parallel Patterns Library (PPL) or the Asynchronous Agents Library if you are new to the Concurrency Runtime.

Every Scheduler object has a default schedule group for every scheduling node. A scheduling node maps to the underlying system topology. The runtime creates one scheduling node for every processor package or Non-Uniform Memory Architecture (NUMA) node, whichever number is larger. If you do not explicitly associate a task with a schedule group, the scheduler chooses which group to add the task to.

The SchedulingProtocol scheduler policy influences the order in which the scheduler executes the tasks in each schedule group. When SchedulingProtocol is set to EnhanceScheduleGroupLocality (which is the default), the Task Scheduler chooses the next task from the schedule group that it is working on when the current task finishes or cooperatively yields. The Task Scheduler searches the current schedule group for work before it moves to the next available group. Conversely, when SchedulingProtocol is set to EnhanceForwardProgress, the scheduler moves to the next schedule group after each task finishes or yields. For an example that compares these policies, see How to: Use Schedule Groups to Influence Order of Execution.

The runtime uses the concurrency::ScheduleGroup class to represent schedule groups. To create a ScheduleGroup object, call the concurrency::CurrentScheduler::CreateScheduleGroup or concurrency::Scheduler::CreateScheduleGroup method. The runtime uses a reference-counting mechanism to control the lifetime of ScheduleGroup objects, just as it does with Scheduler objects. When you create a ScheduleGroup object, the runtime sets the reference counter to one. The concurrency::ScheduleGroup::Reference method increments the reference counter by one. The concurrency::ScheduleGroup::Release method decrements the reference counter by one.

Many types in the Concurrency Runtime let you associate an object together with a schedule group. For example, the concurrency::agent class and message block classes such as concurrency::unbounded_buffer, concurrency::join, and concurrency::timer, provide overloaded versions of the constructor that take a ScheduleGroup object. The runtime uses the Scheduler object that is associated with this ScheduleGroup object to schedule the task.

You can also use the concurrency::ScheduleGroup::ScheduleTask method to schedule a lightweight task. For more information about lightweight tasks, see Lightweight Tasks.


For an example that uses schedule groups to control the order of task execution, see How to: Use Schedule Groups to Influence Order of Execution.

See also

Task Scheduler
Scheduler Instances
How to: Use Schedule Groups to Influence Order of Execution