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test(stargate): add discrete-event routing simulator - #2223

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claude/stargate-routing-sim
Oct 8, 2026
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claude/stargate-routing-sim

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@barrygreengus barrygreengus commented Oct 1, 2026 •

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TL;DR

Adds stargate-routing-sim, a discrete-event simulator that drives the production Stargate load balancers in virtual time against a modeled fleet of Pylon and MockDynamo backends. A full policy sweep (several algorithms, rates and seeds) on a 20-backend, five-region topology takes seconds to minutes instead of cluster hours.

Stacked on #2286, which provides the mock-engine crate the simulated backends run. #1002 (maximum input TPS for Pulsar) and #2288 (Pulsar-WaW) have merged; the remaining stack is #2286, then this PR, then #2321.

Additional Details (optional for docs, build, test, refactor, ci, chore, style, and revert PRs)

Why: each cluster data point costs about 15 minutes of serialized multi-region traffic. Comparing load-balancing policies and their settings across workload regimes needs thousands of runs, so the search has to happen in simulation, with cluster runs validating selected points.

What the simulator models:

  • Stargate:

    • One production LoadBalancer per Stargate, called through the public API. Nothing is reimplemented. Load balancers read virtual time through a backdated received_at.
    • Per-Stargate stats views.
    • Optimistic reservations, cleared when a Pylon update is processed.
    • The proxy routing loop: timed-wait rechecks, no-choice retries within the max-wait budget, and queue-mismatch reroutes.
  • Pylon:

    • Request phases, queue estimates and queue-mismatch admission.
    • The fallback input-throughput window that sets last_mean_input_tps and max_input_tps.
    • Heartbeat publishing, optionally with change-driven publishing and a coalescing window.
  • Engine: the mock-engine crate that MockDynamo also runs. Each backend is a deployment of N GPU workers with:

    • iteration-level batching and chunked prefill;
    • per-worker KV caches that keep a session's prompt and output;
    • perfect KV routing between workers.

    GPU counts can differ per backend.

  • Network: an inter-region RTT matrix.

  • Clients:

    • Fixed-prompt sessions, or growing conversations whose prompts extend each turn up to a context limit.
    • Poisson session arrivals, think time between turns, and a TTFT SLO for goodput.
    • Client timeouts that cancel engine work, and retry of failed turns with backoff.
    • All per-session draws come from the seed, so every policy sees identical conversations.
  • Output: goodput, SLO attainment, TTFT and E2E percentiles, cache reuse, failures by kind, retries, cross-region share, and a per-backend breakdown.

Usage:

cargo run --release -p stargate-routing-sim -- \
  --config crates/stargate-routing-sim/configs/stargate-dev-parity.json \
  --output /tmp/routing-sim.json

Limitations (also in the crate README):

  • Engine step costs and KV capacity are estimates until calibrated against a real engine. Results are relative.
  • Load balancers use an unseeded thread RNG, so repeated runs vary slightly.
  • KV matching is per session key, not per token block.
  • One backend per routed cluster.
  • max_input_work_seconds admission, consider_kv_free_tokens (rejected at validation), and priorities are not modeled. Simulated Pylons publish no KV-cache stats.
  • Without pylon.stats_update_coalesce_ms, Pylons publish only on the heartbeat; production Pylons also publish on request state changes, every 10 ms by default.

For the Reviewer

  • crates/stargate-routing-sim/src/sim.rs: the event loop and the mirrored proxy routing behavior. Check it against stargate/src/http_proxy/run.rs, routing.rs and routing_state/reservations.rs.
  • crates/stargate-routing-sim/src/backend.rs: Pylon bookkeeping and the fallback throughput window. Check it against pylon-lib/src/queue_admission.rs and stats/aggregator.rs.
  • crates/stargate-routing-sim/src/workload.rs: fixed and growing session generation.

For QA (optional for docs, build, test, refactor, ci, chore, style, and revert PRs)

  • cargo test --locked -p stargate-routing-sim: 13 tests pass. They include end-to-end fixed, growing, and short-timeout growing-session runs over three policies, which check that every measured request has one outcome, that timeouts and turn retries occur when expected, and (through debug assertions) that every request resolves and every simulated Pylon and engine ends empty. Config validation rejects values that would hang or misroute. cargo test --locked -p mock-engine: 18 tests pass.
  • cargo clippy --locked -p stargate-routing-sim --all-targets -- -D warnings and cargo fmt --all -- --check: clean.
  • bazel test //src/libraries/rust/stargate/crates/stargate-routing-sim/... //src/libraries/rust/stargate/crates/mock-engine/...: 2 of 2 targets pass.
  • QA: not needed. The simulator is a development tool and is not shipped.

No new third-party dependencies.

Issues

Relates to #2222

Related Pull Requests

Checklist

  • I am familiar with the Contributing Guidelines.
  • I have signed off my commits for Developer Certificate of Origin (DCO) compliance.
  • New or existing tests cover these changes.
  • The documentation is up to date with these changes.

🤖 Generated with Claude Code

Summary by CodeRabbit

  • New Features
    • Added a command-line simulator for comparing Stargate routing policies across configurable traffic, network, backend, and client scenarios. It supports fixed and growing-session workloads, retries, timeouts, detailed performance results, and JSON output.
    • MockDynamo’s H100 profile now uses a batched inference engine with configurable workers, batching, and KV-cache behavior; the legacy model remains available.
    • Stargate’s default PULSAR ranking now weights backends by their maximum observed input throughput. PULSAR wait-and-widen also supports progressive routing fallback controls.
    • OpenBao Helm deployments now support auto-unseal with configurable recovery keys.
    • Gateway error responses now identify their source in a response header and telemetry.
  • Documentation
    • Added simulator usage guidance and modeling limitations, plus updated routing and auto-unseal configuration documentation.

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📝 Walkthrough

Walkthrough

Adds a Rust command-line simulator that runs Stargate load balancers against simulated backends. It supports configurable topology, policies, and fixed or growing workloads, and reports routing, latency, failure, and backend metrics.

Changes

Routing simulation

Layer / File(s) Summary
Crate registration and build targets
MODULE.bazel, src/libraries/rust/stargate/Cargo.toml, src/libraries/rust/stargate/crates/stargate-routing-sim/{BUILD.bazel,Cargo.toml}
Registers the simulator in the Cargo and Bazel workspaces. Defines its binary and test targets and dependencies.
Simulation inputs and workload planning
src/libraries/rust/stargate/crates/stargate-routing-sim/src/{config.rs,time.rs,workload.rs}, src/libraries/rust/stargate/crates/stargate-routing-sim/configs/*
Adds configuration types and validation, time conversions, seeded fixed- and growing-session workload planning, and a development-parity configuration.
Backend modeling and event simulation
src/libraries/rust/stargate/crates/stargate-routing-sim/src/{backend.rs,sim.rs}, src/libraries/rust/stargate/crates/stargate/src/{lib.rs,routing_state/*}
Models backend engine state and statistics. Runs scheduled routing, heartbeat, network, engine, timeout, mismatch-rejection, and retry events. Exposes routing helpers used by the simulator.
Metrics and command-line runs
src/libraries/rust/stargate/crates/stargate-routing-sim/src/{metrics.rs,main.rs}, src/libraries/rust/stargate/crates/stargate-routing-sim/README.md
Calculates run and per-backend metrics. The CLI runs seed, rate, and policy combinations, then prints summaries and optionally writes JSON output. Documents invocation and fidelity limits.

Priority: ⬇️ Low

Estimated code review effort: 4 (Complex) | ~60 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant main
  participant SimConfig
  participant sim_run as sim::run
  participant LoadBalancer
  participant Backend
  participant summarize as metrics::summarize
  main->>SimConfig: Deserialize and validate configuration
  main->>sim_run: Run each seed, rate, and policy
  sim_run->>LoadBalancer: Request a backend route
  LoadBalancer-->>sim_run: Return a backend choice or wait
  sim_run->>Backend: Deliver request and advance engine
  Backend-->>sim_run: Return engine events and backend stats
  sim_run->>summarize: Summarize request records
  summarize-->>main: Return run summary
Loading

Merge Risk: 🔵 Low · up to 9e9a8

The simulator remains mergeable with owner awareness that sufficiently long virtual runs may panic.

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Title check ✅ Passed The title follows Conventional Commits syntax with the valid test(stargate): prefix. It accurately describes the addition of a discrete-event routing simulator used as a development and testing tool…
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Actionable comments posted: 1


  • 🪄 Fix CodeRabbit comments on this PR
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@src/libraries/rust/stargate/crates/stargate-routing-sim/src/config.rs:
- Around line 209-218: Update Config::validate to reject non-finite or negative
values in rtt_ms, intra_region_rtt_ms, stats_relay_delay_ms, and warmup_s, and
reject non-finite or non-positive measure_s. Return validation errors that name
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✅ No security issues found!

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🕐 Last updated: 2026-10-07 02:50:55 UTC | Commit: 8f1fa33

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Actionable comments posted: 1


  • 🪄 Fix CodeRabbit comments on this PR
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- Around line 403-409: Update the received_at calculation in the simulator’s
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virtual elapsed time; use Instant::now() as the fallback while preserving the
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Comment thread src/libraries/rust/stargate/crates/stargate-routing-sim/src/sim.rs
barrygreengus and others added 8 commits October 7, 2026 17:14
Drive the production wait-and-widen, pulsar-wait-and-widen, and power-of-n
load balancers in virtual time so policy configurations can be swept in
seconds instead of cluster hours. The simulator models stale per-Stargate
stats with reservations, the proxy routing wait and retry loop, Pylon
queue-mismatch admission and fallback input-throughput estimates, the
MockDynamo engine and KV cache, inter-region latency, and client timeouts.

Includes a stargate-dev parity config, unit tests, and Bazel targets.

Relates to #2222

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
…rowing sessions

Update the routing simulator to the current model of the system:
- Backends run the mock-engine crate that MockDynamo also uses: batched GPU
  workers with chunked prefill, per-worker KV caches with cached output, and
  perfect KV routing between workers. Per-backend GPU counts are configurable.
- Pylon can publish stats on request state changes with a coalescing window,
  in addition to heartbeats.
- Workloads are either fixed-prompt sessions or growing conversations whose
  prompts extend each turn. Failed turns retry with backoff before a session
  is abandoned.
- Results include a per-backend breakdown.

The mock-engine crate matches the copy in #2286 and drops out of this diff
once that change merges.

Relates to #2222

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
Add tests that run small fixed and growing workloads through
power-of-n, wait-and-widen, and pulsar-wait-and-widen, and assert that
every measured request has exactly one outcome. After each run, a debug
assertion checks that every simulated Pylon is idle, which catches
phase or prompt-work accounting leaks.

Reject consider_kv_free_tokens policies at validation, because simulated
Pylons publish no KV-cache stats and every candidate would be skipped.

Remove the never-empty Option around per-Stargate stats, compute each
backend engine config once, and document the heartbeat-only publish
default, retry scope, growing sessions, and the 1-9 ms retry sleep.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
The end-to-end tests never timed out a request, so the cancel path and
turn retries went unchecked. Add a short-timeout growing-session test
that requires timeouts and retries for every policy and an abandoned
session for at least one. The end-of-run debug checks now also require
every request resolved and every engine empty.

Validation rejects configs that would hang or misroute instead of
failing: a zero heartbeat, non-positive rates, negative or all-zero
traffic weights on Stargate regions, and a zero region gpu_workers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
The routing simulator truncated a load balancer's remaining wait to whole
microseconds. Real-clock jitter in received_at.elapsed() left sub-microsecond
remainders at the end of an affinity window, which became zero and failed the
request as no-route instead of widening. Round the wait up and add a
regression test with no max-wait header.

Review cleanup in the same crate:
- Call the router's own reservation and queue-estimate functions instead of
  local copies, exported from the stargate crate as
  routing::apply_reservation and routing::queue_time_estimate_ms_for_priority.
- Validate delays, RTT symmetry, measure_s, input TPS rates, the fallback
  window, session_zipf_s, and timeout_ms instead of silently clamping them.
- Pass backends to summarize instead of re-deriving their order, drop the
  identity candidate index map, and keep reservation IDs out of metrics.
- Reject --jobs 0 and add run context to simulation errors.
- Document measurement assumptions and match the Bazel test naming.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
A pylon.heartbeat_ms that rounds to zero microseconds rescheduled its
heartbeat at the same instant forever and hung the routing simulator.
Require at least one microsecond.

Second review pass cleanup in the simulator:
- Count a reroute that excluded every cluster as exhausted retries, as the
  proxy does, instead of no-route, and test it on a single backend.
- Make a double resolve panic in release builds instead of wrapping the
  unresolved count and spinning.
- Carry the reservation ID in the BackendReceive event instead of request
  state.
- Pin the wait round-up with a unit test and document the unused rtt_ms
  diagonal.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
The parity config used retired queue-mismatch thresholds and published
Pylon statistics only on heartbeats. Use the current 10,000 ms / 4.0
queue-mismatch defaults and the 10 ms change-driven stats coalescing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
@barrygreengus
barrygreengus force-pushed the claude/stargate-routing-sim branch from 9e9a86e to a818888 Compare October 7, 2026 17:21
@barrygreengus
barrygreengus requested review from a team as code owners October 7, 2026 17:21
@barrygreengus
barrygreengus requested a review from a team as a code owner October 7, 2026 17:21
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Barry Greengus <bgreengus@nvidia.com>

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No P0/P1 findings. Reviewed simulator routing and reservation parity, Pylon bookkeeping, workload generation, validation, metrics, and production API exports. Isolated exact-head cargo test --locked -p stargate-routing-sim passed all 17 tests. Current GitHub checks pass. No live-cluster validation.

@barrygreengus
barrygreengus added this pull request to the merge queue Oct 8, 2026
Merged via the queue into main with commit 69b78e5 Oct 8, 2026
33 checks passed
@barrygreengus
barrygreengus deleted the claude/stargate-routing-sim branch October 8, 2026 19:44
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🎉 This PR is included in src/libraries/rust/stargate/v0.26.0 🎉

The release is available on GitHub release

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This PR is included in version 1.32.1.

The release is available on GitHub release.

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