Feature #20876: Introduce `Fiber::Scheduler#blocking_operation_wait` to avoid stalling the event loop. - Ruby master - Ruby Issue Tracking System

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Feature #20876

closed

Introduce `Fiber::Scheduler#blocking_operation_wait` to avoid stalling the event loop.

Added by ioquatix (Samuel Williams) 16 days ago. Updated 2 days ago.

Status:

Closed

Assignee:

Target version:

[ruby-core:119772]

Description

This is an evolution of the previous proposal: https://bugs.ruby-lang.org/issues/20855

Background¶

The current Fiber Scheduler performance can be significantly impacted by blocking operations that cannot be deferred to the event loop, particularly in high-concurrency environments where Fibers rely on non-blocking operations for efficient task execution.

Proposal¶

Pull Request: https://github.com/ruby/ruby/pull/12016

We will introduce a new fiber scheduler hook called blocking_operation_work:

class MySchduler
  # ...
  def blocking_operation_wait(work)
    # Example (trivial) implementation:
    Thread.new(&work).join
  end
end

We introduce a new flag for rb_nogvl: RB_NOGVL_OFFLOAD_SAFE which indicates that rb_nogvl(func, ...) is a blocking operation that is safe to execute on a different thread or thread pool (or some other context).

When a C extension invokes rb_nogvl(..., RB_NOGVL_OFFLOAD_SAFE), and a fiber scheduler is available, all the arguments will be saved into a instance of a callable object (at this time a Proc) called work and passed to the blocking_operation_wait fiber scheduler hook. When work is #called, it will execute rb_nogvl again with all the same arguments.

The fiber scheduler can decide how to execute that work, e.g. on a separate thread or thread pool, to mitigate the performance impact of the blocking operation on the event loop.

Cancellation¶

rb_nogvl takes several arguments, a func for the actual work, and unblock_func to cancel func if possible. These arguments are preserved in the work proc, and cancellation works the same. However, some extra effort may be required in the fiber scheduler hook, e.g.

class MySchduler
  # ...
  def blocking_operation_wait(work)
    thread = Thread.new(&work)

    thread.join
    thread = nil
  ensure
    thread&.kill
  end
end

Example¶

Using the branch of async gem: https://github.com/socketry/async/pull/352/files and enabling zlib deflate to use this feature, the following performance improvement was achieved:

require "zlib"
require "async"
require "benchmark"

DATA = Random.new.bytes(1024*1024*100)

duration = Benchmark.measure do
  Async do
    10.times do
      Async do
        Zlib.deflate(DATA)
      end
    end
  end
end

# Ruby 3.3.4: ~16 seconds
# Ruby 3.4.0 + PR: ~2 seconds.