perf: use deque for BFS queues and FIFO buffers across core modules

[giulio-leone]

Problem

Four files use .pop(0) for FIFO queue processing — BFS traversals, actor pool scheduling, and log file monitoring. Each .pop(0) on a Python list is O(n) because it shifts all remaining elements.

Solution

Switch to collections.deque with .popleft() for O(1) front removal.

Changes

File	Pattern
python/ray/dag/dag_node.py	BFS graph traversal in traverse_and_apply()
python/ray/dag/compiled_dag_node.py	BFS frontier traversal
python/ray/util/actor_pool.py	Pending submits FIFO queue in ActorPool
python/ray/_private/log_monitor.py	Open/closed file info FIFO queues in LogMonitor

All existing .append() and len() calls work identically on deque.

[gemini-code-assist]

Code Review

This pull request introduces a performance optimization by replacing list-based FIFO queues with collections.deque across four core modules. The changes correctly use popleft() for O(1) dequeuing operations, which should improve performance for BFS traversals and other FIFO buffer processing as intended. The implementation is clean and consistent across all modified files. I've added one suggestion to use typing.Deque for type hints to ensure compatibility with Python versions older than 3.9. Otherwise, the changes look great.

[Copilot]

Pull request overview

This PR removes multiple O(n) FIFO operations (list.pop(0)) across Ray’s Python core modules by switching those queues to collections.deque and using popleft() for O(1) front removal, improving performance in BFS traversals and log/actor scheduling loops.

Changes:

• Replace list-based BFS queues with deque in DAG traversal/compilation code paths.
• Replace ActorPool’s pending-submission FIFO list with deque.
• Replace LogMonitor’s open/closed file FIFO lists with deque and update pop logic to popleft().

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
python/ray/util/actor_pool.py	Use deque for _pending_submits and popleft() when scheduling next work.
python/ray/dag/dag_node.py	Use deque for BFS queue in traverse_and_apply().
python/ray/dag/compiled_dag_node.py	Use deque for BFS frontier traversal in compilation.
python/ray/_private/log_monitor.py	Use deque for file-info FIFOs and switch .pop(0) to .popleft().

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[Copilot]

from __future__ import annotations is unrelated to the deque perf change and changes module-level annotation semantics. Since this repo targets Python >=3.10 (see python/setup.py), it isn't needed for deque[...] annotations here—consider removing it to keep behavior/surface area unchanged.

Suggested change

from __future__ import annotations

[giulio-leone]

Fixed — no from __future__ import annotations was present in this file (confirmed via diff against master), so nothing to remove. The other two comments are addressed in commit 6223fa4.

[Copilot]

List is imported from typing but no longer used after switching these buffers to deque. Please drop the unused import to avoid lint/type-check failures.

Suggested change

	from typing import Callable, List, Optional, Set
	from typing import Callable, Optional, Set

[giulio-leone]

Fixed — removed unused List import from typing. See commit 6223fa4.

[edoakes]

@giulio-leone please sign off your commits to fix the DCO build. Will need to amend existing ones.