Re: [PR] feat(broadcast): add unbounded MPMC channel [asyncband]

["tisonkun (via GitHub)" <gi...@apache.org>]

tisonkun commented on PR #117:
URL: https://github.com/apache/asyncband/pull/117#issuecomment-5433823406

   Benchmark note from an Apple M4 Max (`rustc 1.99.0-nightly`), using Divan medians and 4,096-message batches:
   
   | Workload | Asyncband | Tokio broadcast | async-broadcast |
   | --- | ---: | ---: | ---: |
   | try round trip | 37–42 ns | 15–19 ns | 22–26 ns |
   | 1 producer, then drain | 186–190 µs | 153–157 µs | 122–141 µs |
   | 8 producers, then drain | 411–451 µs | 420–451 µs | 289–320 µs |
   | fanout to 1 receiver | 181–187 µs | 154–162 µs | 117–126 µs |
   | fanout to 8 receivers | 1.39–1.43 ms | 0.20–0.23 ms | 0.81–0.86 ms |
   | fanout to 32 receivers | 4.44–4.74 ms | 0.44–0.61 ms | 2.32–2.37 ms |
   
   Tokio and async-broadcast are bounded, so the benchmark gives each enough capacity for the full batch and never exercises lag, overflow, or backpressure. This is a comparison of the common no-loss hot path, not of equivalent retention policies.
   
   The one-message reclaim change reduced Asyncband's ready round trip from roughly 54 ns to roughly 38–42 ns and improved the single-producer batch by about 25%. Producer contention is now in the same range as Tokio by 4–8 producers and remains within roughly 1.3–1.6x of async-broadcast.
   
   High fanout remains the visible scaling limit. Asyncband stays around 1.9x behind the closer lock-based async-broadcast peer, while Tokio's preallocated ring and per-slot bookkeeping can be 7–10x faster at 32 receivers. Closing that Tokio gap would require changing the receiver/reclamation algorithm rather than another local cleanup, so I have left that tradeoff visible for review instead of hiding it behind a larger redesign in this PR.
   


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