crepererum commented on code in PR #14028:
URL: https://github.com/apache/datafusion/pull/14028#discussion_r1905787146
##########
datafusion/core/src/datasource/physical_plan/file_stream.rs:
##########
@@ -478,7 +478,12 @@ impl<F: FileOpener> FileStream<F> {
reader,
)),
partition_values,
- }
+ };
+ // Return control to the runtime
when we're ready to open the next file
+ // to prevent uncancellable
queries in scenarios with many large files.
+ // This functions similarly to a
`tokio::task::yield_now()`.
+ cx.waker().wake_by_ref();
+ return Poll::Pending;
Review Comment:
For the multi-thread RT (which is the only thing we really care about, I
think it's NOT instantly calling `wake_by_ref` instantly:
1. `yield_now`:
https://github.com/tokio-rs/tokio/blob/bd3e8577377a2b684b50fc0cb50d98f03ad09703/tokio/src/task/yield_now.rs#L57
2. `context::defer`:
https://github.com/tokio-rs/tokio/blob/bd3e8577377a2b684b50fc0cb50d98f03ad09703/tokio/src/runtime/context.rs#L166-L177
3. `with_scheduler`:
https://github.com/tokio-rs/tokio/blob/bd3e8577377a2b684b50fc0cb50d98f03ad09703/tokio/src/runtime/context.rs#L183-L195
4. (via some indirection) `Context::defer`:
https://github.com/tokio-rs/tokio/blob/bd3e8577377a2b684b50fc0cb50d98f03ad09703/tokio/src/runtime/scheduler/mod.rs#L287-L289
5. (via some indirection) `Context::defer` (different `Context` this time):
https://github.com/tokio-rs/tokio/blob/bd3e8577377a2b684b50fc0cb50d98f03ad09703/tokio/src/runtime/scheduler/multi_thread/worker.rs#L766-L768
6. `Defer::defer`:
https://github.com/tokio-rs/tokio/blob/bd3e8577377a2b684b50fc0cb50d98f03ad09703/tokio/src/runtime/scheduler/defer.rs#L15-L26
7. The deferred threads are woken in two places
([1](https://github.com/tokio-rs/tokio/blob/bd3e8577377a2b684b50fc0cb50d98f03ad09703/tokio/src/runtime/scheduler/multi_thread/worker.rs#L513-L516),
[2](https://github.com/tokio-rs/tokio/blob/bd3e8577377a2b684b50fc0cb50d98f03ad09703/tokio/src/runtime/scheduler/multi_thread/worker.rs#L751))
for which the 2nd one seems relevant. The actual waking will happen when the
worker is -- I think -- unparked after a "park timeout". This is definitely
long after `Pending` is returned, because the thread that is returning
`Pending` is the very same worker, i.e. it cannot possibly be parked at this
point in time.
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