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perf: evaluate handoff completion headroom only where admission reads it - #540
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| Check name | Status | Explanation |
|---|---|---|
| Title check |  |
The title precisely states the behavioral change: admission evaluates handoff completion headroom only where required. |
| Description check | |
The description follows the required sections and clearly states the problem, behavior change, review map, risk, contract impact, tests, benchmarks, reproduction details, and tracking references. |
| 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. |
| Cross-Layer Contract | |
PASS. The PR changes only internal Rust scheduler policy seams. policy is a private module, its changed functions and types use pub(super), and vllm exports only VllmCore (crates/core/src/engi… |
| Modeling And Data Evidence | |
The PR provides provenance and reproducible before/after evidence. The description identifies the base commit, workloads, hardware/software, exact reproduction command, five alternating paired rounds,… |
| Compatibility Boundaries | |
No compatibility boundary changed. The PR modifies only three Rust scheduler files; no Python files, schemas, manifests, versions, bindings, or artifact settings changed. The altered policy functions … |
| Review Evidence | |
The PR description names commands and results: cargo test -p aisimulate-core (2,147 passed), cargo fmt --check, and cargo clippy ... (no warnings). It includes negative and boundary validation: … |
Full details: Cross-Layer Contract
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Explanation
PASS. The PR changes only internal Rust scheduler policy seams.
policyis a private module, its changed functions and types usepub(super), andvllmexports onlyVllmCore(crates/core/src/engine/scheduler/vllm/mod.rs:9-15). All non-test callers are updated incore.rs, and repository-wide searches found no Python, CLI, serialization, or documentation consumers. The new callback behavior is tested for both policies inpolicy/tests.rs:399-489; existing integration tests cover activated-waiting and held destination headroom intests.rs:1384-1493. The changed-file inventory contains only these Rust implementation and test files, so no stale cross-layer contract is present.
Full details: Modeling And Data Evidence
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Explanation
The PR provides provenance and reproducible before/after evidence. The description identifies the base commit, workloads, hardware/software, exact reproduction command, five alternating paired rounds, native timing values, unchanged outputs, and an in-run A/A anomaly band. It also states that output comparison excludes host-time fields. The changed selection logic is visible in
core.rs(lines 1196-1205 and 2073-2087) andpolicy.rs(lines 292-353). Added tests verify lazy vLLM evaluation and TRT-LLM headroom subtraction (policy/tests.rs:400-489). This is not binary-only validation.
Full details: Compatibility Boundaries
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Explanation
No compatibility boundary changed. The PR modifies only three Rust scheduler files; no Python files, schemas, manifests, versions, bindings, or artifact settings changed. The altered policy functions remain
pub(super)andpolicyremains private (crates/core/src/engine/scheduler/vllm/mod.rs:12), so the signature changes are internal. The wheel still uses the singleaisimulate-coremanifest through maturin (python/aisimulate/pyproject.toml:104-120), and the crate remains the sameaisimulate-corepackage withrlibandcdyliboutputs (crates/core/Cargo.toml:4-17). Dynamo's published dependency boundary remains separate and version-pinned (ai-dynamo/dynamo, commitb1e4b765d3779cc821267a2d7a59106220441e32,Cargo.toml:54-55;container/deps/requirements.aisimulate.txt:5-6). No changed code adds a Dynamo dependency or alters a serialized or Python contract.
Full details: Review Evidence
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Explanation
The PR description names commands and results:
cargo test -p aisimulate-core(2,147 passed),cargo fmt --check, andcargo clippy ...(no warnings). It includes negative and boundary validation: vLLM must not evaluate the callback, while TRT-LLM admits with two blocks and waits with three; the changed tests implement these cases. It distinguishes simulated results, parity comparison, and native Tyche measurements. It does not cite an inactive nested workflow as hosted CI evidence. The PR diff changes only three Rust files and no workflow files.
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Actionable comments posted: 1
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Inline comments:
Review comments at @crates/core/src/engine/scheduler/vllm/policy.rs:
- Around line 371-375: Add TRT-LLM destination-admission coverage in
trtllm_destination_reservation_protects_completion_headroom by issuing a second
pending ReserveDestination instead of relying on receive. Assert the request
remains pending while the first destination is held and after activation, then
assert it reserves once completion headroom is released; preserve the existing
scheduler-level Vllm coverage.
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crates/core/src/engine/scheduler/vllm/core.rscrates/core/src/engine/scheduler/vllm/policy.rs
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🔀 Multi-repo context ai-dynamo/dynamo
Linked repositories findings
ai-dynamo/dynamo
- TRT-LLM workers explicitly configure
GUARANTEED_NO_EVICT; this remains the policy path that must retain completion-headroom calculations. [::ai-dynamo/dynamo::components/src/dynamo/trtllm/workers/llm_worker.py:382] - Mocker configuration exposes
capacity_scheduler_policyas a TRT-LLM-specific override, so policy dispatch should continue honoring this configuration boundary. [::ai-dynamo/dynamo::components/src/dynamo/mocker/config.py:105] - Release documentation defines TRT-LLM simulation as a
GUARANTEED_NO_EVICTscheduling-policy variant and states that oversized-request rejection propagates through replay drivers. [::ai-dynamo/dynamo::docs/fern/pages/reference/general/releases/dynamo-v1-3-0.mdx:184] - Handoff behavior is shared across offline replay and live bootstrap, with engine-specific admission ordering documented separately; the optimization should not alter lifecycle events or reservation outcomes. [::ai-dynamo/dynamo::docs/fern/pages/reference/general/releases/dynamo-v1-3-0.mdx:189]
- Replay tests preserve rejection semantics by excluding rejected requests from the report. [::ai-dynamo/dynamo::lib/mocker/src/replay/online/tests.rs:683-702]
ai-dynamo/aiconfigurator
No direct references to the changed scheduler policy or handoff-headroom APIs were found.
🔇 Additional comments: 1
crates/core/src/engine/scheduler/vllm/core.rs: 1
1267-1268: 🎯 Functional CorrectnessThe concern is refuted.
decide_waiting_admission_with_costignores handoff headroom forSchedulingPolicy::Vllm; it subtractshandoff_reserved_blocksonly in theTrtllmGuaranteedNoEvictbranch.decide_destination_admissionalso ignores both values for vLLM. Therefore, returning(0, 0)cannot remove a vLLM completion reservation.
jthomson04
changed the title
Every waiting-admission candidate and destination-reservation retry computed the completion tails promised to destination handoffs by walking the worker's whole request map. Only TRT-LLM no-evict admission reads them; vLLM discards the result. On a P/D decode worker every activated handoff stays tracked until it finishes, so the early return for an empty handoff set never fired and each transferred request paid a scan over all resident requests. Return zero tails without scanning when the policy does not reserve them. vLLM admission already treated them as zero, so simulated results are unchanged. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: jthomson04 <jwillthomson19@gmail.com>
Pass the destination handoff completion headroom to admission as a closure that only the TRT-LLM no-evict arms evaluate, replacing the vLLM-only predicate and early return. TRT-LLM decode workers also stop scanning for materialized handoffs, whose admission ignores the result. Find activated handoffs that are not running yet in the waiting queues instead of every resident request. Every request is waiting, connector waiting or running, so the sum is unchanged. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: jthomson04 <jwillthomson19@gmail.com>
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Why and what changed
On P/D decode workers, every waiting-admission candidate and every destination-reservation retry called
destination_handoff_completion_headroom, which walked the worker's whole request map with two hash probes per entry. Only TRT-LLMGUARANTEED_NO_EVICTadmission reads the result. Even there, most decode candidates are activated handoffs with materialized prompts, which are admitted without it. Each activated handoff stays tracked until its request finishes, so the empty-set early return (#289) never fires on a decode worker. At 512 requests in flight, the scan was about a fifth of native replay.decide_waiting_admission_with_costanddecide_destination_admissionnow take the headroom as a closure and evaluate it only in their TRT-LLM arms. vLLM never computes it. TRT-LLM computes it only for fresh-KV candidates and destination reservations.Simulated results are unchanged for both policies.
Review map
destination_handoff_completion_headroomand its two closure call sites incrates/core/src/engine/scheduler/vllm/core.rs, then the TRT-LLM arms inpolicy.rs.Evidence
vllm-pd-c512: vLLM P/D, tiny Llama on h200, fixed 1 ms timing: 4 prefill + 1 decode (max 512 seqs), synthetic 1024/32, concurrency 512, 262k requeststrtllm-pd-c256: TRT-LLM 1.3.0rc20 P/D, same setup at decode max 256 seqs, concurrency 256, 262k requestsMedian of 5 paired, alternating rounds; band ±3.7% from an in-run A/A on
vllm-pd-c512; outputs compared excluding host-time fields.cargo test -p aisimulate-core(2,147 passed),cargo fmt --check, andcargo clippy -p aisimulate-core --all-targets(no warnings in the changed files). New policy tests check two things: vLLM admission never evaluates the headroom, and TRT-LLM admission subtracts it.trtllm_same_prefix_generation_destinations_need_fresh_full_footprintscovers the activated-waiting tail (it fails if that tail is dropped), andtrtllm_destination_reservation_protects_completion_headroomcovers the held tail.4730b2c1006e.aisim-e2e run --site tyche --arm base=4730b2c1006ec19b9513f09e4c9ad93e67c41265 --arm lazy-headroom=4730b2c1006ec19b9513f09e4c9ad93e67c41265:lazy-headroom.patch --workload vllm-pd-c512=vllm-pd-c512.yaml --workload-args vllm-pd-c512='--set traffic.stop.requests=262144' --workload trtllm-pd-c256=trtllm-pd-c256.yaml --workload-args trtllm-pd-c256='--set traffic.stop.requests=262144' --rounds 5 --control aaTracking
Workloads
Workloads
vllm-pd-c512andtrtllm-pd-c256:aisimulate predict --config config.yaml --set traffic.stop.requests=262144vllm-pd-c512:trtllm-pd-c256:🤖 Generated with Claude Code