bench(engine): single-producer vs multi-producer WorkQueue overhead (#1572)#4209
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…1572) Adds `crates/engine/benches/sp_vs_mp_workqueue.rs` with two Criterion groups that both move 100K items through the concurrent delta work queue: - `sp/1p/100k`: one producer thread pushes 100K pre-allocated DeltaWork items via the default-build `Send + !Clone` WorkQueueSender path. - `mp/4p/100k`: four producer threads each push 25K pre-allocated items via the gated `Clone` impl on WorkQueueSender. Compiled only when `--features multi-producer` is set. Both groups report `Throughput::Elements(100_000)` so items/sec figures compare directly regardless of how the work is split. Inputs are pre-allocated outside the timed section via `iter_batched`, matching the discipline used by the parallel_dispatch_overhead bench. The MP group is feature-gated behind the engine crate's existing `multi-producer` feature - no Cargo.toml dependency change is required; the gate already exists at `crates/engine/Cargo.toml:91`. The top-of- file documentation cross-references the audit at `docs/audits/workqueue-sender-multi-producer-audit.md` (PR #4173), #4203 sync_channel bench, and #4206 parallel_dispatch_overhead bench, and spells out the decision criteria (>=15% SP-vs-MP delta) that this bench informs.
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…#4212) Document the four CAPACITY_MULTIPLIER sites and the two duplicate hard-coded `2`s in delta_pipeline.rs, justify each against the recent dispatch benches (#4203 channel overhead, #4204 reorder memory, #4206 dispatch decomposition, #4209 sp vs mp), and recommend keeping the default at 2 with one follow-up bench specified to challenge it.
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…1405) (#4224) Replaces the prior multi-root-focused note at the same path with a focused #1405 design discussion for parallel generator fan-in. Surveys the current SP / MP shape (feature-gated Clone at work_queue/multi_producer.rs), the candidate use cases, the three adjacent designs (Clone via #1569, Arc-wrap via #1610, explicit producer-vector constructor), and the ordering, capacity, and bench evidence that would have to land before flipping the default. Recommends keeping WorkQueueSender Send + !Clone in default builds and the multi-producer cargo feature opt-in. Documents the cost so reviewers can reject naive MP refactors. Promotion to default-on is gated on PR #4209 SP-vs-MP throughput parity, PR #4214 drain bench showing no regression, and an actual fan-in caller materialising.
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…#4212) Document the four CAPACITY_MULTIPLIER sites and the two duplicate hard-coded `2`s in delta_pipeline.rs, justify each against the recent dispatch benches (#4203 channel overhead, #4204 reorder memory, #4206 dispatch decomposition, #4209 sp vs mp), and recommend keeping the default at 2 with one follow-up bench specified to challenge it.
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…1572) (#4209) Adds `crates/engine/benches/sp_vs_mp_workqueue.rs` with two Criterion groups that both move 100K items through the concurrent delta work queue: - `sp/1p/100k`: one producer thread pushes 100K pre-allocated DeltaWork items via the default-build `Send + !Clone` WorkQueueSender path. - `mp/4p/100k`: four producer threads each push 25K pre-allocated items via the gated `Clone` impl on WorkQueueSender. Compiled only when `--features multi-producer` is set. Both groups report `Throughput::Elements(100_000)` so items/sec figures compare directly regardless of how the work is split. Inputs are pre-allocated outside the timed section via `iter_batched`, matching the discipline used by the parallel_dispatch_overhead bench. The MP group is feature-gated behind the engine crate's existing `multi-producer` feature - no Cargo.toml dependency change is required; the gate already exists at `crates/engine/Cargo.toml:91`. The top-of- file documentation cross-references the audit at `docs/audits/workqueue-sender-multi-producer-audit.md` (PR #4173), #4203 sync_channel bench, and #4206 parallel_dispatch_overhead bench, and spells out the decision criteria (>=15% SP-vs-MP delta) that this bench informs.
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…1405) (#4224) Replaces the prior multi-root-focused note at the same path with a focused #1405 design discussion for parallel generator fan-in. Surveys the current SP / MP shape (feature-gated Clone at work_queue/multi_producer.rs), the candidate use cases, the three adjacent designs (Clone via #1569, Arc-wrap via #1610, explicit producer-vector constructor), and the ordering, capacity, and bench evidence that would have to land before flipping the default. Recommends keeping WorkQueueSender Send + !Clone in default builds and the multi-producer cargo feature opt-in. Documents the cost so reviewers can reject naive MP refactors. Promotion to default-on is gated on PR #4209 SP-vs-MP throughput parity, PR #4214 drain bench showing no regression, and an actual fan-in caller materialising.
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…#4212) Document the four CAPACITY_MULTIPLIER sites and the two duplicate hard-coded `2`s in delta_pipeline.rs, justify each against the recent dispatch benches (#4203 channel overhead, #4204 reorder memory, #4206 dispatch decomposition, #4209 sp vs mp), and recommend keeping the default at 2 with one follow-up bench specified to challenge it.
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…1572) (#4209) Adds `crates/engine/benches/sp_vs_mp_workqueue.rs` with two Criterion groups that both move 100K items through the concurrent delta work queue: - `sp/1p/100k`: one producer thread pushes 100K pre-allocated DeltaWork items via the default-build `Send + !Clone` WorkQueueSender path. - `mp/4p/100k`: four producer threads each push 25K pre-allocated items via the gated `Clone` impl on WorkQueueSender. Compiled only when `--features multi-producer` is set. Both groups report `Throughput::Elements(100_000)` so items/sec figures compare directly regardless of how the work is split. Inputs are pre-allocated outside the timed section via `iter_batched`, matching the discipline used by the parallel_dispatch_overhead bench. The MP group is feature-gated behind the engine crate's existing `multi-producer` feature - no Cargo.toml dependency change is required; the gate already exists at `crates/engine/Cargo.toml:91`. The top-of- file documentation cross-references the audit at `docs/audits/workqueue-sender-multi-producer-audit.md` (PR #4173), #4203 sync_channel bench, and #4206 parallel_dispatch_overhead bench, and spells out the decision criteria (>=15% SP-vs-MP delta) that this bench informs.
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May 18, 2026
…1405) (#4224) Replaces the prior multi-root-focused note at the same path with a focused #1405 design discussion for parallel generator fan-in. Surveys the current SP / MP shape (feature-gated Clone at work_queue/multi_producer.rs), the candidate use cases, the three adjacent designs (Clone via #1569, Arc-wrap via #1610, explicit producer-vector constructor), and the ordering, capacity, and bench evidence that would have to land before flipping the default. Recommends keeping WorkQueueSender Send + !Clone in default builds and the multi-producer cargo feature opt-in. Documents the cost so reviewers can reject naive MP refactors. Promotion to default-on is gated on PR #4209 SP-vs-MP throughput parity, PR #4214 drain bench showing no regression, and an actual fan-in caller materialising.
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Summary
crates/engine/benches/sp_vs_mp_workqueue.rs(Criterion bench) with two groups that both move 100K items through the concurrent delta work queue:sp/1p/100k: one producer thread pushes 100K pre-allocatedDeltaWorkitems via the default-buildSend + !CloneWorkQueueSenderpath.mp/4p/100k: four producer threads each push 25K pre-allocated items via the gatedCloneimpl onWorkQueueSender. Compiled only when--features multi-produceris set.Throughput::Elements(100_000)so items/sec compare directly regardless of producer count. Inputs are pre-allocated outside the timed section viaiter_batched, matching the discipline used by theparallel_dispatch_overheadbench (bench(engine): decompose parallel dispatch overhead at 100K items (#1551) #4206).multi-producer = []atcrates/engine/Cargo.toml:91. Only the[[bench]]entry is added.Why
PR #4173 audited every live
WorkQueueSendersite and concluded that all three production producer sites are correctly single-producer today. Task #1572 produces the quantitative SP-vs-MP overhead figure that gates whether themulti-producerfeature should stay opt-in or graduate to default-on if a future caller wants MP fan-in. The bench's top-of-file documentation spells out the decision criteria (>=15% delta threshold, matching the buffer-pool sharded-benchmark gate) and cross-references the #4173 audit, the #4203sync_channelbench, and the #4206parallel_dispatch_overheadbench so reviewers can read all four data points together.Test plan
cargo bench -p engine --bench sp_vs_mp_workqueuebuilds and runs the SP group on the default build (locally verified viacargo fmtonly; benches deferred to CI per repo policy).cargo bench -p engine --features multi-producer --bench sp_vs_mp_workqueuebuilds and runs both groups when the feature is enabled.