Scheduler

The scheduler interface is used to intercept blocking operations. A typical implementation would be a wrapper for a gem like EventMachine or Async. This design provides separation of concerns between the event loop implementation and application code. It also allows for layered schedulers which can perform instrumentation.

Interface

This is the interface you need to implement.

class Scheduler
  # Wait for the given file descriptor to match the specified events within
  # the specified timeout.
  # @parameter event [Integer] A bit mask of `IO::READABLE`,
  #   `IO::WRITABLE` and `IO::PRIORITY`.
  # @parameter timeout [Numeric] The amount of time to wait for the event in seconds.
  # @returns [Integer] The subset of events that are ready.
  def io_wait(io, events, timeout)
  end

  # Sleep the current task for the specified duration, or forever if not
  # specified.
  # @param duration [Numeric] The amount of time to sleep in seconds.
  def kernel_sleep(duration = nil)
  end

  # Block the calling fiber.
  # @parameter blocker [Object] What we are waiting on, informational only.
  # @parameter timeout [Numeric | Nil] The amount of time to wait for in seconds.
  # @returns [Boolean] Whether the blocking operation was successful or not.
  def block(blocker, timeout = nil)
  end

  # Unblock the specified fiber.
  # @parameter blocker [Object] What we are waiting on, informational only.
  # @parameter fiber [Fiber] The fiber to unblock.
  # @reentrant Thread safe.
  def unblock(blocker, fiber)
  end

  # Intercept the creation of a non-blocking fiber.
  # @returns [Fiber]
  def fiber(&block)
    Fiber.new(blocking: false, &block)
  end

  # Invoked when the thread exits.
  def close
    self.run
  end

  def run
    # Implement event loop here.
  end
end

Additional hooks may be introduced in the future, we will use feature detection in order to enable these hooks.

Non-blocking Execution

The scheduler hooks will only be used in special non-blocking execution contexts. Non-blocking execution contexts introduce non-determinism because the execution of scheduler hooks may introduce context switching points into your program.

Fibers

Fibers can be used to create non-blocking execution contexts.

Fiber.new do
  puts Fiber.current.blocking? # false

  # May invoke `Fiber.scheduler&.io_wait`.
  io.read(...)

  # May invoke `Fiber.scheduler&.io_wait`.
  io.write(...)

  # Will invoke `Fiber.scheduler&.kernel_sleep`.
  sleep(n)
end.resume

We also introduce a new method which simplifies the creation of these non-blocking fibers:

Fiber.schedule do
  puts Fiber.current.blocking? # false
end

The purpose of this method is to allow the scheduler to internally decide the policy for when to start the fiber, and whether to use symmetric or asymmetric fibers.

IO

By default, I/O is non-blocking. Not all operating systems support non-blocking I/O. Windows is a notable example where socket I/O can be non-blocking but pipe I/O is blocking. Provided that there is a scheduler and the current thread is non-blocking, the operation will invoke the scheduler.

Mutex

The Mutex class can be used in a non-blocking context and is fiber specific.

ConditionVariable

The ConditionVariable class can be used in a non-blocking context and is fiber-specific.

Queue / SizedQueue

The Queue and SizedQueue classses can be used in a non-blocking context and are fiber-specific.

Thread

The Thread#join operation can be used in a non-blocking context and is fiber-specific.

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