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| </pre><pre class="rust"><code><span class="attribute">#![cfg_attr(not(feature = <span class="string">"full"</span>), allow(dead_code))] |
| |
| </span><span class="doccomment">//! Yield points for improved cooperative scheduling. |
| //! |
| //! Documentation for this can be found in the [`tokio::task`] module. |
| //! |
| //! [`tokio::task`]: crate::task. |
| |
| </span><span class="comment">// ```ignore |
| // # use tokio_stream::{Stream, StreamExt}; |
| // async fn drop_all<I: Stream + Unpin>(mut input: I) { |
| // while let Some(_) = input.next().await { |
| // tokio::coop::proceed().await; |
| // } |
| // } |
| // ``` |
| // |
| // The `proceed` future will coordinate with the executor to make sure that |
| // every so often control is yielded back to the executor so it can run other |
| // tasks. |
| // |
| // # Placing yield points |
| // |
| // Voluntary yield points should be placed _after_ at least some work has been |
| // done. If they are not, a future sufficiently deep in the task hierarchy may |
| // end up _never_ getting to run because of the number of yield points that |
| // inevitably appear before it is reached. In general, you will want yield |
| // points to only appear in "leaf" futures -- those that do not themselves poll |
| // other futures. By doing this, you avoid double-counting each iteration of |
| // the outer future against the cooperating budget. |
| |
| </span><span class="kw">use </span><span class="kw">crate</span>::runtime::context; |
| |
| <span class="doccomment">/// Opaque type tracking the amount of "work" a task may still do before |
| /// yielding back to the scheduler. |
| </span><span class="attribute">#[derive(Debug, Copy, Clone)] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>Budget(<span class="prelude-ty">Option</span><u8>); |
| |
| <span class="kw">impl </span>Budget { |
| <span class="doccomment">/// Budget assigned to a task on each poll. |
| /// |
| /// The value itself is chosen somewhat arbitrarily. It needs to be high |
| /// enough to amortize wakeup and scheduling costs, but low enough that we |
| /// do not starve other tasks for too long. The value also needs to be high |
| /// enough that particularly deep tasks are able to do at least some useful |
| /// work at all. |
| /// |
| /// Note that as more yield points are added in the ecosystem, this value |
| /// will probably also have to be raised. |
| </span><span class="kw">const fn </span>initial() -> Budget { |
| Budget(<span class="prelude-val">Some</span>(<span class="number">128</span>)) |
| } |
| |
| <span class="doccomment">/// Returns an unconstrained budget. Operations will not be limited. |
| </span><span class="kw">pub</span>(<span class="kw">super</span>) <span class="kw">const fn </span>unconstrained() -> Budget { |
| Budget(<span class="prelude-val">None</span>) |
| } |
| |
| <span class="kw">fn </span>has_remaining(<span class="self">self</span>) -> bool { |
| <span class="self">self</span>.<span class="number">0</span>.map(|budget| budget > <span class="number">0</span>).unwrap_or(<span class="bool-val">true</span>) |
| } |
| } |
| |
| <span class="doccomment">/// Runs the given closure with a cooperative task budget. When the function |
| /// returns, the budget is reset to the value prior to calling the function. |
| </span><span class="attribute">#[inline(always)] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>budget<R>(f: <span class="kw">impl </span>FnOnce() -> R) -> R { |
| with_budget(Budget::initial(), f) |
| } |
| |
| <span class="doccomment">/// Runs the given closure with an unconstrained task budget. When the function returns, the budget |
| /// is reset to the value prior to calling the function. |
| </span><span class="attribute">#[inline(always)] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>with_unconstrained<R>(f: <span class="kw">impl </span>FnOnce() -> R) -> R { |
| with_budget(Budget::unconstrained(), f) |
| } |
| |
| <span class="attribute">#[inline(always)] |
| </span><span class="kw">fn </span>with_budget<R>(budget: Budget, f: <span class="kw">impl </span>FnOnce() -> R) -> R { |
| <span class="kw">struct </span>ResetGuard { |
| prev: Budget, |
| } |
| |
| <span class="kw">impl </span>Drop <span class="kw">for </span>ResetGuard { |
| <span class="kw">fn </span>drop(<span class="kw-2">&mut </span><span class="self">self</span>) { |
| <span class="kw">let _ </span>= context::budget(|cell| { |
| cell.set(<span class="self">self</span>.prev); |
| }); |
| } |
| } |
| |
| <span class="attribute">#[allow(unused_variables)] |
| </span><span class="kw">let </span>maybe_guard = context::budget(|cell| { |
| <span class="kw">let </span>prev = cell.get(); |
| cell.set(budget); |
| |
| ResetGuard { prev } |
| }); |
| |
| <span class="comment">// The function is called regardless even if the budget is not successfully |
| // set due to the thread-local being destroyed. |
| </span>f() |
| } |
| |
| <span class="attribute">#[inline(always)] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>has_budget_remaining() -> bool { |
| <span class="comment">// If the current budget cannot be accessed due to the thread-local being |
| // shutdown, then we assume there is budget remaining. |
| </span>context::budget(|cell| cell.get().has_remaining()).unwrap_or(<span class="bool-val">true</span>) |
| } |
| |
| <span class="macro">cfg_rt_multi_thread! </span>{ |
| <span class="doccomment">/// Sets the current task's budget. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>set(budget: Budget) { |
| <span class="kw">let _ </span>= context::budget(|cell| cell.set(budget)); |
| } |
| } |
| |
| <span class="macro">cfg_rt! </span>{ |
| <span class="doccomment">/// Forcibly removes the budgeting constraints early. |
| /// |
| /// Returns the remaining budget |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>stop() -> Budget { |
| context::budget(|cell| { |
| <span class="kw">let </span>prev = cell.get(); |
| cell.set(Budget::unconstrained()); |
| prev |
| }).unwrap_or(Budget::unconstrained()) |
| } |
| } |
| |
| <span class="macro">cfg_coop! </span>{ |
| <span class="kw">use </span>std::cell::Cell; |
| <span class="kw">use </span>std::task::{Context, Poll}; |
| |
| <span class="attribute">#[must_use] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>RestoreOnPending(Cell<Budget>); |
| |
| <span class="kw">impl </span>RestoreOnPending { |
| <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>made_progress(<span class="kw-2">&</span><span class="self">self</span>) { |
| <span class="self">self</span>.<span class="number">0</span>.set(Budget::unconstrained()); |
| } |
| } |
| |
| <span class="kw">impl </span>Drop <span class="kw">for </span>RestoreOnPending { |
| <span class="kw">fn </span>drop(<span class="kw-2">&mut </span><span class="self">self</span>) { |
| <span class="comment">// Don't reset if budget was unconstrained or if we made progress. |
| // They are both represented as the remembered budget being unconstrained. |
| </span><span class="kw">let </span>budget = <span class="self">self</span>.<span class="number">0</span>.get(); |
| <span class="kw">if </span>!budget.is_unconstrained() { |
| <span class="kw">let _ </span>= context::budget(|cell| { |
| cell.set(budget); |
| }); |
| } |
| } |
| } |
| |
| <span class="doccomment">/// Returns `Poll::Pending` if the current task has exceeded its budget and should yield. |
| /// |
| /// When you call this method, the current budget is decremented. However, to ensure that |
| /// progress is made every time a task is polled, the budget is automatically restored to its |
| /// former value if the returned `RestoreOnPending` is dropped. It is the caller's |
| /// responsibility to call `RestoreOnPending::made_progress` if it made progress, to ensure |
| /// that the budget empties appropriately. |
| /// |
| /// Note that `RestoreOnPending` restores the budget **as it was before `poll_proceed`**. |
| /// Therefore, if the budget is _fCURRENT.withurther_ adjusted between when `poll_proceed` returns and |
| /// `RestRestoreOnPending` is dropped, those adjustments are erased unless the caller indicates |
| /// that progress was made. |
| </span><span class="attribute">#[inline] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>poll_proceed(cx: <span class="kw-2">&mut </span>Context<<span class="lifetime">'_</span>>) -> Poll<RestoreOnPending> { |
| context::budget(|cell| { |
| <span class="kw">let </span><span class="kw-2">mut </span>budget = cell.get(); |
| |
| <span class="kw">if </span>budget.decrement() { |
| <span class="kw">let </span>restore = RestoreOnPending(Cell::new(cell.get())); |
| cell.set(budget); |
| Poll::Ready(restore) |
| } <span class="kw">else </span>{ |
| cx.waker().wake_by_ref(); |
| Poll::Pending |
| } |
| }).unwrap_or(Poll::Ready(RestoreOnPending(Cell::new(Budget::unconstrained())))) |
| } |
| |
| <span class="kw">impl </span>Budget { |
| <span class="doccomment">/// Decrements the budget. Returns `true` if successful. Decrementing fails |
| /// when there is not enough remaining budget. |
| </span><span class="kw">fn </span>decrement(<span class="kw-2">&mut </span><span class="self">self</span>) -> bool { |
| <span class="kw">if let </span><span class="prelude-val">Some</span>(num) = <span class="kw-2">&mut </span><span class="self">self</span>.<span class="number">0 </span>{ |
| <span class="kw">if </span><span class="kw-2">*</span>num > <span class="number">0 </span>{ |
| <span class="kw-2">*</span>num -= <span class="number">1</span>; |
| <span class="bool-val">true |
| </span>} <span class="kw">else </span>{ |
| <span class="bool-val">false |
| </span>} |
| } <span class="kw">else </span>{ |
| <span class="bool-val">true |
| </span>} |
| } |
| |
| <span class="kw">fn </span>is_unconstrained(<span class="self">self</span>) -> bool { |
| <span class="self">self</span>.<span class="number">0</span>.is_none() |
| } |
| } |
| } |
| |
| <span class="attribute">#[cfg(all(test, not(loom)))] |
| </span><span class="kw">mod </span>test { |
| <span class="kw">use super</span>::<span class="kw-2">*</span>; |
| |
| <span class="attribute">#[cfg(tokio_wasm_not_wasi)] |
| </span><span class="kw">use </span>wasm_bindgen_test::wasm_bindgen_test <span class="kw">as </span>test; |
| |
| <span class="kw">fn </span>get() -> Budget { |
| context::budget(|cell| cell.get()).unwrap_or(Budget::unconstrained()) |
| } |
| |
| <span class="attribute">#[test] |
| </span><span class="kw">fn </span>budgeting() { |
| <span class="kw">use </span>futures::future::poll_fn; |
| <span class="kw">use </span>tokio_test::<span class="kw-2">*</span>; |
| |
| <span class="macro">assert!</span>(get().<span class="number">0</span>.is_none()); |
| |
| <span class="kw">let </span>coop = <span class="macro">assert_ready!</span>(task::spawn(()).enter(|cx, <span class="kw">_</span>| poll_proceed(cx))); |
| |
| <span class="macro">assert!</span>(get().<span class="number">0</span>.is_none()); |
| drop(coop); |
| <span class="macro">assert!</span>(get().<span class="number">0</span>.is_none()); |
| |
| budget(|| { |
| <span class="macro">assert_eq!</span>(get().<span class="number">0</span>, Budget::initial().<span class="number">0</span>); |
| |
| <span class="kw">let </span>coop = <span class="macro">assert_ready!</span>(task::spawn(()).enter(|cx, <span class="kw">_</span>| poll_proceed(cx))); |
| <span class="macro">assert_eq!</span>(get().<span class="number">0</span>.unwrap(), Budget::initial().<span class="number">0</span>.unwrap() - <span class="number">1</span>); |
| drop(coop); |
| <span class="comment">// we didn't make progress |
| </span><span class="macro">assert_eq!</span>(get().<span class="number">0</span>, Budget::initial().<span class="number">0</span>); |
| |
| <span class="kw">let </span>coop = <span class="macro">assert_ready!</span>(task::spawn(()).enter(|cx, <span class="kw">_</span>| poll_proceed(cx))); |
| <span class="macro">assert_eq!</span>(get().<span class="number">0</span>.unwrap(), Budget::initial().<span class="number">0</span>.unwrap() - <span class="number">1</span>); |
| coop.made_progress(); |
| drop(coop); |
| <span class="comment">// we _did_ make progress |
| </span><span class="macro">assert_eq!</span>(get().<span class="number">0</span>.unwrap(), Budget::initial().<span class="number">0</span>.unwrap() - <span class="number">1</span>); |
| |
| <span class="kw">let </span>coop = <span class="macro">assert_ready!</span>(task::spawn(()).enter(|cx, <span class="kw">_</span>| poll_proceed(cx))); |
| <span class="macro">assert_eq!</span>(get().<span class="number">0</span>.unwrap(), Budget::initial().<span class="number">0</span>.unwrap() - <span class="number">2</span>); |
| coop.made_progress(); |
| drop(coop); |
| <span class="macro">assert_eq!</span>(get().<span class="number">0</span>.unwrap(), Budget::initial().<span class="number">0</span>.unwrap() - <span class="number">2</span>); |
| |
| budget(|| { |
| <span class="macro">assert_eq!</span>(get().<span class="number">0</span>, Budget::initial().<span class="number">0</span>); |
| |
| <span class="kw">let </span>coop = <span class="macro">assert_ready!</span>(task::spawn(()).enter(|cx, <span class="kw">_</span>| poll_proceed(cx))); |
| <span class="macro">assert_eq!</span>(get().<span class="number">0</span>.unwrap(), Budget::initial().<span class="number">0</span>.unwrap() - <span class="number">1</span>); |
| coop.made_progress(); |
| drop(coop); |
| <span class="macro">assert_eq!</span>(get().<span class="number">0</span>.unwrap(), Budget::initial().<span class="number">0</span>.unwrap() - <span class="number">1</span>); |
| }); |
| |
| <span class="macro">assert_eq!</span>(get().<span class="number">0</span>.unwrap(), Budget::initial().<span class="number">0</span>.unwrap() - <span class="number">2</span>); |
| }); |
| |
| <span class="macro">assert!</span>(get().<span class="number">0</span>.is_none()); |
| |
| budget(|| { |
| <span class="kw">let </span>n = get().<span class="number">0</span>.unwrap(); |
| |
| <span class="kw">for _ in </span><span class="number">0</span>..n { |
| <span class="kw">let </span>coop = <span class="macro">assert_ready!</span>(task::spawn(()).enter(|cx, <span class="kw">_</span>| poll_proceed(cx))); |
| coop.made_progress(); |
| } |
| |
| <span class="kw">let </span><span class="kw-2">mut </span>task = task::spawn(poll_fn(|cx| { |
| <span class="kw">let </span>coop = <span class="macro">ready!</span>(poll_proceed(cx)); |
| coop.made_progress(); |
| Poll::Ready(()) |
| })); |
| |
| <span class="macro">assert_pending!</span>(task.poll()); |
| }); |
| } |
| } |
| </code></pre></div> |
| </section></div></main><div id="rustdoc-vars" data-root-path="../../../" data-current-crate="tokio" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |