fix(router): add an absolute margin to the prefix_hash load check

[slin1237]

Description

Problem

prefix_hash sends a request to the worker its prefix hashes to, unless that
worker looks overloaded, in which case it walks the ring to a less loaded one.
The overload test is purely relative:

let avg_load = (total_load + 1) as f64 / num_workers as f64;
let threshold = avg_load * self.config.load_factor;
(worker_load as f64) <= threshold

Because the test is a ratio, the load level it fires at scales with the counts
it is handed — and those counts get smaller as you add router replicas. Each
replica only counts the requests it routed itself, so with N replicas every
worker's observed in-flight count is about 1/N of its true one. Request arrivals
are Poisson, so the noise on an observed count of mean λ has relative width
1/√λ: dividing the counts by N multiplies the noise by √N.

At that point the margin is measuring noise rather than imbalance. For an
observed mean of 10 and the default load_factor of 1.25, P(load > 1.25 × avg)
is about 18% under a Poisson model with no real imbalance at all. Every one of
those requests abandons its hashed worker and lands somewhere with no warm
prefix, so the policy loses cache affinity in proportion to how many replicas
you run — the opposite of what a load guard should do.

Observed on an 8-replica deployment with the load spread evenly across 2,000
workers: the per-router view of a worker averaged 10.6 in-flight against a true
84.7, and the load_balance_walk branch of
smg_prefix_hash_policy_branch_total accounted for 21.8% of routing decisions
against 18.3% predicted from sampling noise alone. Engine queue depths were flat
at the same time (num_requests_running 63-64 across a 48-engine sample,
waiting-queue CoV 0.27), so there was no imbalance for the walk to be
responding to.

cache_aware does not have this problem: its imbalance test requires an
absolute gap as well as a relative one
(model_gateway/src/policies/cache_aware.rs:451). prefix_hash has no such
companion term.

Solution

Require the load to clear an absolute margin as well as the relative one:

let threshold = (avg_load * self.config.load_factor)
    .max(avg_load + self.config.balance_abs_threshold as f64);

balance_abs_threshold defaults to 10 requests. The absolute term binds while
the average is small — exactly where the sampling noise lives — and the relative
term takes over once avg × load_factor exceeds avg + balance_abs_threshold
(above an average of 40 at the default settings). Behavior under genuine load is
unchanged; setting balance_abs_threshold to 0 restores the previous check
exactly.

Changes

• model_gateway/src/policies/prefix_hash.rs — balance_abs_threshold on
  PrefixHashConfig (default 10), applied in load_ok
• model_gateway/src/config/types.rs — field on PolicyConfig::PrefixHash
  with a serde default, so existing configs keep parsing
• model_gateway/src/main.rs — --prefix-hash-balance-abs-threshold, wired
  through parse_policy
• model_gateway/src/policies/factory.rs,
  model_gateway/src/config/validation.rs — pass-through
• bindings/python/ — constructor parameter, RouterArgs field and CLI flag,
  docstring

The Go SDK does not expose prefix_hash (bindings/golang/src/policy.rs:330),
so it needs no change.

Test Plan

Unit tests in model_gateway/src/policies/prefix_hash.rs:

• test_absolute_margin_absorbs_small_count_noise — at an average of 10.25 a
  load of 13 clears the relative margin (12.8) but not the absolute one, so it
  is no longer treated as overloaded; 21 still is.
• test_relative_margin_still_binds_at_high_load — at an average of 200.25 the
  relative margin (250.3) exceeds the absolute one (210.25) and is the binding
  constraint: 240 passes, 260 does not.
• test_load_ok_calculation — pins balance_abs_threshold to 0 and keeps the
  original assertions, showing the previous behavior is recoverable.
• test_absolute_margin_defaults_on — the default is 10, not 0.

$ cargo test -p smg prefix_hash
test result: ok. 14 passed; 0 failed

$ cargo test -p smg
test result: ok. 2107 passed; 0 failed

$ cargo +nightly fmt --all
$ cargo clippy -p smg --all-targets -- -D warnings
$ cargo build -p smg-python

(--all-features pulls in opencv, whose build script does not run in my
environment, so clippy was run over every target without it; nothing here is
behind a feature gate.)

Before/after on a deployment routing to 2,000 workers behind 8 router replicas,
read from smg_prefix_hash_policy_branch_total: load_balance_walk was 21.8%
of decisions with flat engine queue depths. With a per-router observed mean of
10.6, an absolute margin of 10 requires a load of 20.6 rather than 13.3 to
trigger the walk, which takes the noise-driven rate under a Poisson model from
18.3% to 0.3%.

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

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between e5e9a2b and aa3df43.

📒 Files selected for processing (2)

• model_gateway/src/config/types.rs
• model_gateway/src/policies/prefix_hash.rs

🚧 Files skipped from review as they are similar to previous changes (2)

• model_gateway/src/policies/prefix_hash.rs
• model_gateway/src/config/types.rs

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📝 Walkthrough

Summary by CodeRabbit

• New Features

  • Added a configurable absolute load threshold for PrefixHash routing.
  • Added Python and command-line options for prefix-hash-balance-abs-threshold, defaulting to 10.

• Improvements

  • PrefixHash overload detection now considers both relative load and the configured absolute threshold.
  • Updated configuration and usage documentation to describe the new setting.

• Tests

  • Added coverage for low-load noise, high-load relative limits, and default threshold behavior.

Walkthrough

Changes

PrefixHash overload detection now requires both relative and absolute load margins. The absolute threshold defaults to 10 and is available through Rust CLI configuration and Python router interfaces.

PrefixHash threshold configuration

Layer / File(s)	Summary
PrefixHash overload detection model_gateway/src/config/types.rs, model_gateway/src/policies/prefix_hash.rs	PrefixHash uses the greater of the relative threshold and average load plus balance_abs_threshold. Tests cover low-load noise, high-load relative limits, and the default value.
Rust configuration and factory wiring model_gateway/src/main.rs, model_gateway/src/config/validation.rs, model_gateway/src/policies/factory.rs, model_gateway/tests/common/test_config.rs	Rust configuration and CLI parsing accept the threshold and pass it into PrefixHashConfig. Factory and shared test configurations provide the new field.
Python Router configuration bindings/python/src/lib.rs, bindings/python/src/smg/router_args.py, bindings/python/src/smg/router.py	Python constructors and CLI arguments accept the threshold, store it in Router, pass it to PrefixHash, and document its behavior.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Mergeability Score: 🔵 Low · up to aa3df

Python router-prefixed configuration may ignore a supplied backend threshold and use the default value of 10 instead, causing configured routing behavior to differ from the caller’s intent. The PR is mergeable with explicit owner awareness and follow-up on this bounded integration risk.

Sequence Diagram(s)

sequenceDiagram
  participant CLI as Router or model_gateway CLI
  participant Config as PolicyConfig::PrefixHash
  participant Factory as PolicyFactory
  participant Policy as PrefixHash
  CLI->>Config: provide balance_abs_threshold
  Config->>Factory: pass balance_abs_threshold
  Factory->>Policy: create PrefixHashConfig
  Policy->>Policy: combine relative and absolute load margins

Loading

Possibly related PRs

• smg-project/smg#1589: Modifies Python RouterArgs and PrefixHash configuration paths for CLI policy exposure.
• smg-project/smg#2144: Extends Python Router and RouterArgs with tunable PrefixHash configuration.
• smg-project/smg#2148: Modifies PrefixHash routing and configuration documentation.

Suggested reviewers: catherinesue, key4ng

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: adding an absolute margin to the prefix_hash load check.
Description check	✅ Passed	The description directly explains the prefix_hash load-check problem, solution, configuration changes, and test coverage.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% 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

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• Create PR with unit tests
• Commit unit tests in branch fix/prefix-hash-absolute-load-margin

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[claude]

Clean, well-tested change. The dual-margin threshold logic is correct, defaults are consistent across all config surfaces, serde defaults preserve backward compatibility, and the pattern properly mirrors cache_aware's imbalance test. No issues found.

[coderabbitai]

Actionable comments posted: 2

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Inline comments:
In `@bindings/python/src/smg/router_args.py`:
- Around line 408-416: Change the prefixed argument definition for
prefix_hash_balance_abs_threshold to default to None, allowing
RouterArgs.from_cli_args to fall back to the unprefixed value; retain the
dataclass default of 10 when neither argument is supplied. Add a regression test
covering the unprefixed fallback when use_router_prefix=True.

In `@model_gateway/src/config/types.rs`:
- Around line 569-572: Update the public PrefixHash policy documentation near
balance_abs_threshold to state that overload requires both the relative
load_factor threshold and the absolute balance_abs_threshold, and that routing
selects the least-loaded healthy worker rather than walking the ring.

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---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: a328aacb-c23d-417f-be1d-72674d9e0e44

📥 Commits

Reviewing files that changed from the base of the PR and between ff7657d and e5e9a2b.

📒 Files selected for processing (9)

• bindings/python/src/lib.rs
• bindings/python/src/smg/router.py
• bindings/python/src/smg/router_args.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/prefix_hash.rs
• model_gateway/tests/common/test_config.rs