[ishandhanani]

Part of #16543 (PR 2 of 5, on top of #16616)

Summary

The policy-class queue decided "does this worker have room?" with worker_is_busy on a prefill-only token snapshot, at three call sites, and the offline replay router kept a copy of the same logic. Worker selection and classifiers read load from LOAD rows, so the queue and plugins could see different numbers for the same worker.

BusyThresholds::has_room(&WorkerLoadInput, WorkerCapacity) is now the one capacity check: a pure function of a worker's LOAD row, its advertised capacity, and the thresholds. Anyone holding those three gets the router's answer.

LOAD row      per worker, per request     WorkerInputs::LOAD (declared)
capacity      per worker, static          WorkerInfo (always available)
thresholds    per request (its class)     PolicyClassConfig::busy_thresholds (one field); classifiers read them in #16619
                       |
          BusyThresholds::has_room(load, info.capacity())

• New WorkerInfo: a worker rank's identity plus the max_num_batched_tokens and total_kv_blocks its runtime config advertises. These change only when a worker registers again, so every component reads them without declaring an input group: WorkerCandidate::info() for filters, scorers and classifier rows, WorkerInputView::info(row) for pickers.
• WorkerInfo is not stored in CandidateData. Rows are rebuilt and copied several times per candidate per selection, so growing them slows every policy, including ones that never read capacity. The host looks WorkerInfo up only when a component calls info(): a filter reads the config already in hand; scorers and pickers look the worker up in the selection's worker map; classifier rows own a copy.
• Every host path builds LOAD rows through WorkerLoadInput::for_worker, which flags a rank without a projection unavailable. The queue and replay pass an idle fallback over project_worker_loads(None); selection passes the request's unobserved-worker load. BusyThresholds::worker_is_full owns the per-rank loop that the queue, replay, and feat(router): enforce busy-reject thresholds in the KV router queue #16619's reject scan share, so replay no longer needs a doc-hidden conversion.
• PolicyClassConfig::worker_is_busy is replaced by the busy_thresholds field plus has_room. WorkerCapacity::prefill_budget is the one home for the DEFAULT_MAX_BATCHED_TOKENS fallback (the constant is still re-exported from scheduling::queue), and WorkerCapacity holds Option<u64> instead of a sentinel.

Behavior is unchanged.

Validation

• New worker_info_reaches_every_component_without_declaration: a filter, scorer and picker read info() without declaring anything. classifier_reads_declared_worker_inputs runs has_room on its own rows with info().capacity(): the worker with 64 booked prefill tokens is full at prefill_busy_threshold_frac: 0.5 of a reported 100, and the idle worker has room.
• cargo test -p dynamo-kv-router --features standalone-indexer --lib: 1175 passed, including gate_snapshot_reuses_the_actor_scratch_across_admissions, the new prefill_budget_defaults_when_unreported, unobserved_rank_row_is_unavailable_and_idle, and worker_is_full_only_when_every_rank_is_full. plugin_api_compat: passed. cargo test -p dynamo-mocker --lib: 261 passed. cargo test -p dynamo-custom-policy-builtin: passed. Clippy (-D warnings) and fmt: clean.
• size_of::<CandidateData>() is 112 bytes before and after. Classifier rows carry each rank's WorkerCapacity (32 bytes), not a full WorkerInfo.
• The class-queue gate refills one actor-owned scratch map instead of allocating a 48-byte-per-rank projection per admission. selection_core gained a no_hints_queue_gate case; five pinned alternating rounds, medians: 64 workers 26.3 -> 26.0 us, 512 workers 71.6 -> 66.4 us (-7.2%), 4096 workers 617.8 -> 595.4 us (-3.6%); the gate-off control moved -1.0%.
• Review round (sentinel -> Option<u64> capacity, shared row builder, worker_is_full): custom_worker_selection/10_000 A/B from one target dir with both binaries copied out, three alternating rounds, medians: custom_no_filter 649.6 -> 637.3 us, custom_keep_all_filter 891.4 -> 877.6 us, custom_capacity_filter 926.3 -> 927.1 us (different host than the table below; no regression). cargo clippy --workspace --all-targets -- -D warnings: clean.
• Selection benchmarks on an H100 host, one pinned core excluded from numad, medians of three alternating runs (one disturbed first run dropped), 10k workers:

Bench	#16616	This PR	Change
custom policy, no filter	309.8 us	303.4 us	-2.1%
custom policy, keep-all filter	417.6 us	418.9 us	+0.3%
unused preferred taints (0 / 1 / 4)	300.4 / 300.0 / 300.7 us	296.2 / 295.5 / 297.4 us	-1.1% to -1.5%
default policy wrapper, 1024 workers	21.57 us	21.47 us	-0.4%

Reading WorkerInfo costs about 2.4 ns per call in a filter and about 9.5 ns in a scorer or picker (a worker-map lookup): a scorer that reads every candidate at 10k workers adds about 95 us per selection. Copying capacity into every row instead cost every policy +4.5% to +15%.

Stack

1. feat(router): expose worker inputs to request classifiers #16616 feat(router): expose worker inputs to request classifiers
2. refactor(router): judge queue capacity on the shared LOAD row #16617 refactor(router): judge queue capacity on the shared LOAD row (this PR)
3. feat(router): add a decode busy threshold to policy-class queues #16618 feat(router): add a decode busy threshold to policy-class queues
4. feat(router): enforce busy-reject thresholds in the KV router queue #16619 feat(router): enforce busy-reject thresholds in the KV router queue
5. refactor(router): unify classifier inputs; deprecate absolute busy thresholds #16620 refactor(router): rename classifier contexts; deprecate absolute busy thresholds

Summary by CodeRabbit

• New Features

  • Worker selection policies can now use worker identity and capacity information when filtering, scoring, and choosing workers.
  • Routing and queue admission now account for projected worker load, KV capacity, and batch-token limits.
  • Busy thresholds support absolute and percentage-based prefill limits, improving admission decisions.
  • Added capacity-aware benchmarking for large worker pools.

• Documentation

  • Added guidance for accessing worker information and applying capacity-aware admission checks in custom components.

[github-actions]

🌿 Fern Docs Preview: https://nvidia-preview-01a17be7-ce4b-72fc-89a0-3f3addeca40a.docs.buildwithfern.com/dynamo/dev

[devin-ai-integration]

✅ Dynamo PR CI passed — run 40457086504 (attempt 1) on 6d16913b95

Gate checks: ✅ backend-status-check · ✅ deploy-status-check · ✅ dynamo-status-check

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[ishandhanani]

has_room as the single check is the right shape, and the side-buffer decision is well argued by the numbers. Confirmed the "behavior unchanged" claim: the old active_tokens snapshot and the new row's active_prefill_tokens both read load.active_tokens(decay_now).

The inline notes are about finishing the consolidation this PR starts: the projection-to-row conversion and the rank loop still exist in two or three copies, and a few structures carry duplicate state.

[coderabbitai]

Walkthrough

Worker selection now exposes worker capacity metadata to filters, scorers, and pickers. Queue admission and offline replay use projected worker loads and capacity-aware busy thresholds to evaluate whether workers have room.

Changes

Worker capacity and admission

Layer / File(s)	Summary
Expose worker metadata to selection lib/kv-router/src/plugins/worker_selection/inputs.rs, lib/kv-router/src/plugins/worker_selection.rs, lib/kv-router/src/scheduling/selector/*, lib/kv-router/benches/worker_selection.rs, docs/fern/pages/developer-guide/knowledge-base/modular-components/router/custom-worker-selection.mdx	Worker candidates and picker rows can access WorkerInfo capacity data. Selector policies pass this metadata to filters, scorers, and pickers. A benchmark adds capacity-based filtering.
Define capacity-aware busy thresholds lib/kv-router/src/plugins/worker_selection/inputs.rs, lib/kv-router/src/scheduling/policy_config.rs, lib/kv-router/src/scheduling/mod.rs, lib/kv-router/src/scheduling/local.rs, docs/fern/pages/developer-guide/knowledge-base/modular-components/router/custom-worker-selection.mdx	BusyThresholds::has_room evaluates active prefill tokens against configured absolute and fractional limits, using reported worker capacity or the default batch-token limit. Tests cover threshold boundaries and worker room classification.
Apply capacity checks during admission lib/kv-router/src/scheduling/queue.rs, lib/mocker/src/replay/offline/extensions/kv_router/mod.rs, docs/fern/pages/developer-guide/knowledge-base/modular-components/router/custom-admission-control.md	Queue admission, dispatch, and offline replay evaluate projected worker loads against busy thresholds and worker capacity. The admission-control guide documents the check.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: 🔵 Low · up to 36b95

The capacity check is not shown to fail at this head, but its public API should be tightened before merge to follow the project’s compatibility guidance.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 57.81% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 64 functions across 10 files. (2 skipped:…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: judging router queue capacity using the shared LOAD row.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Description check		The description is detailed and directly covers the change, implementation details, validation, performance results, and related issue reference. It does not use the template headings exactly and does…

Full details: Description check

Explanation

The description is detailed and directly covers the change, implementation details, validation, performance results, and related issue reference. It does not use the template headings exactly and does not identify specific reviewer starting files, but the required information is mostly present.

Full details: Docstring Coverage

Explanation

Docstring coverage is 57.81% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 64 functions across 10 files. (2 skipped: 2 unsupported.)

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

✨ Finishing Touches: 💡 1

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[coderabbitai]

Actionable comments posted: 1

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• 🪄 Fix comments: Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @lib/kv-router/src/scheduling/policy_config.rs:
- Around line 76-84: Change both fields of BusyThresholds,
prefill_busy_threshold and prefill_busy_threshold_frac, from public visibility
to pub(crate). Do not add constructors or accessors; preserve the existing
PolicyClassConfig::busy_thresholds() API and in-crate test construction.

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📥 Commits

Reviewing files that changed from the base of the PR and between 7035301 and 36b95db.

📒 Files selected for processing: 12

• docs/fern/pages/developer-guide/knowledge-base/modular-components/router/custom-admission-control.md
• docs/fern/pages/developer-guide/knowledge-base/modular-components/router/custom-worker-selection.mdx
• lib/kv-router/benches/worker_selection.rs
• lib/kv-router/src/plugins/worker_selection.rs
• lib/kv-router/src/plugins/worker_selection/inputs.rs
• lib/kv-router/src/scheduling/local.rs
• lib/kv-router/src/scheduling/mod.rs
• lib/kv-router/src/scheduling/policy_config.rs
• lib/kv-router/src/scheduling/queue.rs
• lib/kv-router/src/scheduling/selector/mod.rs
• lib/kv-router/src/scheduling/selector/policy.rs
• lib/mocker/src/replay/offline/extensions/kv_router/mod.rs

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[jthomson04]

One perf note on the gate snapshot this adds to the actor's admission path.

[jthomson04]

Gate scratch and queue-path numbers verified on 2a92808.