Enhance engine snapshot management and API lifecycle

[fuhefei]

Purpose

Add an opt-in Engine Snapshot deep-sleep path that keeps the API frontend, HTTP listener, and launcher alive while snapshotting and exiting the EngineCoreProc subtree.

On wake, vLLM validates the snapshot and runtime environment, restores the EngineCore and CUDA state, adopts the restored process, reconnects the frontend, and reopens request admission only after health and semantic checks pass.

Design, roadmap, compatibility constraints, and complete experimental results are documented in RFC #52125: vLLM Native Reusable Engine Snapshot Sleep (L3).

This work overlaps with #51360 in its use of CRIU and CUDA Checkpoint/Restore for an initialized engine. The snapshot boundary and user-facing lifecycle differ:

• [Frontend] Add reusable TP1 initialized-engine snapshots #51360 captures the complete initialized service process tree before the HTTP listener is bound and restores it during instance startup.
• This PR implements runtime deep sleep. The frontend and HTTP listener remain available for status and wake requests while only the EngineCore subtree is captured, terminated, restored, and reattached.

Change Summary

• Add the Engine Snapshot lifecycle: READY → DRAINING → PREPARING → SNAPSHOTTING → HIBERNATED and HIBERNATED → RESTORING → ATTACHING → VERIFYING → READY.
• Add idle-only request admission control and readiness handling around snapshot transitions.
• Add EngineCore detach, expected-exit handling, restored-process adoption, and frontend reconnection.
• Add fake and criu_cuda snapshot providers.
• Add atomic snapshot artifact commit, manifest validation, diagnostics, rollback, and cleanup.
• Expose two L3 resource policies:
  • full: preserve weights, KV cache, and runtime state in the CUDA image.
  • minimized: discard weights and KV cache before capture, then reload weights from model files and rebuild KV cache after restore.
• Add development-mode CLI configuration and the Level 3 sleep, wake, and status endpoints.
• Add allocator, worker, process-lifecycle, provider, manager, API, middleware, CLI, and serving tests.
• Document prerequisites, supported scope, endpoints, snapshot contents, and deployment restrictions.

Current Scope

The initial implementation supports:

• Same-machine restore.
• One host and one GPU.
• TP=1, PP=1, and DP=1.
• One Python API server using UniProcExecutor.
• Idle-only capture with no in-flight request preservation.
• fake and CRIU/CUDA providers.
• Development-mode enablement through VLLM_SERVER_DEV_MODE=1.

The feature is opt-in and does not change the default vllm serve process topology or request path.

Cross-machine restore, 1→N restore, multi-GPU snapshot groups, distributed topologies, and in-flight request preservation remain follow-up work described in the RFC.

Test Plan

• Run Ruff on the changed Python files.
• Run Python 3.10 mypy on the changed typed modules.
• Run the snapshot unit-test suite: pytest -q tests/snapshot
• Run the Engine Snapshot CLI parser tests: pytest -q tests/entrypoints/openai/test_cli_args.py -k engine_snapshot
• Exercise the lifecycle and failure paths with the fake provider.
• Run repeated real CRIU/CUDA capture and restore cycles for the full and minimized policies.
• Verify restored inference output, process cleanup, physical HBM release, snapshot artifacts, and phase-level timing.

Test Result

Test environment:

• NVIDIA H20, single GPU.
• Qwen3-8B BF16.
• TP=1, PP=1, DP=1, one API server.
• Non-eager execution.
• Model, compilation cache, CRIU images, and snapshots stored on tmpfs.
• Six runs per group: one warmup followed by five measured runs.
• minimized was repeated across three groups for 15 measured runs.

Median results:

L3 policy	Measured runs	Checkpoint time	Restore time	Artifact size
minimized	15	2.837 s	5.948 s	4.251 GiB
full	5	27.242 s	22.990 s	49.659 GiB

Restore time is measured from the /wake_up request to the first non-empty token from a fixed streaming request, with the final response content also checked.

Correctness and resource results:

• All reported full and minimized runs restored successfully and passed the fixed semantic checks.
• Every pre-restore model page-residency check reported 100%.
• Engine read_bytes delta was 0 in all reported runs.
• Every reported HIBERNATED check showed 0 MiB for both project-process HBM and full-GPU HBM.
• Ruff passed.
• Python 3.10 mypy passed.
• Snapshot tests: 75 passed.
• CLI parser tests: 3 passed, 30 deselected.

These validation results predate the final public-policy simplification commit and are not presented as a rerun of the current tree.

AI Assistance

AI assistance was used for implementation, testing, analysis, and drafting. The submitter reviewed the changes and owns the final submission.

[claude]

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Documentation preview: https://vllm--52132.org.readthedocs.build/en/52132/

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This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @fuhefei.

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This pull request has merge conflicts that must be resolved before it can be
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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @fuhefei.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @fuhefei.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork