feat(runtime): add Velo TCP and UCX response transport by jthomson04 · Pull Request #15231 · ai-dynamo/dynamo

jthomson04 · 2026-09-23T19:17:48Z

Summary

Add an optional Velo response plane with TCP and UCX/RDMA support. Velo handles batching, flow control, and stream lifecycle; Dynamo preserves typed responses, errors, cancellation, and shared Runtime ownership. The default response transport remains TCP.

DYN_RESPONSE_PLANE=velo selects Velo.
DYN_VELO_RESPONSE_TRANSPORT=tcp|ucx selects the transport. UCX requires the velo-ucx build feature.
DYN_VELO_RESPONSE_BATCH_MS defaults to 5 ms. Set 0 for greedy flushing or a positive integer for another maximum delay. Invalid values fail at startup.

Use the published Velo 0.19.0 and velo-ext 0.5.5 releases, including the listener shutdown fix. Frontends and workers must use matching response protocol versions.

Performance

Tyche, 2,048 speedup-10 mock workers, two NUMA frontends, and concurrency 6,144. One measured run per arm using repository-default release builds.

Metric	QUIC	Velo TCP · 5 ms	Velo RDMA · 5 ms	Dynamo TCP
Successful requests/s	4,794	5,011	5,044	1,847
Output tokens/s	4,407,661	4,598,940	4,631,254	1,730,522
Frontend CPU ms/successful request	26.445	26.766	26.730	27.884
Frontend CPU µs/output token	28.761	29.162	29.111	29.757
Host softirq cores	15.105	14.845	17.788	61.871
Physical RX+TX packets/output token	1.244	1.242	1.199	2.054
p99 TTFT (ms)	311.209	262.100	284.323	6,174.886
p99 request latency (ms)	12,921.576	12,378.099	11,974.767	11,812.401
p99 per-request mean ITL (ms)	6.562	3.104	3.073	2.363

Both 5 ms Velo arms meet the agreed 5% limits against QUIC in this directional comparison. The Velo runs used different nodes from the references, and clients were nearly saturated. Dynamo TCP had 30,000 HTTP 504 errors (8.22%), which affects its comparison. Packet counts cover total frontend traffic.

Measurements used Dynamo c71acb2af72 with Velo 18f51dba95687dc5408d960f90e134b013c2fb12 and benchmark-only ZMQ socket-capacity patch; the Velo runs added the fixed 5 ms policy now exposed by the environment setting. Full exports were retained, including client drain-credit warnings. Benchmark files remain local.

Validation

Local Runtime, response lifecycle, UCX-over-TCP, and Python configuration tests passed, with Clippy and formatting. Two-node Tyche checks passed on the equivalent Velo Git revision (ef29a4e7) for TCP and RDMA (rc_mlx5), including producer cancellation and clean shutdown after request drain.

devin-ai-integration · 2026-10-08T00:38:26Z

✅ Dynamo PR CI passed — run 38508626853 (attempt 1) on `cbae3d88a9`

Gate checks: ✅ backend-status-check · ✅ deploy-status-check · ✅ dynamo-status-check

_{Posted automatically by Devin for run 38508626853. Updated on every full-CI run of this PR.}

coderabbitai · 2026-10-08T17:07:32Z

Walkthrough

The pull request adds Velo as a response-plane option. It implements a Velo response service and streaming protocol, connects them to runtime and router lifecycle operations, and updates configuration entry points and tests.

Changes

Velo response plane

Layer / File(s)	Summary
Velo dependency and response-plane contract `Cargo.toml`, `lib/runtime/Cargo.toml`, `lib/bindings/python/Cargo.toml`, `lib/kvbm-*/Cargo.toml`, `lib/runtime/src/config/environment_names.rs`, `lib/runtime/src/pipeline/network.rs`, `lib/bindings/python/rust/lib.rs`, `lib/bindings/python/src/dynamo/_core.pyi`	The workspace pins Velo to a Git revision and enables the required features. Runtime mode parsing, transport lookup, environment configuration, and Python bindings recognize Velo.
Velo service ownership and registration `lib/runtime/src/pipeline/network/velo_response.rs`, `lib/runtime/src/runtime.rs`, `lib/runtime/src/distributed.rs`, `lib/runtime/src/pipeline/network.rs`	The Velo response service configures transports, manages shared runtime ownership, registers streams, and tracks endpoint-instance cancellation. Runtime and ingress code acquire the service and shut it down.
Velo stream protocol and router integration `lib/runtime/src/pipeline/network/velo_response.rs`, `lib/runtime/src/pipeline/network/egress/addressed_router.rs`, `lib/runtime/src/pipeline/network/ingress/push_handler.rs`, `lib/runtime/src/pipeline/network.rs`, `lib/runtime/src/pipeline/network/{quic_response.rs,tcp/*}`	The Velo sender and receiver handle response frames and stream lifecycle. The addressed router and ingress publisher use Velo for response streams. TCP and QUIC receiver construction uses the shared receiver type.
Velo selection in runtime and CLI entry points `components/src/dynamo/common/backend/`, `components/src/dynamo/common/configuration/groups/runtime_args.py`, `components/src/dynamo/common/utils/runtime.py`, `components/src/dynamo/frontend/frontend_args.py`, `components/src/dynamo/mocker/`, `components/src/dynamo/common/tests/configuration/*`, `components/src/dynamo/mocker/tests/unit/test_config.py`	Runtime validation and backend, frontend, and mocker argument parsing accept Velo. Configuration tests check that the option parses to `velo`.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~45 minutes

Merge Risk: 🟡 Moderate · up to 0a579

When the Velo response plane is selected, a worker with a misconfigured Velo transport still registers its endpoints. Requests sent to it can then stall instead of failing quickly. Initialize the Velo service at startup before merging. Setup failures should also be counted in the response-stream error metrics. The default TCP path is unaffected.

Pre-merge checks | 4 | 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 22.11% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 95 functions across 22 files. (6 skipped:…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Title check		The title clearly and concisely describes the main change: adding Velo TCP and UCX response transport support to the runtime.
Description check		The description provides an overview, implementation details, performance results, validation results, and the required issue reference. It does not use the template headings for reviewer starting poi…

Full details: Docstring Coverage

Explanation

Docstring coverage is 22.11% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 95 functions across 22 files. (6 skipped: 6 unsupported.)

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Actionable comments posted: 2

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Inline comments:
Review comments at @lib/runtime/src/distributed.rs:
- Around line 353-357: When ResponsePlaneMode::Velo is selected during startup,
initialize the service by awaiting distributed_runtime.velo_response_service()
and propagate initialization errors before the worker handles requests. Keep the
existing register_metrics call in the Velo startup branch.

Review comments at @lib/runtime/src/pipeline/network/ingress/push_handler.rs:
- Around line 955-968: Update the Velo sender-setup error handling in the
response-stream path to increment the RESPONSE_STREAM error counter when
`service.sender(...)` fails, matching the TCP and QUIC branches. Also count
failures from `self.velo_response_service()` before propagating them, so both
Velo setup failure points are reflected in worker metrics.

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📥 Commits

Reviewing files that changed from the base of the PR and between 5cbdc71 and 0a579b9.

⛔ Files ignored due to path filters (4)

Cargo.lock is excluded by !**/*.lock
lib/bindings/kvbm/Cargo.lock is excluded by !**/*.lock
lib/bindings/python/Cargo.lock is excluded by !**/*.lock
lib/runtime/examples/Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (28)

Cargo.toml
components/src/dynamo/common/backend/sample_diffusion_engine.py
components/src/dynamo/common/backend/sample_engine.py
components/src/dynamo/common/backend/worker.py
components/src/dynamo/common/configuration/groups/runtime_args.py
components/src/dynamo/common/tests/configuration/test_kv_router_args.py
components/src/dynamo/common/tests/configuration/test_runtime_args.py
components/src/dynamo/common/utils/runtime.py
components/src/dynamo/frontend/frontend_args.py
components/src/dynamo/mocker/args.py
components/src/dynamo/mocker/tests/unit/test_config.py
lib/bindings/python/Cargo.toml
lib/bindings/python/rust/lib.rs
lib/bindings/python/src/dynamo/_core.pyi
lib/kvbm-config/Cargo.toml
lib/kvbm-engine/Cargo.toml
lib/kvbm-physical/Cargo.toml
lib/runtime/Cargo.toml
lib/runtime/src/config/environment_names.rs
lib/runtime/src/distributed.rs
lib/runtime/src/pipeline/network.rs
lib/runtime/src/pipeline/network/egress/addressed_router.rs
lib/runtime/src/pipeline/network/ingress/push_handler.rs
lib/runtime/src/pipeline/network/quic_response.rs
lib/runtime/src/pipeline/network/tcp/client.rs
lib/runtime/src/pipeline/network/tcp/server.rs
lib/runtime/src/pipeline/network/velo_response.rs
lib/runtime/src/runtime.rs

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jthomson04

Review of 0a579b9d. The typed-prologue decoding issue (velo_response.rs:125) is confirmed: I reproduced it with a standalone rmp_serde round-trip. The other comments come from reading the code.

One finding falls outside the diff:

lib/backend-common/src/worker.rs still rejects velo. RuntimeConfig accepts only tcp|quic for DYN_RESPONSE_PLANE, both in the clap value_parser (L84) and in to_distributed_config (L115-L119). With DYN_RESPONSE_PLANE=velo set cluster-wide, every binary that flattens CommonArgs (the TRT-LLM/SGLang/vLLM sidecars and the backend-common mocker) fails at argument parsing with invalid value 'velo' for '--response-plane'. Passing response_plane=Some("velo") fails with expected 'tcp' or 'quic'.

Signed-off-by: jthomson04 <jwillthomson19@gmail.com>

…w of 32 The Velo response sender called `register_peer` and `wait_for_handler(peer, "_stream_stop")` for every request. In the pinned velo, `wait_for_handler` refreshes the handler list with a `_hello` round trip through the frontend's messenger. That put a control-plane round trip through the frontend in front of every request's first token. The service now keeps the set of frontends it has prepared and does both steps once per frontend. An instance id names one process, so a restarted frontend is prepared again. Response streams now use a per-stream credit window of 32, set explicitly. When the frontend is the bottleneck, every stream runs at its window, and a larger window fills the shared path from a worker to the frontend, so a new stream's first token waits behind all of it. On the mocker rig with a saturated 24-core frontend, a window of 256 put TTFT p50 at 135 to 355 ms, and 32 put it at 88 to 102 ms. Throughput was within noise, and ITL p99 was no higher. Signed-off-by: Ryan Olson <rolson@nvidia.com> (cherry picked from commit 18a7697) Signed-off-by: jthomson04 <jwillthomson19@gmail.com>

The TCP response transport is built with lanes(4). Each lane is its own TCP connection, read by its own task on the frontend, and the velo mux spreads the response streams over the lanes. One connection is limited by the task that reads it. On the two-node mocker rig (8 x 64 workers, concurrency 8,192, ISL 1,024, OSL 900), 4 lanes against 1, both on velo 0.18.0: 2,406 against 1,359 requests/s, ITL p99 4.9 against 15.0 ms, end-to-end p99 4.5 against 13.5 s, and 20.0 against 37.4 ms of frontend CPU per request. tcp_responses_use_four_lanes builds the transport and checks its lane count; it fails when the lanes() call is removed. UCX has no lanes and is unchanged. Signed-off-by: Ryan Olson <rolson@nvidia.com> (cherry picked from commit e55a5c5) Signed-off-by: jthomson04 <jwillthomson19@gmail.com>

Adapted from 0b4ecc6 in PR #15320. Preserve main's endpoint response-plane binding. Signed-off-by: jthomson04 <jwillthomson19@gmail.com>

Signed-off-by: jthomson04 <jwillthomson19@gmail.com>

Encode response prologues as JSON bytes inside the typed Velo frame and require protocol v3. Preserve cancellation deferral, terminal error checks, and worker association semantics. Separate finalized cleanup from cancellation and correct response setup and cancellation metrics. Signed-off-by: jthomson04 <jwillthomson19@gmail.com>

Create the shared Velo service during distributed runtime startup and accept velo in the shared Rust backend configuration. Check invalid transport settings, missing UCX support, listener construction failure, and CLI-to-runtime selection. Signed-off-by: jthomson04 <jwillthomson19@gmail.com>

Signed-off-by: jthomson04 <jwillthomson19@gmail.com>

jthomson04 · 2026-10-13T16:57:31Z

/ok to test 662f23d

Runtime shutdown awaited the Velo response service with no overall bound. A hung transport close or a panic during Velo teardown kept Phase 3 from cancelling the main token, leaving NATS/etcd connected and the process alive. Run the teardown in its own task with a 10 s timeout and log failures before proceeding. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: jthomson04 <jwillthomson19@gmail.com>

prepare_peer took a shard write lock via entry() on every request, so all requests from one frontend serialized on the same shard. Check for an initialized cell under a read lock first. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: jthomson04 <jwillthomson19@gmail.com>

jthomson04 · 2026-10-13T17:11:09Z

/ok to test cacab67

…ation Signed-off-by: jthomson04 <jwillthomson19@gmail.com>

pull-request-size Bot added the size/XXL label Sep 23, 2026

github-actions Bot added feat documentation Improvements or additions to documentation frontend `python -m dynamo.frontend` and `dynamo-run in=http|text|grpc` labels Sep 23, 2026

jthomson04 force-pushed the jthomson04/velo-response branch from c973431 to e2c45cf Compare September 23, 2026 19:42

jthomson04 mentioned this pull request Sep 24, 2026

feat(streaming): add native stop and ticket lifecycle ai-dynamo/velo#90

Merged

ryanolson mentioned this pull request Sep 28, 2026

fix(runtime): prepare Velo peers once and close owned response services #15320

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ryanolson added this pull request to stack #15364 September 29, 2026 01:58

pull-request-size Bot added size/XL and removed size/XXL labels Oct 7, 2026

jthomson04 force-pushed the jthomson04/velo-response branch from 3c0f506 to 0a579b9 Compare October 8, 2026 16:48

pull-request-size Bot added size/XXL and removed size/XL labels Oct 8, 2026

jthomson04 marked this pull request as ready for review October 8, 2026 16:59

jthomson04 requested review from a team as code owners October 8, 2026 16:59

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jthomson04 commented Oct 8, 2026

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This was referenced Oct 9, 2026

feat(runtime): add Velo TCP and UCX response transport with configurable batching #16026

Open

fix(tcp,uds): wait for the accept loop during shutdown ai-dynamo/velo#121

Merged

jthomson04 force-pushed the jthomson04/velo-response branch from c7117ae to a73598b Compare October 10, 2026 04:12

jthomson04 and others added 2 commits October 13, 2026 09:49

feat(runtime): add standalone Velo response transport

8a82df5

Signed-off-by: jthomson04 <jwillthomson19@gmail.com>

ryanolson and others added 9 commits October 13, 2026 09:49

fix(runtime): own and shut down mux-only Velo response services

a11000a

Adapted from 0b4ecc6 in PR #15320. Preserve main's endpoint response-plane binding. Signed-off-by: jthomson04 <jwillthomson19@gmail.com>

build: pin merged Velo 0.19 and update all lockfiles

bbc77b5

Signed-off-by: jthomson04 <jwillthomson19@gmail.com>

test(runtime): keep observable Velo peer preparation coverage

04ff197

Signed-off-by: jthomson04 <jwillthomson19@gmail.com>

feat(runtime): configure Velo response batching with a 5ms default

7e6af3b

Signed-off-by: jthomson04 <jwillthomson19@gmail.com>

fix(build): pin Velo listener shutdown fix

ffe879f

Signed-off-by: jthomson04 <jwillthomson19@gmail.com>

build: use published Velo 0.19 crates

662f23d

Signed-off-by: jthomson04 <jwillthomson19@gmail.com>

jthomson04 force-pushed the jthomson04/velo-response branch from 77358c7 to 662f23d Compare October 13, 2026 16:54

jthomson04 and others added 2 commits October 13, 2026 10:10

jthomson04 mentioned this pull request Oct 13, 2026

Velo response plane follow-ups #16316

Open

4 of 7 tasks

spa5k mentioned this pull request Oct 13, 2026

TCP registration repeatedly scans unexpired worker-removal records #16340

Open

test(runtime): trim redundant Velo checks and synchronize stream isol…

cbae3d8

…ation Signed-off-by: jthomson04 <jwillthomson19@gmail.com>

jthomson04 mentioned this pull request Oct 14, 2026

fix(runtime): Velo response plane follow-ups #16396

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feat(runtime): add Velo TCP and UCX response transport - #15231

feat(runtime): add Velo TCP and UCX response transport#15231
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