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Concurrency Runtime (CRT): The Task Scheduler

Dispatching Tasks

The Scheduler tries to search for work to execute. "Work" can be:

  • Unblocked context
  • Lightweight tasks
  • Tasks in work-stealing queues

Again, all these tasks are stored in queues managed by Schedule groups, and each group is managed by a scheduling ring.

When a virtual processor is allocated to the scheduler, a ThreadProxy class is created and associated to this processor. After the creation the Dispatch method of the ThreadProxy is invoked.

As shown in Figure 18, the CRT uses abstract classes to enforces low coupling, and the real dispatch invoked depends of the implementation chosen by the runtime -- this selection is made according to the scheduler policy.

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Figure 18

The concrete implementation of Dispatch invokes the Dispatch method of the execution context.

Figure 19 shows the methods invoked by concrete implementation of Context::Dispatch method:

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Figure 19

So the algorithm for searching for the next task to execute is implemented by the WorkSearchContext class. Let's uncover all the classes used directly by WorkSearchContext to achieve its goal:

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Figure 20

The responsibility of WorkSearchContext is to give us a WorkItem to execute; it could be InternalContextBase, RealizedChore, or _UnrealizedChore. To understand the collaboration between these classes, let's search for methods directly used by WorkSearchContext:

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Figure 21

The WorkSearchContext iterates on the SchedulingRing and ScheduleGroup classes via SchedulerBase methods. And for each ScheduleBase, you search for RunnableContext, realizedChore, or UnrealizedChore.

The WorkSearchContext class is created by the VirtualProcessor class, and as shown in Figure 22, the algorithm used is specified when the VirtualProcessor is initialized, and it asks the Scheduler for the SchedulingProtocol which describes the scheduling algorithm that will be used for the Scheduler.

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Figure 22

WorkSearchContext will discover the algorithm to use by passing it a value from Algorithm enum.

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Figure 23

So two algorithms are implemented by this class to find a task:

Cache Local algorithm. This algorithm will look for a runnable context within the current schedule group, then realized chores and unrealized chores. If there's no more work in the current schedule group, it looks for the next group in the same schedule ring. And when it finishes all works in the current schedule ring, it looks in the next schedule ring.

So the scheduler prefers to continue to work on tasks within the current schedule group before moving to another schedule group.

This algorithm is implemented by the WorkSearchContext::SearchCacheLocal method, and as shown by this dependency graph, this method search invoke other methods to search for runnable contexts, RealizedChore or _UnrealizedChore.

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Figure 24

Another specificity of this algorithm is that the unblocked contexts are cached per virtual-processor and are typically scheduled in a last-in first-out (LIFO) fashion by the virtual processor that unblocked them. To verify this behavior, here's a dependency graph of methods invoked when searching for runnable context:

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Figure 25

This algorithm is the default algorithm used by the Scheduler if one is not specified..

Fair algorithm. In this case the scheduler prefers to round-robin through schedule groups after executing each task. Unblocked contexts are typically scheduled in a first-in-first-out (FIFO) fashion. Virtual processors do not cache unblocked contexts.

This algorithm is implemented by the WorkSearchContext::SearchFair method, and as shown by this dependency graph, this method search invokes other methods to search for runnable contexts, RealizedChore or _UnrealizedChore.

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Figure 26

From this analysis, it is fair to say that the CRT Task Scheduler appears to be well-designed, is characterized by a high cohesion and very low coupling, is very flexible, and provides by default an optimized algorithm to exploit better hardware resources.

Lahlali Issam is the developer of CppDepend code analysis tool for C/C++.

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