This document describes the role of contexts in the Concurrency Runtime. A thread that is attached to a scheduler is known as an execution context, or just context. The concurrency::wait function and the concurrency::Context class enable you to control the behavior of contexts. Use the wait function to suspend the current context for a specified time. Use the Context class when you need more control over when contexts block, unblock, and yield, or when you want to oversubscribe the current context.
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.
The wait Function
The concurrency::wait function cooperatively yields the execution of the current context for a specified number of milliseconds. The runtime uses the yield time to perform other tasks. After the specified time has elapsed, the runtime reschedules the context for execution. Therefore, the wait function might suspend the current context longer than the value provided for the milliseconds parameter.
Passing 0 (zero) for the milliseconds parameter causes the runtime to suspend the current context until all other active contexts are given the opportunity to perform work. This lets you yield a task to all other active tasks.
For an example that uses the wait function to yield the current context, and thus allow for other contexts to run, see How to: Use Schedule Groups to Influence Order of Execution.
The Context Class
The concurrency::Context class provides a programming abstraction for an execution context and offers two important features: the ability to cooperatively block, unblock, and yield the current context, and the ability to oversubscribe the current context.
The Context class lets you block or yield the current execution context. Blocking or yielding is useful when the current context cannot continue because a resource is not available.
The concurrency::Context::Block method blocks the current context. A context that is blocked yields its processing resources so that the runtime can perform other tasks. The concurrency::Context::Unblock method unblocks a blocked context. The Context::Unblock method must be called from a different context than the one that called Context::Block. The runtime throws concurrency::context_self_unblock if a context attempts to unblock itself.
To cooperatively block and unblock a context, you typically call concurrency::Context::CurrentContext to retrieve a pointer to the Context object that is associated with the current thread and save the result. You then call the Context::Block method to block the current context. Later, call Context::Unblock from a separate context to unblock the blocked context.
You must match each pair of calls to Context::Block and Context::Unblock. The runtime throws concurrency::context_unblock_unbalanced when the Context::Block or Context::Unblock method is called consecutively without a matching call to the other method. However, you do not have to call Context::Block before you call Context::Unblock. For example, if one context calls Context::Unblock before another context calls Context::Block for the same context, that context remains unblocked.
The concurrency::Context::Yield method yields execution so that the runtime can perform other tasks and then reschedule the context for execution. When you call the Context::Block method, the runtime does not reschedule the context.
For an example that uses the Context::Block, Context::Unblock, and Context::Yield methods to implement a cooperative semaphore class, see How to: Use the Context Class to Implement a Cooperative Semaphore.
The default scheduler creates the same number of threads as there are available hardware threads. You can use oversubscription to create additional threads for a given hardware thread.
For computationally intensive operations, oversubscription typically does not scale because it introduces additional overhead. However, for tasks that have a high amount of latency, for example, reading data from disk or from a network connection, oversubscription can improve the overall efficiency of some applications.
Enable oversubscription only from a thread that was created by the Concurrency Runtime. Oversubscription has no effect when it is called from a thread that was not created by the runtime (including the main thread).
To enable oversubscription in the current context, call the concurrency::Context::Oversubscribe method with the _BeginOversubscription parameter set to true. When you enable oversubscription on a thread that was created by the Concurrency Runtime, it causes the runtime to create one additional thread. After all tasks that require oversubscription finish, call Context::Oversubscribe with the _BeginOversubscription parameter set to false.
You can enable oversubscription multiple times from the current context, but you must disable it the same number of times that you enable it. Oversubscription can also be nested; that is, a task that is created by another task that uses oversubscription can also oversubscribe its context. However, if both a nested task and its parent belong to the same context, only the outermost call to Context::Oversubscribe causes the creation of an additional thread.
The runtime throws concurrency::invalid_oversubscribe_operation if oversubscription is disabled before it is enabled.
For an example that uses oversubscription to offset the latency that is caused by reading data from a network connection, see How to: Use Oversubscription to Offset Latency.