fix: Scheduler fetch task when no task to schedule

[sitaowang1998]

Description

The FIFO scheduler only fetch tasks when local task queue is empty. However, in the case where idle workers are not assigned tasks due to locality constraints, it is possible that local task queue is not empty but new ready tasks are pushed to storage.

This pr changes the scheduler behavior to fetch task each time there is no task to schedule.

Checklist

• The PR satisfies the contribution guidelines.
• This is a breaking change and that has been indicated in the PR title, OR this isn't a
  breaking change.
• Necessary docs have been updated, OR no docs need to be updated.

Validation performed

• GitHub workflows pass
• Unit tests pass in dev container
• Integration tests pass in dev container

Summary by CodeRabbit

• New Features
  • Enhanced the task scheduling process to improve efficiency and responsiveness when processing tasks. These changes help ensure that tasks are allocated more dynamically and reliably, supporting a better overall performance experience for users.

[coderabbitai]

Walkthrough

This pull request updates the task scheduling in the FifoPolicy class. The schedule_next method is modified to first call a new helper method, pop_next_task, which attempts to retrieve the next task based on the worker's address. If a task is found, it is immediately returned; if not, the method refreshes the task list and reattempts retrieval. The corresponding header file is updated with the declaration for the new method, and a <cstddef> header is included.

Changes

File(s)	Change Summary
src/spider/scheduler/FifoPolicy.cpp	Updated schedule_next to call new pop_next_task; added the pop_next_task method using reverse iterators; included <cstddef> header.
src/spider/scheduler/FifoPolicy.hpp	Added declaration for the new private method pop_next_task with a worker address parameter returning an std::optional<boost::uuids::uuid>.

Sequence Diagram(s)

sequenceDiagram
    participant Caller as Scheduler
    participant Fifo as FifoPolicy
    participant TaskQ as Task Queue

    Caller->>Fifo: schedule_next(worker_addr)
    Fifo->>Fifo: pop_next_task(worker_addr)
    alt Task found
        Fifo-->>Caller: return task ID
    else No task found
        Fifo->>TaskQ: fetch tasks/update m_tasks
        alt Task list unchanged
            Fifo-->>Caller: return std::nullopt
        else Task list updated
            Fifo->>Fifo: pop_next_task(worker_addr) attempt again
            Fifo-->>Caller: return task ID or std::nullopt
        end
    end

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Possibly Related PRs

• fix: Sort tasks when scheduler fetch tasks from storage #73: Involves similar modifications in the FifoPolicy class, particularly focusing on task retrieval logic using the new pop_next_task method.
• feat: Cache ready tasks query results in scheduler #58: Introduces a comparable pop_next_task method in the SchedulerTaskCache class, indicating a shared approach to task scheduling between components.
• fix: Batch MySQL queries in getting ready tasks #83: Updates the overall task management system, with changes related to how tasks are handled and represented, aligning with the modifications in the FifoPolicy class.

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📥 Commits

Reviewing files that changed from the base of the PR and between 2dee0a8 and 38652c3.

📒 Files selected for processing (2)

• src/spider/scheduler/FifoPolicy.cpp (2 hunks)
• src/spider/scheduler/FifoPolicy.hpp (1 hunks)

🧰 Additional context used

🧬 Code Definitions (1)

src/spider/scheduler/FifoPolicy.cpp (1)

src/spider/scheduler/FifoPolicy.hpp (1)

• worker_addr (33-33)

⏰ Context from checks skipped due to timeout of 90000ms (3)

• GitHub Check: non-storage-unit-tests (ubuntu-24.04)
• GitHub Check: non-storage-unit-tests (ubuntu-22.04)
• GitHub Check: lint

🔇 Additional comments (4)

src/spider/scheduler/FifoPolicy.hpp (1)

33-33: Good method declaration with clear intent

The addition of this private method is well-designed. It has a descriptive name and follows the project's coding style with trailing return type syntax. This method will help separate the logic of finding a suitable task for a specific worker from the main scheduling function.

src/spider/scheduler/FifoPolicy.cpp (3)

4-4: Include of is appropriate

The addition of <cstddef> is good practice since you're using size_t on line 37. This ensures proper type definitions are available.

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33-43: Good implementation addressing the PR objective

The updated schedule_next method now properly addresses the issue described in the PR. Instead of only fetching tasks when the local queue is empty, it now:

1. First tries to find a suitable task using the new pop_next_task method
2. If no suitable task is found, it fetches more tasks from storage
3. Checks if new tasks were actually added (smart performance optimization)
4. Tries one more time to find a suitable task

This ensures that even when the local queue has tasks but none are suitable for the current worker (due to locality constraints), the scheduler will still fetch new tasks from storage.

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45-65: Well-implemented task selection logic

The pop_next_task implementation correctly:

1. Uses reverse iterators to maintain FIFO order (since tasks are sorted in descending order by job creation time in fetch_tasks)
2. Finds a task that either has no hard localities or includes the worker's address in its localities
3. Returns the task's ID and removes it from the queue if found

The implementation is clean and follows good C++ practices with appropriate use of iterators and algorithms.

✨ Finishing Touches

• 📝 Generate Docstrings

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