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feat(stargate): use maximum input TPS for Pulsar weighting - #1002
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Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
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Included review availability: This review used your included allowance. Your plan provides up to 12 included reviews per hour; 10 remain after this review. 📝 WalkthroughWalkthroughModel statistics now carry a backend generation’s maximum input TPS. Stargate aggregates valid maxima across active backends, and Pulsar uses the maximum as its default rendezvous weight, with mean-TPS weighting available as an option. Benchmark configuration and rollout documentation also change. ChangesGeneration maximum TPS and Pulsar weighting
Priority: ➖ Normal Estimated code review effort: 4 (Complex) | ~50 minutes Change: Bug fix · Severity of issue fixed: Medium Sequence Diagram(s)sequenceDiagram
participant MockDynamo
participant PylonStatsCollector
participant PylonRegistration
participant StargateK8sRouter
participant Stargate
participant Pulsar
MockDynamo->>PylonStatsCollector: input-token counters after cache processing
PylonStatsCollector->>PylonRegistration: generation maximum in model stats
PylonRegistration->>StargateK8sRouter: registration with max_input_tps
StargateK8sRouter->>Stargate: forward model statistics
Stargate->>Pulsar: aggregated maximum input TPS
Suggested reviewers: Merge Risk: 🟡 Moderate · up to Pulsar now defaults to maximum-TPS weighting, while the linked issue asks for mean weighting to stay the default. Existing deployments with older Pylons that do not report a maximum could lose eligible clusters. Confirm this default change is intended, or restore mean as the default, before merging. 🚥 Pre-merge checks | ✅ 3 | ❌ 2❌ Failed checks (2 warnings)
✅ Passed checks (3 passed)
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🛡️ CodeQL Analysis🚨 Found 2 issue(s) Severity Breakdown:
📋 Top Issues🔗 View full details in Security tab 🕐 Last updated: 2026-08-19 15:47:50 UTC | Commit: f00fe01 |
Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
Refresh the existing peak-throughput change onto main, retain maxima across fallback and engine observations, and preserve explicit mean mode. Aggregate shared-engine peaks without double counting and document the Pylon-first rollout. Closes #1001 Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
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Document the relay upgrade requirement and verify the optional maximum survives gRPC forwarding. Relates to #1001 Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
Keep input-work estimates mean-based while counting only candidates eligible under the selected Pulsar capacity statistic. Relates to #1001 Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
This was referenced Sep 30, 2026
barrygreengus
marked this pull request as ready for review
October 2, 2026 20:32
Resolve the MockDynamo stream conflict by keeping input counter emission at modeled prefill completion and adopting main's include_usage chat role chunk. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Reviewed the Pylon max collection, wire/relay propagation, shared-cluster aggregation, and Pulsar selector/cache wiring. The plumbing looks correct and well tested. Two P1s and one P2 are inline, mainly about how robust the new default signal is and the rollout failure mode.
…ed-tps-weight Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
…store The weight mode is fixed for each Pulsar load balancer, so the ranking store now takes it at construction instead of receiving it on every lookup and insert. Rename the remaining selector parameters to rendezvous_weight to match the config field and type, and drop a test helper that only restated the default weight. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
The benchmark renderers are asserted not to emit --benchmark-pin-input-tps, but no code in the workspace defines or emits that flag, so the assertions cannot fail. Also restore the blank line between imports and constants in the stats collector. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
rendezvous_weight exists only on detailed algorithm objects. Name-only entries, the top-level default, and built-in defaults always use the maximum, so a mixed-version rollout must set the legacy weight on each detailed Pulsar entry. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
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@src/libraries/rust/stargate/crates/stargate/src/load_balancer/pulsar/ranking.rs:
- Line 324: Update the default selector used to initialize rendezvous_weight so
it remains LastMeanInputTps; use MaxInputTps only when explicitly selected,
preserving eligible clusters when older Pylon versions omit max_input_tps.
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src/libraries/rust/stargate/README.mdsrc/libraries/rust/stargate/crates/pylon-lib/src/stats/collector.rssrc/libraries/rust/stargate/crates/stargate-bench/src/k8s/tests.rssrc/libraries/rust/stargate/crates/stargate-bench/src/orchestrator.rssrc/libraries/rust/stargate/crates/stargate/src/load_balancer/pulsar.rssrc/libraries/rust/stargate/crates/stargate/src/load_balancer/pulsar/ranking.rssrc/libraries/rust/stargate/crates/stargate/src/load_balancer/tests.rssrc/libraries/rust/stargate/docs/load-balancer-configuration.md
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- src/libraries/rust/stargate/crates/stargate-bench/src/k8s/tests.rs
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- src/libraries/rust/stargate/crates/pylon-lib/src/stats/collector.rs
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The invalid-maximum tests built candidates whose mean was also invalid, so they passed in mean mode too. Keep the mean valid and make only the maximum invalid. The ranking-cache invalidation test now runs in both weight modes and changes only the selected field, so mean mode keeps coverage. Also record selected_inst.max_input_tps on the request span, since the default Pulsar weight no longer comes from last_mean_input_tps. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
Pulsar weights backends by maximum input TPS, so the routing expression test backend from main must advertise it to remain selectable. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
barrygreengus
added this pull request to stack #2338
October 7, 2026 02:35
The generation maximum ratcheted on every raw input-throughput sample, including engine counter deltas taken before the mean has enough data. One outlier tick, such as a prefix-cache hit reported over a short interval, could set the Pulsar rendezvous weight for the whole generation. Raise the maximum only when Pylon publishes a new last_mean_input_tps, so it is the greatest smoothed mean of the generation. Fallback-mode behavior is unchanged because its published mean is already the windowed rate. Replace the raw-sample tests with an engine-stream outlier case and restore the input-only update assertion the raw path had loosened. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
Summing per-Pylon means but taking the greatest per-Pylon maximum lowers a shared cluster's weight against single-Pylon clusters when each Pylon measures only its share of the engine. State this in the multi-backend cluster doc, and point the load balancer doc there instead of repeating the aggregation rule. Also fold the no-fallback weight test into the maximum weight test and drop the backend-order check from the shared-cluster maximum test; the aggregate is a plain maximum. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
A shared cluster's maximum is the greatest per-Pylon value, so per-Pylon --initial-input-tps contributions are not summed either. Say so in the multi-backend cluster doc, and reduce the README note to a pointer to the load balancer doc, which owns the rollout guidance. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
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CodeRabbit summary and pre-merge items, handled as of 913b43a:
Other changes in this pass: the Pylon maximum now follows the published smoothed mean instead of raw samples (6851813), and the shared-cluster weight caveat is documented (19641e9, 913b43a). |
The prefill counter tests use start_paused and tokio::time::advance, which need tokio's test-util feature. They only compiled when built with pylon-lib, which enables the feature, so cargo test -p mock-dynamo failed on its own. Add the feature as a dev-dependency and refresh the Bazel lockfile hash for the changed manifest. No new third-party dependency: tokio is already a workspace dependency. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
The mean dilutes an outlier sample but does not remove it, so say that instead of claiming one sample cannot set the maximum. Note that a Pulsar default algorithm keeps unlisted models on maximum weighting during a mixed-version rollout, and rename the maximum weight test to say it covers the default. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: Barry Greengus <bgreengus@nvidia.com>
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TL;DR
Pulsar and Pulsar wait-and-widen weight rendezvous ownership by each backend's last mean input TPS. That mean falls when traffic falls or caches warm, so ownership weights shift with load rather than with capacity, and affinity rankings churn. This change makes the per-generation maximum input TPS the default weight and adds a
rendezvous_weightselector so the current mean weighting stays available.Why
The rendezvous weight is meant to express how much input work a backend can absorb. A live mean measures recent demand as much as capacity. When a backend is lightly loaded, or when cache hits shorten prefill, its mean drops and it loses ownership share even though its capacity is unchanged. Rankings then move between backends, which breaks cache affinity at the load levels where reuse matters most.
The highest input throughput observed in a model generation is a steadier capacity signal. It only rises when the backend proves it can do more work, and it resets when the model generation changes.
What changed
Pylon:
Wire format:
ModelStats.max_input_tpsfield. It is additive, so older readers ignore it.Stargate:
rendezvous_weighttopulsarandpulsar-wait-and-widenconfigs:max-input-tps(default) orlast-mean-input-tps(current behavior).Benchmark fixtures:
stargate-benchprofiles dropregistration.last_mean_input_tps. The now-requiredservice_time_ms.prefill_tokens_per_sseeds Pylon--initial-input-tpsand the scoring capacity, so the seed and the mock's prefill rate cannot disagree.Observability:
pylon_model_max_input_tpsgauge when the maximum is known. The series is removed on model removal, or on replacement with unknown state. Existing metrics are unchanged.selected_inst.max_input_tpsbesideselected_inst.last_mean_input_tps.Customer Release Notes
Pulsar routing now weights backends by generation maximum input throughput by default. Upgrade Pylons and registration relays before Stargates so every active backend publishes and forwards the new statistic. Set
rendezvous_weight: last-mean-input-tpsto keep mean weighting during a staged rollout.Plan Summary
Not applicable. No infrastructure changes. Rollout order matters: Pylons and registration relays first, then Stargates.
Usage
Omit
rendezvous_weightonpulsarorpulsar-wait-and-widenconfigs to usemax-input-tps. Setrendezvous_weight: last-mean-input-tpsfor mean weighting. The field exists only on detailed algorithm objects: the top-leveldefault, name-onlymodelsandrequest_algorithmsentries, and built-in defaults always usemax-input-tps. Queue bounds, concurrency admission, affinity keys, and request deadlines are unchanged.For the Reviewer
Start with maximum collection in
pylon-lib/src/stats/aggregator.rsandprojection.rs. Then review wire propagation, shared-engine aggregation, and the selector and cache signatures understargate/src/load_balancer/pulsar/.No public invocation API or nvcf-cli changes are required.
Testing
cargo test --locked -p stargate,-p pylon-lib, and-p stargate-bench, each run alone: all pass (stargate 398 unit and 150 integration, pylon-lib 539, stargate-bench 164).cargo fmt --all -- --checkandcargo clippy --locked -p <crate> --all-targets -- -D warningsfor those crates pass.bazel testfor thestargate,pylon-lib, andstargate-benchpackages: 8 of 8 targets pass.cargo test --workspace --lockedalso passed.Routing comparison
Release Stargate, registration relay, and Pylon images from this branch ran on a five-region, 20-backend MockDynamo deployment. One arm set
last-mean-input-tps; the other omitted the selector to use the new default. All other settings matched.Maximum versus mean: successful throughput 2.45% higher, TTFT p99 14.86% lower, end-to-end p99 12.00% lower. Terminal failures rose from 4 to 73 and retries from 8,183 to 13,389.
Maximum weighting held cache reuse through the 180 RPS step, where mean weighting collapsed. Both collapsed at 200 RPS. Reuse stayed low on the return to 180 RPS and recovered at 160 RPS.
Reused-input share during the final 120 seconds of each step:
Workload: 768 fixed sessions and workers, 6,661 input tokens, up to 256 output tokens, 10s SLO and maximum wait, 30s client timeout. Each arm started cold and kept caches across five 180s steps. Engine concurrency and calibration maximum 25, startup calibration on, periodic canaries off, Pylon fallback statistics with the engine stats stream disabled.
Pulsar wait-and-widen settings: queue bounds 4000/4000 ms,
max_queued=4,n=2, TTFT bucket 20 ms, unlock factor 0.25, required affinity keys and input tokens,consider_kv_free_tokens=false.Successful RPS includes final drain. Reuse divides successful cached tokens by all admitted input tokens, including failures. Latency p99 uses successful requests and includes client retries. RPS values in the step table are caps.
This is a single pair with no repeatability range, and it does not isolate a sole cause of the cache behavior. A production recommendation depends on real-engine QA.
Issues
Closes #1001
Dependencies
None. No license or NOTICE changes.
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