[KV Connector][NIXL] Coalesce host-buffer KV copies across cache groups

[beenpow]

Purpose

Addresses the coalescing item on the NIXL P/D roadmap (#33702) and the TODO it
came from, in base_worker.py:

# TODO (NickLucche) D2H<>H2D ops could benefit from coalescing io across groups

When NIXL cannot register device memory (TPU, XPU, or kv_buffer_device=cpu on
CUDA), KV moves through a CPU staging buffer. Both directions issued one copy per
KV cache group:

for group_block_ids in local_block_ids:
    self.copy_blocks(src, dst, group_block_ids, group_block_ids, "h2d")

copy_kv_blocks applies the given ids to every layer, so this costs
groups x layers copy operations. All KV cache groups allocate from the same
BlockPool, so block ids are unique across groups and the lists can be
concatenated into a single call: the same rows are written to the same layers,
and the launch count drops to layers.

The bytes copied are unchanged. This is a launch-overhead change, not a bandwidth
one, and it only matters for models with more than one KV cache group.

Applies to both sync_recved_kv_to_device (h2d, decode side) and
save_kv_to_host (d2h, prefill side), which share the same loop. The TODO names
both directions.

Not a duplicate. No issue or PR tracks this item. Checked with:

gh issue view 33702 --repo vllm-project/vllm --comments
gh pr list --repo vllm-project/vllm --state open --search "33702 in:body"
gh pr list --repo vllm-project/vllm --state all --search "coalescing io across groups"
gh pr list --repo vllm-project/vllm --state all --search "copy_blocks groups nixl host buffer"
gh pr list --repo vllm-project/vllm --state all --search "sync_recved_kv_to_device"
gh pr list --repo vllm-project/vllm --state all --search "save_kv_to_host"

# and, for each open PR that touches nixl/base_worker.py, whether it edits these functions
gh api "repos/vllm-project/vllm/pulls/$PR/files?per_page=100" \
  --jq '.[] | select(.filename|test("nixl/base_worker.py")) | .patch' \
  | grep -E "^[+-].*(sync_recved_kv_to_device|save_kv_to_host|copy_blocks)"

main still carries the per-group loop and the TODO comment, so nothing has
landed for this item.

Scope note: this coalesces across groups only, as the TODO says. save_kv_to_host
still iterates requests, and requests are not merged.

The gpu_sync_allowed() marker from #43107 is left as it is. Merging calls does
not change whether the copies are synchronous, and it reduces the number of syncs
rather than adding any.

Test Plan

# unit
.venv/bin/python -m pytest tests/v1/kv_connector/unit/test_nixl_connector_hma.py \
  -m cpu_test -q

# regression across the connector suite
.venv/bin/python -m pytest tests/v1/kv_connector/unit/ -m cpu_test -q

# lint
.venv/bin/pre-commit run --files \
  vllm/distributed/kv_transfer/kv_connector/v1/nixl/base_worker.py \
  tests/v1/kv_connector/unit/test_nixl_connector_hma.py \
  benchmarks/kernels/benchmark_host_buffer_kv_copy.py

# benchmark, RTX 4090 (timings and device-op counts)
.venv/bin/python benchmarks/kernels/benchmark_host_buffer_kv_copy.py \
  --num-layers 16 --blocks-per-request 6 --iters 200
.venv/bin/python benchmarks/kernels/benchmark_host_buffer_kv_copy.py \
  --num-layers 16 --blocks-per-request 6 --iters 20 --op-counts

--blocks-per-request 6 keeps the run in the regime this change acts on. At the
default (60) each sync moves ~31 MB and bandwidth dominates, leaving launch
overhead as a small fraction of the total.

The new test fails on main for the multi-group and empty-group cases (three and
two calls respectively, where one is expected) and passes with this change.

Test Result

Unit and regression, rebased onto 5707355209:

tests/v1/kv_connector/unit/test_nixl_connector_hma.py    76 passed, 1 deselected
tests/v1/kv_connector/unit/                             366 passed, 2 skipped

Lint: all pre-commit hooks pass, including ruff, mypy, and the torch.cuda
API check.

Latency per request-sync, minimum of 200 runs, RTX 4090:

KV cache groups	direction	before (us)	after (us)	saved
1 (control)	h2d	1330	1337	-0.5%
1 (control)	d2h	1405	1400	+0.3%
2	h2d	2035	1306	35.8%
2	d2h	2143	1311	38.8%
3	h2d	2802	1259	55.1%
3	d2h	3178	1342	57.8%
6	h2d	4953	1274	74.3%
6	d2h	5978	1381	76.9%

The "before" column grows with the group count while "after" stays flat, which is
the point of the change: one call regardless of how many groups there are.

One group is the control: both paths run identical code there, so the delta must
be zero, and it measures within 0.5%.

Device-op counts per request-sync, --num-layers 16, from torch.profiler:

KV cache groups	operation	before	after
2	Memcpy HtoD (Pageable -> Device)	32	16
3	Memcpy HtoD (Pageable -> Device)	48	16
6	Memcpy HtoD (Pageable -> Device)	96	16
6	Memcpy DtoH (Device -> Pageable)	96	16
6	Memcpy HtoD (Pinned -> Device) (index upload)	6	1
6	gather / index CUDA kernels	96	16

Exactly groups x layers -> layers, with index uploads going from one per group
to one.

No model evaluation is included: the change copies the same bytes from the same
source rows to the same destination rows, so decoding output is unaffected.

---

AI assistance was used for this change (see the Assisted-by trailer on the
commit). I have reviewed every changed line and run the tests above myself.

[claude]

Claude Code Review

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[beenpow]

@NickLucche this is the roadmap item you left the TODO for:

[ ] More efficient h2d copy_blocks operations for HMA groups

Opened #54483. With 16 layers and 6 groups the Memcpy count per request-sync goes from 96 to 16.

Happy to adjust the scope if this isn't the direction you had in mind.

[xuechendi]

LGTM, my only concern is this PR brings up memory utilization
wondering what is the worst case you saw, how many groups here might be?

[beenpow]

@xuechendi

Thanks for the review!

Measured on a real P/D run (kv_buffer_device=cpu) with gemma-3-1b, 7 KV cache groups (4 full + 22 sliding-window layers). Peak GPU memory in sync_recved_kv_to_device, before → after:

prompt	blocks per group	before	after	ratio
512 tok	[32×7]	512 KiB	4.0 MiB	8.0×
2K	[32×6, 97]	1.8 MiB	4.8 MiB	2.7×
8K	[32×6, 481]	7.5 MiB	10.5 MiB	1.4×

• after = total blocks of the request × per-layer block bytes, so it's bounded by the request's own KV footprint regardless of group count
• outputs identical before/after
• worst group count I found in-tree is 7 (Gemma-3); typical hybrids are 2

Let me know if there's anything else you'd like me to check.

[xuechendi]

I see, data makes sense to me, Thanks

[xuechendi]

LGTM

[beenpow]

/ci run

[github-actions]

❌ This PR is 1320 commits behind upstream main. Your branch must contain every commit currently on upstream main. No new CI build was started. Merge or rebase onto the latest main, then rerun /ci run. To test this branch at your own risk, use /ci run --allow-stale.

[beenpow]

/ci run

[github-actions]

✅ Triggered Buildkite CI #91076 for commit ddb1fedf975b.

[beenpow]

@NickLucche This has Chendi's approval and green CI.
Could you merge when you get a chance? Thanks!!

[mergify]

Hi @beenpow, 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.

[beenpow]

/ci run

[github-actions]

❌ This PR is 6 commits behind upstream main. Your branch must contain every commit currently on upstream main. No new CI build was started. Merge or rebase onto the latest main, then rerun /ci run. To test this branch at your own risk, use /ci run --allow-stale.

[beenpow]

/ci run

[github-actions]

✅ Triggered Buildkite CI #92261 for commit 13cf1b1d57ea.

[beenpow]

The v1-executor-plus-worker failures are unrelated to this PR: test_eagle3_aux_hidden_states_pp.py is currently broken on main by #58648 (fix in #59508). This PR only inherited it by merging main. I'll update and re-run CI once the fix lands.

[beenpow]

/ci run

[github-actions]

❌ This PR is 1 commit behind upstream main. Your branch must contain every commit currently on upstream main. No new CI build was started. Merge or rebase onto the latest main, then rerun /ci run. To test this branch at your own risk, use /ci run --allow-stale.

[beenpow]

/ci run

[github-actions]

✅ Triggered Buildkite CI #92301 for commit 0fc59f8d43ff.

[beenpow]

@xuechendi The earlier CI failures were from an unrelated test on main (fixed in #59508, not this PR). After the branch update CI is green. Could you merge when you get a chance? Thanks!