feat(router): per-worker waiting-queue cap for least_load

[slin1237]

Description

Problem

least_load is a pure argmin: when every worker is backlogged it still picks the least-bad one and keeps deepening the queue. Nothing in the configuration bounds an individual worker's waiting queue — the router-level admission cap (--max-concurrent-requests) only bounds the aggregate, so skew still passes through it: a stale poll window, or a sink worker that accepts quickly and therefore looks attractive to load-based scoring, can pile requests behind one engine well past its batch capacity.

Solution

Add max_waiting_requests to PolicyConfig::LeastLoad (--least-load-max-waiting-requests, default 0 = disabled). Selection skips workers whose reported waiting requests, plus requests dispatched to them since their last poll (the same since-poll accounting the in-flight correction uses, converted at mean_prefill_tokens per request), have reached the cap. Workers without a load snapshot stay eligible — there is no queue evidence to veto on, and a dark fleet must keep routing. When every candidate is at the cap, selection returns none and the request falls to the router's admission queue instead of deepening a backlog. The single-healthy-worker shortcut no longer bypasses the veto when a cap is configured. Set the cap below the engine's max batch size.

Changes

• policies/least_load.rs: max_waiting_requests field + veto in select_min_expected_wait; single-worker shortcut guarded; tuning-knob docs
• config/types.rs: max_waiting_requests on PolicyConfig::LeastLoad, #[serde(default)]
• config/validation.rs: destructure the new field
• main.rs: --least-load-max-waiting-requests CLI flag, wired into the policy construction
• policies/factory.rs: pass the cap through with_params
• bindings/python: Router parameter, RouterArgs field + --least-load-max-waiting-requests argparse entry, arg-parser test field list

Test Plan

• cargo test -p smg — full suite green (lib + all integration binaries)
• New tests: waiting_queue_veto_skips_capped_worker, waiting_queue_veto_all_capped_returns_none, waiting_queue_veto_counts_since_poll_dispatches, waiting_queue_veto_ignores_workers_without_snapshots, waiting_queue_veto_applies_to_single_worker, waiting_queue_cap_zero_disables_veto; hinted_policy_inherits_operator_least_load_config extended to the new knob
• cargo build -p smg-python — bindings compile
• cargo +nightly fmt --check, cargo clippy -p smg --all-targets -- -D warnings — clean

Checklist

• cargo +nightly fmt passes
• cargo clippy --all-targets --all-features -- -D warnings passes
• (Optional) Documentation updated
• (Optional) Please join us on Slack #sig-smg to discuss, review, and merge PRs

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Reviewing files that changed from the base of the PR and between 157bd6c and e02d827.

📒 Files selected for processing (4)

• bindings/python/src/lib.rs
• bindings/python/src/smg/router_args.py
• bindings/python/tests/test_arg_parser.py
• model_gateway/src/policies/least_load.rs

🚧 Files skipped from review as they are similar to previous changes (4)

• bindings/python/src/smg/router_args.py
• bindings/python/tests/test_arg_parser.py
• bindings/python/src/lib.rs
• model_gateway/src/policies/least_load.rs

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📝 Walkthrough

Summary by CodeRabbit

• New Features

  • Added an optional per-worker waiting-request limit for least-load routing, configurable through Python settings and command-line options.
  • Requests are deferred to the admission queue when all eligible workers reach the configured limit.
  • Workers without recent load data remain eligible for routing.

• Improvements

  • Power-of-two routing now selects workers based on expected wait time, including queue length, throughput, in-flight work, and KV pressure.

• Documentation

  • Updated policy descriptions and load-monitoring documentation to reflect expected-wait routing behavior.

Walkthrough

The PR adds a configurable per-worker waiting-request cap to least-load routing. It propagates the setting through Rust and Python configuration paths. PowerOfTwo now uses LeastLoadPolicy expected-wait scoring for sampled workers.

Changes

Routing policy updates

Layer / File(s)	Summary
Queue-cap configuration and propagation model_gateway/src/config/types.rs, model_gateway/src/main.rs, model_gateway/src/policies/factory.rs, bindings/python/src/lib.rs, bindings/python/src/smg/router_args.py, model_gateway/src/config/validation.rs, model_gateway/src/policies/registry.rs, bindings/python/tests/test_arg_parser.py	Adds max_waiting_requests with a default of 0. Rust and Python CLI and constructor paths forward the value into LeastLoadPolicy.
Least-load capped selection model_gateway/src/policies/least_load.rs	Filters workers at the waiting-request cap, includes since-poll dispatches, scopes expected-wait calculations to candidates, and returns no worker when all candidates are capped.
PowerOfTwo expected-wait scoring model_gateway/src/policies/power_of_two.rs	Delegates sampled-worker scoring and load state handling to LeastLoadPolicy. Tests cover queue tokens, KV pressure, snapshots, in-flight credit, and dark workers.
Routing documentation and monitor contracts bindings/python/src/smg/router.py, model_gateway/src/config/types.rs, model_gateway/src/worker/monitor.rs	Updates documentation for expected-wait scoring and load-aware policy polling.

Estimated code review effort: 4 (Complex) | ~45 minutes

Merge Risk: ⚪ Minimal · up to e02d8

This adds an opt-in per-worker waiting-queue cap for least-load routing while preserving existing behavior by default; no actionable merge-blocking risk remains beyond normal checks and review.

Sequence Diagram(s)

sequenceDiagram
  participant WorkerMonitor
  participant PowerOfTwoPolicy
  participant LeastLoadPolicy
  participant Workers
  WorkerMonitor->>LeastLoadPolicy: update load snapshots
  WorkerMonitor->>PowerOfTwoPolicy: update load-aware policy state
  PowerOfTwoPolicy->>Workers: sample two healthy workers
  PowerOfTwoPolicy->>LeastLoadPolicy: score sampled workers
  LeastLoadPolicy->>Workers: read queue and in-flight state
  LeastLoadPolicy-->>PowerOfTwoPolicy: return lower expected-wait worker

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Possibly related PRs

• smg-project/smg#554: Introduces related least-load and PowerOfTwo load-scoring behavior.
• smg-project/smg#1629: Introduces the LeastLoad policy extended by this PR.
• smg-project/smg#2187: Adds the shared expected-wait scorer used by this PR.

Suggested reviewers: catherinesue, key4ng

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: adding a per-worker waiting-queue cap for the least-load router policy.
Description check	✅ Passed	The description directly explains the problem, solution, configuration changes, implementation scope, and tests for the waiting-queue cap.
Docstring Coverage	✅ Passed	Docstring coverage is 96.23% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches 💡 1

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• Resolve merge conflict in branch feat/least-load-waiting-cap

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• Commit on current branch

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• Create PR with unit tests
• Commit unit tests in branch feat/least-load-waiting-cap

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Actionable comments posted: 2

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Inline comments:
In `@bindings/python/src/smg/router_args.py`:
- Line 70: Append least_load_max_waiting_requests after
upstream_pool_idle_timeout_secs in bindings/python/src/smg/router_args.py:70-70,
bindings/python/src/lib.rs:913-913, and bindings/python/src/lib.rs:1052-1052 so
dataclass, PyO3 signature, and fn new positional orders remain aligned. In
bindings/python/tests/test_arg_parser.py:1111-1111, move the
EXPECTED_FIELD_SEQUENCE entry to the end and add the field to the
test_new_fields_appended_after_positional_reserve tuple.

In `@model_gateway/src/policies/least_load.rs`:
- Around line 248-277: Update the waiting-queue veto around inflight_tokens so
since_poll represents the number of requests dispatched since each worker’s last
poll, not token work divided by mean_prefill_tokens. Track and use a separate
per-worker dispatch counter for this cap, preserving the existing eligibility
behavior for workers without load snapshots and the max_waiting_requests == 0
bypass.

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Configuration used: Organization UI

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📥 Commits

Reviewing files that changed from the base of the PR and between 52affd8 and 157bd6c.

📒 Files selected for processing (12)

• bindings/python/src/lib.rs
• bindings/python/src/smg/router.py
• bindings/python/src/smg/router_args.py
• bindings/python/tests/test_arg_parser.py
• model_gateway/src/config/types.rs
• model_gateway/src/config/validation.rs
• model_gateway/src/main.rs
• model_gateway/src/policies/factory.rs
• model_gateway/src/policies/least_load.rs
• model_gateway/src/policies/power_of_two.rs
• model_gateway/src/policies/registry.rs
• model_gateway/src/worker/monitor.rs

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