[Perf][CLI] Improve overall startup time

[aarnphm]

This PR performs lazy loading for importing vllm, where it tries to cleanup a lot of eager heavy imports to help improve general CLI startup experience

This PR does:

• Lazy load imports vllm/__init__.py to avoid unwanted side-effect. I have checked that most of these imports will be run during serve/engine initialization
• Move a lot of the configuration validator into @model_validator hooks
• Cached the compute kwargs in arg_utils.py, given that these are currently being called multiples times (we only need this for CLI, so most of the time, it is safe to have this cache. I used a lru_cache size of 30 preemptively to make sure we have enough room for future configuration if needed)
• Lazy load all CLI imports, given that we don't need to setup eagerly during startup.
• remove triton_utils side effect in utils, given that this doesn't seem to do anything (for all of the vllm.triton_utils import, it should validate HAS_TRITON accordingly).

With this PR, it reduces the vllm CLI by around 2 seconds which we still have a lot of room for improvement (a part of #19824 to improve general startup UX)

Here is the hyperfine for vllm -h. Python 3.11, 8xH100 instances on LambdaLabs:

with this PR

➜ hyperfine 'vllm -h' --warmup 3 --runs 100
Benchmark 1: vllm -h
  Time (mean ± σ):      6.705 s ±  0.016 s    [User: 14.560 s, System: 0.571 s]
  Range (min … max):    6.676 s …  6.798 s    100 runs

➜ hyperfine 'python -c "import vllm"' --warmup 3 --runs 100
Benchmark 1: python -c "import vllm"
  Time (mean ± σ):      1.344 s ±  0.004 s    [User: 9.608 s, System: 0.181 s]
  Range (min … max):    1.336 s …  1.356 s    100 runs

On current main

➜ hyperfine 'vllm -h' --warmup 3 --runs 100
Benchmark 1: vllm -h
  Time (mean ± σ):      8.635 s ±  0.020 s    [User: 16.423 s, System: 0.637 s]
  Range (min … max):    8.544 s …  8.730 s    100 runs

➜ hyperfine 'python -c "import vllm"' --warmup 3 --runs 100
Benchmark 1: python -c "import vllm"
  Time (mean ± σ):      5.194 s ±  0.060 s    [User: 13.067 s, System: 0.552 s]
  Range (min … max):    5.139 s …  5.568 s    100 runs

Signed-off-by: Aaron Pham [email protected]

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[gemini-code-assist]

Summary of Changes

Hello @aarnphm, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces significant performance improvements, particularly for the CLI startup time, by implementing a comprehensive lazy loading strategy across the vllm package and its command-line interface. It also refactors configuration validation and optimizes argument parsing, contributing to a more efficient and responsive user experience.

Highlights

• Core Lazy Loading Implementation: The primary vllm/__init__.py file has been refactored to use a __getattr__ mechanism. This defers the import of various vLLM components (like EngineArgs, LLMEngine, SamplingParams, etc.) until they are actually accessed, significantly reducing initial startup overhead.
• CLI Subcommand Lazy Loading: All major CLI subcommands (benchmark, collect-env, openai, run-batch, serve) now lazily load their respective modules and dependencies. Imports previously at the top of these files have been moved into their main or subparser_init functions, ensuring they are only loaded when a specific subcommand is invoked.
• Configuration Validation Refactoring: The vllm/config.py file has been updated to leverage Pydantic's model_validator(mode='after') decorator for argument validation. This removes explicit calls to _verify_args() in __post_init__ methods, making validation an inherent part of the model's lifecycle and potentially deferring some checks.
• Optimized Argument Parsing: The get_kwargs function in vllm/engine/arg_utils.py has been optimized with functools.lru_cache to cache computation results and now returns a copy.deepcopy to prevent unintended modifications of cached data, improving performance for repeated calls.
• Cleanup of Eager Imports and Logging: Several unnecessary eager imports have been removed across various files (e.g., vllm.triton_utils from vllm/utils.py). Logging initialization has been standardized across CLI entrypoints, and some logging messages have been improved for clarity.

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[gemini-code-assist]

Code Review

The code changes introduce lazy loading for vllm imports to improve CLI startup time. The changes also include refactoring and improvements to configuration and argument parsing.