rt: batch pop from injection queue when idle

[carllerche]

In the multi-threaded scheduler, when no tasks are on the local queue, a worker will attempt to pull tasks from the injection queue. Previously, the worker would only attempt to poll one task from the injection queue and then continue trying to find work from other sources. This can result in the injection queue backing up when many tasks are scheduled outside the runtime.

This patch updates the worker to try to poll more than one task from the injection queue when it has no more local work. Note that we also don't want a single worker to poll all tasks on the injection queue, as that would result in work becoming unbalanced.

Benchmarks:

I added some new benchmarks. All benchmark results are the same except for spawn_many_remote_busy

Before:

test spawn_many_remote_busy ... bench: 10,140,229 ns/iter (+/- 1,142,091)

After:

test spawn_many_remote_busy ... bench: 8,143,800 ns/iter (+/- 426,671)

[Noah-Kennedy]

This will be fun to experiment with.

[Noah-Kennedy]

I'm specifically looking at this PR from a performance perspective and leaving the review of things like the atomic ops to others.

[hawkw]

Overall, this looks good to me. I had some minor style suggestions.

[hawkw]

style nit: why not just

Suggested change

	let maybe_task = self.next_local_task();
	if maybe_task.is_some() {
	return maybe_task;
	}
	if let task @ Some(_) = self.next_local_task() {
	return task;
	}

[carllerche]

Honestly, I'm not too fond of that style. I find @ syntax there harder to read

[hawkw]

it could also be:

Suggested change

	let maybe_task = self.next_local_task();
	if maybe_task.is_some() {
	return maybe_task;
	}
	if let Some(task) = self.next_local_task() {
	return Some(task);
	}

this isn't super important to me, but the reason i'm commenting on it is that the current code had me searching for the place where maybe_task was referenced again outside of the if, but it never actually is referenced again. not a big deal, but i found that slightly weird. 🤷‍♀️

[carllerche]

@Darksonn let me know if you are good w/ it