Implement NIO scheduler for ZIO

https://nurmohammed840.github.io/posts/announcing-nio/

/bounty $2500

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Attempt	Started (GMT+0)	Solution
🔴 @maq796113	Nov 30, 2024, 3:58:06 PM	WIP
🔴 @ahmedtariqkhan	Nov 30, 2024, 4:06:01 PM	WIP
🟢 @promisingcoder	Dec 14, 2024, 8:54:21 AM	WIP
🟢 @HollandDM	Dec 25, 2024, 10:57:44 AM	WIP
🔴 @SAIKIRANSURAPALLI	Dec 31, 2024, 7:32:13 AM	WIP
🟢 @abdelfetah18	Mar 10, 2025, 5:37:03 PM	#9684

/attempt #9356 Plan is going to be as straightforward as it could be, where ill be implementing NIO with respect to checking performances with new techniques.

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[!NOTE] The user @maq796113 is already attempting to complete issue #9356 and claim the bounty. We recommend checking in on @maq796113's progress, and potentially collaborating, before starting a new solution.

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/attempt #9356

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The bounty is up for grabs! Everyone is welcome to /attempt #9356 🙌

Hi @jdegoes @kyri-petrou I was going through the project, I didn't see any specific Spec file for ZScheduler, only RuntimeSpec and ExecutorSpec. If one implements a NIO-LLScheduler, how should they test the correctness? is there any expectation on that like how tests should be shaped?

/attempt #9356

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/attempt #9356

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/attempt #9356

Algora profile	Completed bounties	Tech	Active attempts	Options
@abdelfetah18	2 bounties from 1 project	Java, C++		Cancel attempt

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Is this still available?

/attempt #9356

/attempt #9356

Hi all, seems that the CI is failing on a step of action [Run re-actors/alls-green@release/v1](https://github.com/zio/zio/actions/runs/19160514157/job/54773148810#annotation:2:91) is this a known issue? Everything else (linter, localPublish and tests) pass correctly.

EDIT: issue with PR resolved, waiting for review

Hey! Really interesting challenge here. I've been looking into the Nio scheduler approach and think we can adapt something similar for ZIO.

/attempt #9356

So here's what I'm thinking - the Least-Loaded scheduler from Nio is pretty elegant compared to work-stealing. Instead of all that complex bookkeeping, we just track which worker has the least work and send new fibers there. Simple but effective.

My plan:

First, I need to understand how ZIO currently does scheduling. I'll dig into the Runtime internals and see where we can hook in. The main thing is figuring out how to measure "load" - probably queue size plus active fibers running on each worker.

Then build the core scheduler logic. The Rust version is only 150 lines, so the ZIO version shouldn't be too crazy either. Main challenge will be making it thread-safe without killing performance.

Testing is going to be key. Those Nio benchmarks looked promising - 3x better in some cases! I want to set up similar tests with ZIO workloads and see if we get comparable improvements.

I'm particularly curious about how this will handle ZIO's interruption model and whether we need any special handling for different fiber types.

Will start poking around the codebase this week and report back with what I find. If anyone has insights on the current scheduler internals or performance pain points they've seen, would love to hear them!