carols10cents commented on code in PR #15030:
URL: https://github.com/apache/datafusion/pull/15030#discussion_r1983660228
##########
datafusion/physical-plan/src/execution_plan.rs:
##########
@@ -260,13 +260,30 @@ pub trait ExecutionPlan: Debug + DisplayAs + Send + Sync {
/// used.
/// Thus, [`spawn`] is disallowed, and instead use [`SpawnedTask`].
///
+ /// To enable timely cancellation, the [`Stream`] that is returned must not
+ /// pin the CPU and must yield back to the tokio runtime regularly. This
can
+ /// be achieved by manually returning [`Poll::Pending`] in regular
intervals,
+ /// or the use of [`tokio::task::yield_now()`]. Cooperative scheduling may
also
+ /// be a way to achieve this goal, as [tokio support for it
improves][coop].
+ /// Determination for "regularly" may be made using a timer (being careful
with
+ /// the overhead-heavy syscall needed to take the time) or by counting
rows or
+ /// batches.
+ ///
+ /// The goal is for `datafusion`-provided operator implementation to
+ /// strive for [the guideline of not spending a long time without reaching
Review Comment:
I'm not sure if I'm understanding what you're saying correctly-- but I
believe datafusion _does_ need this as demonstrated by [the cancellation
benchmarks I recently added](https://github.com/apache/datafusion/pull/14818)
that show there's at least one case where it takes 32ms (on @alamb's machine)
to cancel/drop the runtime because there are operations that aren't yielding
often enough (the guidelines I've seen suggest aiming for 1ms of work between
yield points).
There are also issues such as
https://github.com/apache/datafusion/issues/14036 where queries aren't able to
be cancelled, and the root cause also appears to be not yielding often enough.
Could you elaborate on why you _don't_ think datafusion needs manual yields,
based on the behavior of the benchmarks and uncancellable query issues?
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