[ruby-core:119646] [Ruby master Feature#20855] Introduce `Fiber::Scheduler#blocking_region` to avoid stalling the event loop.

Issue #20855 has been reported by ioquatix (Samuel Williams).

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Feature #20855: Introduce `Fiber::Scheduler#blocking_region` to avoid stalling the event loop.
https://bugs.ruby-lang.org/issues/20855

* Author: ioquatix (Samuel Williams)
* Status: Open
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The fiber scheduler performance can be negatively affected when performing blocking operations that cannot be deferred to the event loopThe 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 IO for efficient task execution.

## Problem Description

Fibers in Ruby are designed to improve performance and responsiveness by allowing concurrent tasks to proceed without blocking one another. However, certain operations inherently block the Fiber Scheduler, leading to delayed execution across other Fibers. When blocking operations are inevitable, such as system or CPU bound operations without event-loop support, they create bottlenecks that degrade the Scheduler's performance.

## Proposed Solution

The proposed solution in PR #11963 introduces a blocking_region hook in the Fiber Scheduler to improve handling of blocking operations. This addition allows code that releases the GVL (Global VM Lock) code to be lifted out of the event loop, reducing the performance impact on the Scheduler during blocking calls. By isolating these operations from the primary event loop, this enhancement aims to improve Fiber-based concurrency, especially in cases where non-deferrable blocking operations would otherwise delay the Fiber Scheduler.

The new Fiber Scheduler hook blocking_region is designed to accept an opaque callable object, which encapsulates work that can be offloaded to a thread pool for execution. By isolating these blocking tasks from the main event loop, this hook allows the Scheduler to maintain better responsiveness. This addition offers an efficient way to handle tasks that cannot run asynchronously within the event loop itself, leveraging multi-threading where necessary to keep Fibers running smoothly and minimize blocking impact.



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