[Core] Use fastsafetensors ParallelLoader for weight loading

[gitbisector]

Replace the manual per-shard SafeTensorsFileLoader loop with ParallelLoader (added in fastsafetensors 0.2.0), routing through fastsafetensors' copier registry so future copier improvements are picked up automatically.

Adds VLLM_FASTSAFETENSORS_QUEUE_SIZE as an opt-in pipelining knob (default 0 preserves existing peak VRAM; queue_size=k costs k extra shard-sized buffers at peak during loading).

Purpose

Switch vLLM's fastsafetensors path to ParallelLoader (added in
fastsafetensors 0.2.0). On the currently released fastsafetensors
(0.2.2) this alone is ~10 % faster loading a 75 GB model, and it
routes through fastsafetensors' copier registry so future copier
improvements — e.g. the unified-memory copier in
fastsafetensors#60
— reach vLLM automatically once a new release is pinned.

Pipelining is opt-in via VLLM_FASTSAFETENSORS_QUEUE_SIZE (default
0 preserves existing peak VRAM; queue_size=k adds k shard-sized
buffers at peak during loading).

Test Plan

pytest tests/model_executor/model_loader/fastsafetensors_loader/test_weight_utils.py -v

The existing correctness test is now parametrized over
queue_size=[0, 1] so both paths run against the reference
safetensors_weights_iterator.

End-to-end perf: vllm serve Qwen/Qwen3-Coder-Next-FP8 --load-format fastsafetensors … on DGX Spark, reading the Loading weights took X seconds log line from default_loader.py:384.

Test Result

Qwen3-Coder-Next-FP8, 75 GB / 40 shards, DGX Spark (GB10):

fastsafetensors	queue_size	Load time
0.2.2, manual loop (current)	n/a	93.3 s
0.2.2, this PR	0	84.4 s
0.2.2, this PR	1	83.5 s
main (post-#60), this PR	0	27.2 s
main (post-#60), this PR	1	23.8 s

Mergeable today on released fastsafetensors (~10 % win, no VRAM
regression). The ~4× cliff shows up once fastsafetensors' unified
copier lands in a release — this PR is the prerequisite.

No documentation updates.

AI-assisted: drafted with Claude, reviewed end-to-end and benchmarked
by the submitter. Commit carries Co-authored-by: and
Signed-off-by: trailers.

[claude]

Claude Code Review

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[gemini-code-assist]

Code Review

This pull request refactors the fastsafetensors weight loading logic to use ParallelLoader, enabling pipelined loading via a new VLLM_FASTSAFETENSORS_QUEUE_SIZE environment variable. The changes include updated tests and a fallback mechanism for systems without GPUDirect Storage. Feedback was provided to ensure that progress bars are only displayed on the lead rank in distributed environments to prevent log clutter.

[gitbisector]

@mgoin, this switches vLLM's fastsafetensors path from a manual per-shard loop to ParallelLoader, routing through the copier registry so copier improvements get picked up automatically. Small and self-contained (+57/−56, 3 files); the existing correctness test is parametrized over queue_size=[0,1] so both paths run against the reference iterator.

It's also a natural prerequisite for #29410, once loading goes through ParallelLoader, making fastsafetensors the default is a smaller step. Already ~10% faster on currently-released fastsafetensors (75 GB model on DGX Spark, no VRAM regression), and it sets up the bigger loader speedups from newer fastsafetensors releases.

CI hasn't run since there's no ready label — could you add it and take a look when you have a moment? Happy to adjust.

[mergify]

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

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, @gitbisector.

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

[mergify]

Hi @gitbisector, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

[DarkLight1337]

Thanks

[gitbisector]

Fixed the Model Executor failure: the ParallelLoader switch left the GDS→nogds fallback only around construction. Now wraps iteration too, falling back only before the first tensor is yielded. Synced with main. That amd-multi-modal-models-standard-2-qwen3-plus-gemm failure might be due to #29520 (failing in other recent main builds).