westonpace opened a new pull request #10258:
URL: https://github.com/apache/arrow/pull/10258
Previously a future's callbacks would always run synchronously, either as
part of `Future::MarkFinished` or as part of `Future::AddCallback`.
`Executor::Transfer` made it possible to schedule continuations on a new thread
but it would only take effect if the transferred future's callbacks were added
before the source future finished. There are times when the desired behavior
is to spawn a new thread task even if the source future is finished already.
This PR adds four scheduling options:
* Never - The default (and existing) behavior, never spawn a new task
* IfUnfinished - The default behavior for transfers, spawn a new task only
if the future isn't already finished
* IfIdle - A new behavior, spawn a new task as long as there is a core in
the destination thread pool that can process it
* Always - Always spawn a new task, on both finished and unfinished futures,
regardless of destination thread pool idleness.
This PR also adds a best effort "IsIdle" mechanism to the thread pool. It
is best effort because logic like...
```
if (pool->IsIdle()) {
pool->Submit(...);
}
```
...could still end up over-subscribing if tasks were added after the
`IsIdle` check and before the `Submit` check. However, this should be ok. An
alternative would be to do something like `TrySpawn` but that seemed like more
complexity than needed at the moment.
Leaving in draft until ARROW-12004 is merged as this one is built on top of
that one.
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