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perf: count MTP+G2 prefill headroom from the allocation watermark - #813
With native MTP and G2 host offload on an aggregated vLLM worker, every scheduler pass sizes the headroom that running chunked prefills still need (VllmCore::remaining_prefill_blocks). Since #519 that subtracts BlockRequestLease::physical_owned_block_count(), which walks every lease entry, one per prompt block. A prompt of P tokens with token budget B and block size b costs about (P/B) × (P/b) entry visits: 440k per request for 120k-token prompts at a 2,048-token budget.
This change takes the count from the lease's allocation watermark instead. A scalar token-only lease, the only kind native MTP runs on, binds its blocks in order, so its resident entries are exactly the blocks below the watermark. watermark_owned_block_count asserts that it equals the scan in debug builds. The headroom is the same number, so admission decisions and simulated outputs are unchanged.
Review map
Risk level: low
Start with: BlockRequestLease::watermark_owned_block_count in crates/core/src/engine/kv_manager/vllm_backend.rs, then its one caller in crates/core/src/engine/scheduler/vllm/core.rs.
Public or serialized contract changed: no
Feature Support Matrix: unaffected
Compatibility or rollback concern: none
Evidence
Workload
Base
Head
Change
mtp-g2-v032-120k-b2048
12,065 ms
10,715 ms
-11.7%
mtp-g2-120k-b2048
19,103 ms
18,226 ms
-3.7%
Native replay time, medians of 5 paired rounds on one exclusive Tyche node, measured at 912e91755a23 before the rebase. Noise band ±2.1% from an in-run A/A. Outputs are equal.
remaining_prefill_blocks falls from 5.4% of native replay cycles to 0.04% on the first workload and from 3.7% to under 0.01% on the second.
Two contrast workloads are inside the noise band: 30k-token prompts at an 8,192-token budget (-1.2%), and G2 without MTP (+0.2%), which takes the branch this change leaves alone.
Tests: cargo test -p aisim-core --lib engine:: in a debug build, so the new assertion runs in the MTP+G2 scheduler tests. Added assertions cover allocation, capacity exhaustion, retained slots, preemption and an H2D-loaded prefix.
mtp-g2-120k-b2048, run with aisimulate predict --config config.yaml. mtp-g2-v032-120k-b2048 is the same config with connector: offloading_v0_32 added under host_offload.
No actionable comments were generated in the recent review.
Summary
Risk: Low to moderate. The three areas for human attention are the watermark-to-owned-block invariant, MTP prefill headroom accounting, and H2D-loaded prefixes with speculative-slot ownership.
Changed behavior: The MTP path in remaining_prefill_blocks now subtracts watermark_owned_block_count instead of scanning physical ownership. The new count uses allocated_tokens.div_ceil(block_size) plus opaque copies, with a debug assertion against physical_owned_block_count(). The non-MTP calculation is unchanged.
Contracts: The supplied diff changes no public or exported declarations. The change affects internal MTP prefill headroom accounting. The claimed lack of changes to serialized contracts and the Feature Support Matrix is consistent with the narrow diff, but those artifacts were not independently checked.
Technical evidence: The diff adds assertions for speculative allocation exhaustion and success, retained capacity, binding retained capacity to a new logical block, preemption, and a fully H2D-loaded prefix. The objectives report equal outputs and performance gains on two long-prompt MTP+G2 workloads. They also report passing targeted debug tests with assertions. Current-head test, build, and CI results are not available.
Review evidence: The objectives report that CodeRabbit completed a review, but no findings were supplied. This is not approval. Current review findings and severity counts are not available.
Merge readiness: Technical evidence is limited to the supplied diff and reported results. Current-head checks and review findings are not available, so merge readiness cannot be established.
The request lease derives watermark-owned block counts from allocated tokens and opaque copies. The MTP scheduler uses this count to calculate remaining prefill blocks. Tests cover allocation, retained capacity, preemption, and full-H2D behavior.
The lease derives block counts from the allocated-token watermark and opaque copies. A debug assertion compares the count with physical ownership. Tests check speculative allocation, retained-capacity binding, and preemption.
ai-dynamo/aisimulate#519: Adds MTP speculative-slot allocation and prefill-headroom accounting, which this PR updates.
Merge Risk: Minimal · up to 4a26d
No merge-blocking behavior change was identified. The additional admission-boundary test can be a follow-up.
Pre-merge checks: 8
Check name
Status
Explanation
Title check
The title precisely states the behavioral change: MTP+G2 prefill headroom is counted from the allocation watermark.
Description check
The description covers the rationale, behavior change, review map, risk, compatibility, benchmarks, tests, provenance, and tracking. Fast CI, Full CI, and reviewed-commit fields are blank, but the des…
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
The diff changes an internal block count, not a public input, schema, default, or output. Native MTP is limited to scalar token-only leases (core.rs:3030-3037; vllm_backend.rs:681-684). The new co…
Modeling And Data Evidence
The PR provides a reproducible comparison against the previous entry-scan algorithm. vllm_backend.rs:219-239 compares the new watermark count with the physical count in debug builds. Tests check exp…
Compatibility Boundaries
The PR changes only crate-private scheduler accounting and test assertions. It does not change public surfaces, schemas, defaults, dependencies, versions, bindings, or artifact configuration. Native M…
Review Evidence
The description names test and replay commands, reports results, and covers allocation exhaustion and preemption. It identifies the replay environment and does not cite nested workflows as hosted CI e…
Full details: Description check
Explanation
The description covers the rationale, behavior change, review map, risk, compatibility, benchmarks, tests, provenance, and tracking. Fast CI, Full CI, and reviewed-commit fields are blank, but the description is otherwise mostly complete.
Full details: Cross-Layer Contract
Explanation
The diff changes an internal block count, not a public input, schema, default, or output. Native MTP is limited to scalar token-only leases (core.rs:3030-3037; vllm_backend.rs:681-684). The new count checks equivalence with the previous count in debug builds (vllm_backend.rs:233-239). Lease and H2D tests cover the new count, and the existing MTP offload test exercises chunked prefill (mtp_offload_tests.rs:203-263). The PR also reports equal replay outputs.
Full details: Modeling And Data Evidence
Explanation
The PR provides a reproducible comparison against the previous entry-scan algorithm. vllm_backend.rs:219-239 compares the new watermark count with the physical count in debug builds. Tests check explicit counts for allocation, retained capacity, preemption, and H2D activation. The PR description also supplies the workload configuration, paired timing medians, and an A/A noise estimate.
Full details: Compatibility Boundaries
Explanation
The PR changes only crate-private scheduler accounting and test assertions. It does not change public surfaces, schemas, defaults, dependencies, versions, bindings, or artifact configuration. Native MTP uses the new count only when the KV manager supports scalar token-only speculative slots.
Full details: Review Evidence
Explanation
The description names test and replay commands, reports results, and covers allocation exhaustion and preemption. It identifies the replay environment and does not cite nested workflows as hosted CI evidence.
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Additional context used
Code guidelines: 2
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REVIEW.md — configured
Path-based instructions: 4
Review scheduler state transitions and event ordering, including cancel, preempt, drain, retry, duplicate, empty, and terminal paths.
CodeRabbit configuration file
Files:
crates/core/src/engine/kv_manager/vllm_backend.rs
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CodeRabbit configuration file
Files:
crates/core/src/engine/kv_manager/vllm_backend.rs
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Files:
crates/core/src/engine/kv_manager/vllm_backend.rs
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crates/core/src/engine/kv_manager/vllm_backend.rs
Multi-repo context ai-dynamo/dynamo
ai-dynamo/dynamo
Dynamo pins the AISimulate wheel and Rust core to matching exact versions; adoption requires coordinated pin updates. [::ai-dynamo/dynamo::container/deps/requirements.aisimulate.txt:4-6] [::ai-dynamo/dynamo::Cargo.toml:69-70]
Dynamo’s agentic replay test rejects speculative decoding combined with host offload, while accepting speculation after offload is removed. This limits downstream coverage of the combined path. [::ai-dynamo/dynamo::components/src/dynamo/replay/tests/test_simulation.py:837-859]
Separate G2 replay tests exercise H2D restoration and assert reused-token counts and transfer latency across trace formats and offload scopes. [::ai-dynamo/dynamo::components/src/dynamo/replay/tests/test_agentx_g2.py:246-276]
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Actionable comments posted: 1
Fix comments: Fix CodeRabbit comments on this PR
Prompt to fix all issues
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Inline comments:
Review comments at @crates/core/src/engine/scheduler/vllm/core.rs:
- Line 1816: Add a scheduler test for H2D load admission during a running MTP
prefill, covering both cases where remaining headroom fits the lease count
exactly and where it is one block short; assert the admission decision in each
case and exercise the watermark_owned_block_count calculation using the
configured block size.
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Review details
CI failures not shown inline: 7
GitHub Actions: Fast CI / 0_Fast CI Success.txt: perf: count MTP+G2 prefill headroom from the allocation watermark
Dynamo pins both the AISimulate wheel and Rust core to the same exact development version; adopting this optimization requires coordinated pin updates. [::ai-dynamo/dynamo::container/deps/requirements.aisimulate.txt:4-6] [::ai-dynamo/dynamo::Cargo.toml:69-70]
Dynamo documents replay of captured request traces through AISimulate with vLLM, prefix caching, and configurable KV block size—a downstream parity scenario. [::ai-dynamo/dynamo::benchmarks/request_trace/README.md:47-70]
With native MTP and G2 host offload, every scheduler pass sized the
headroom of running chunked prefills by counting the resident entries of
each lease, one entry per prompt block. A 120k-token prompt at a
2,048-token budget walked 7,500 entries on each of about 59 passes.
A scalar token-only lease's resident entries are exactly the blocks below
its allocation watermark, so take the count from the watermark and the
opaque slots instead. A debug assertion checks it against the scan.
Simulated results are unchanged.
Signed-off-by: jthomson04 <jwillthomson19@gmail.com>
Co-Authored-By: Claude <noreply@anthropic.com>
A scalar token-only lease never allocates past its known tokens, and it
holds one entry per known block, so the watermark cannot exceed the entry
count. Assert against physical_owned_block_count, the method this replaces.
Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>
Signed-off-by: jthomson04 <jwillthomson19@gmail.com>
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APPROVE · 9.3/10 · high confidence
What this PR does
Speeds up native MTP simulation with G2 host offload by calculating a running prefill's owned KV blocks from its existing allocation watermark instead of scanning every prompt block. It adds lifecycle assertions while keeping the existing admission calculation.
Keeping the former ownership scan as a debug equality oracle, with assertions for exhausted allocation, retained speculative slots, preemption, and H2D-loaded prefixes, gives this small optimization a concrete correctness check.
Why this score
9.3/10, APPROVE. The production caller is limited to scalar token-only MTP leases, whose contiguous allocation and transfer paths preserve the count invariant. All 1365 debug engine tests passed, and exact-head Fast CI and Full CI succeeded. No actionable findings or comments are recommended. This is a local technical recommendation; CODEOWNER approval remains a separate merge gate.
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jasonqinzhou
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Why and what changed
With native MTP and G2 host offload on an aggregated vLLM worker, every scheduler pass sizes the headroom that running chunked prefills still need (
VllmCore::remaining_prefill_blocks). Since #519 that subtractsBlockRequestLease::physical_owned_block_count(), which walks every lease entry, one per prompt block. A prompt of P tokens with token budget B and block size b costs about (P/B) × (P/b) entry visits: 440k per request for 120k-token prompts at a 2,048-token budget.This change takes the count from the lease's allocation watermark instead. A scalar token-only lease, the only kind native MTP runs on, binds its blocks in order, so its resident entries are exactly the blocks below the watermark.
watermark_owned_block_countasserts that it equals the scan in debug builds. The headroom is the same number, so admission decisions and simulated outputs are unchanged.Review map
BlockRequestLease::watermark_owned_block_countincrates/core/src/engine/kv_manager/vllm_backend.rs, then its one caller incrates/core/src/engine/scheduler/vllm/core.rs.Evidence
mtp-g2-v032-120k-b2048mtp-g2-120k-b2048912e91755a23before the rebase. Noise band ±2.1% from an in-run A/A. Outputs are equal.remaining_prefill_blocksfalls from 5.4% of native replay cycles to 0.04% on the first workload and from 3.7% to under 0.01% on the second.cargo test -p aisim-core --lib engine::in a debug build, so the new assertion runs in the MTP+G2 scheduler tests. Added assertions cover allocation, capacity exhaustion, retained slots, preemption and an H2D-loaded prefix.Modeling or data provenance
N/A
Tracking
Workloads
mtp-g2-120k-b2048, run withaisimulate predict --config config.yaml.mtp-g2-v032-120k-b2048is the same config withconnector: offloading_v0_32added underhost_offload.🤖 Generated with Claude Code