[Core][Perf] Prefer contiguous KV block runs for disaggregated transfer

[anish-shanbhag]

Overview

Prefer physically contiguous KV block-ID runs for large allocations so disaggregated KV transfers can coalesce more NIXL descriptors. The policy is opt-in with VLLM_KV_CONTIG_ALLOC=1; the default allocator remains unchanged.

The motivating workload is MiniMax-M3-NVFP4 with 128-token KV blocks, one TP2 prefill replica, three TP2 decode replicas, and 60 concurrent long-context agentic sessions.

Claims

• For allocations of at least eight blocks, the opt-in policy searches a bounded 4 * num_blocks prefix of the free queue, takes the longest queue-adjacent physical-ID runs in ascending order, then fills any remainder in normal eviction order.
• It changes only which already-free physical block IDs are returned. Reference counts, prefix-cache eviction, hashes, metrics, and allocation size retain their existing paths.
• The default-off path and allocations below eight blocks continue to use popleft_n directly.
• Blocks released by the current unpin_blocks API remain last-resort candidates and are excluded from contiguous-run selection; they are reused only by the ordinary fallback when necessary.

Author-provided end-to-end measurements against the same source base and serving configuration showed:

Run	p90 output tok/s/user	Total tok/s/GPU	Delta
Control	123.591	47,289.91	--
Contiguous allocation A	129.297	49,698.86	+4.62% / +5.09%
Contiguous allocation B	127.553	48,828.00	+3.21% / +3.25%

A representative per-rank READ reduced native descriptors from 23,252 to 8,926-10,222, descriptor posting time from 79.6 ms to about 32 ms, and transfer time from 117 ms to about 53 ms. Results were workload-dependent: at 48 sessions the change was +1.58% / +0.43%, and a heterogeneous TP2-prefill/TP4-decode point was within run-to-run variation at +1.22% / +0.29%. This variability and the bounded scan/LRU tradeoff are why the policy is not the default.

Validation

Full pr-checklist audit: read the condensed report.

Before the rebase, the original eight focused unit tests passed:

VLLM_TARGET_DEVICE=cpu .venv/bin/python -m pytest -q \
  tests/v1/core/test_block_pool_contig.py
# 8 passed in 0.85s

After rebasing onto current main, the suite gained a ninth regression test for the new unpin_blocks last-resort contract. All changed-file pre-commit hooks passed:

.venv/bin/pre-commit run --files \
  tests/v1/core/test_block_pool_contig.py \
  vllm/envs.py \
  vllm/v1/core/block_pool.py

This included Ruff check/format, mypy, Buildkite test tethering, SPDX, forbidden-import, root-lazy-import, and environment-default validation.

The post-rebase suite was run on exact head 0f79acbc62e6339e141f4ef72e88d455bf21520b in a supported Linux ARM64 CPU environment (Python 3.12.14, PyTorch 2.13.0+cpu):

VLLM_TARGET_DEVICE=cpu python -m pytest -q \
  tests/v1/core/test_block_pool_contig.py
# 9 passed, 15 warnings in 2.14s

All nine tests passed, including the rebase-driven last-resort reuse regression. The warnings were one source-tree version-metadata warning and 14 unrelated PyTorch torch.jit.script_method deprecation warnings.

The full-duration treatment run completed 9,591 requests over 3,629 seconds with no recorded request errors, cancellations, or output-length mismatches. A model-quality evaluation was not run because the change affects physical free-block selection, not cached values, model computation, or sampling.

Details

Implementation and tradeoffs

The bounded window preserves approximate eviction age rather than globally optimizing packing. Within that window, selecting longer runs can bypass older free blocks; if useful runs do not satisfy the request, allocation resumes from the queue head. Descending runs are normalized because transfer coalescing requires matching address direction at both endpoints.

The implementation adds 52 non-test lines to the core allocator and isolates the selection logic in one helper. The environment variable is registered in vllm.envs and excluded from torch.compile factors because it changes allocator policy rather than generated code.

Duplicate-work check

Open-PR searches for contiguous KV allocation, physical block-ID runs, NIXL descriptor merging, and BlockPool changes found no overlapping allocator optimization. Draft PR #32290 deliberately emulates strict contiguous-allocation fragmentation by delaying work when a contiguous range is unavailable; this PR instead performs a best-effort bounded preference with ordinary fallback. Connector packing/range-loading PRs operate at the transfer or layout layer rather than changing BlockPool.get_new_blocks().

AI assistance

OpenAI Codex assisted with the rebase, compatibility audit, test addition, validation, and description update. The human author supplied the optimization direction and measurement evidence and must review the rebased diff before requesting maintainer attention.

Checklist

• Purpose, target workload, and performance evidence are documented.
• Default behavior, compatibility, tradeoffs, and fallback behavior are explicit.
• Focused regression coverage includes run ordering, fallback, queue integrity, bounded search, environment activation, and last-resort unpinned blocks.
• Changed-file pre-commit hooks pass on the rebased commit.
• Duplicate-work search and AI assistance are documented.
• No model evaluation is required because model outputs and computation are unchanged.
• The post-rebase nine-test suite passes on a supported Linux CPU environment.
• Human author has reviewed every changed line.

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[mergify]

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

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