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task: add consume_budget for cooperative scheduling
For cpu-only computations that do not use any Tokio resources, budgeting does not really kick in in order to yield and prevent other tasks from starvation. The new mechanism - consume_budget, performs a budget check, consumes a unit of it, and yields only if the task exceeded the budget. That allows cpu-intenstive computations to define points in the program which indicate that some significant work was performed. It will yield only if the budget is gone, which is a much better alternative to unconditional yielding, which is a potentially heavy operation.
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use std::task::Poll; | ||
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/// Consumes a unit of budget and returns the execution back to the Tokio | ||
/// runtime *if* the task's coop budget was exhausted. | ||
/// | ||
/// The task will only yield if its entire coop budget has been exhausted. | ||
/// This function can can be used in order to insert optional yield points into long | ||
/// computations that do not use Tokio resources like sockets or semaphores, | ||
/// without redundantly yielding to the runtime each time. | ||
/// | ||
/// **Note**: This is an [unstable API][unstable]. The public API of this type | ||
/// may break in 1.x releases. See [the documentation on unstable | ||
/// features][unstable] for details. | ||
/// | ||
/// # Examples | ||
/// | ||
/// Make sure that a function which returns a sum of (potentially lots of) | ||
/// iterated values is cooperative. | ||
/// | ||
/// ``` | ||
/// async fn sum_iterator(input: &mut impl std::iter::Iterator<Item=i64>) -> i64 { | ||
/// let mut sum: i64 = 0; | ||
/// while let Some(i) = input.next() { | ||
/// sum += i; | ||
/// tokio::task::consume_budget().await | ||
/// } | ||
/// sum | ||
/// } | ||
/// ``` | ||
/// [unstable]: crate#unstable-features | ||
#[cfg_attr(docsrs, doc(cfg(feature = "rt")))] | ||
pub async fn consume_budget() { | ||
let mut status = Poll::Pending; | ||
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crate::future::poll_fn(move |cx| { | ||
if status.is_ready() { | ||
return status; | ||
} | ||
status = crate::coop::poll_proceed(cx).map(|restore| { | ||
restore.made_progress(); | ||
}); | ||
status | ||
}) | ||
.await | ||
} |
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