docs(refit): design the NCCL M2N collective refit path - #659
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Specifies a push-style collective refit data plane built on nccl.m2n.reshard, standalone from the NIXL pull path: separate proto service, separate Rust module, separate client package, no shared types. Follows the V2 RL Refit API design section 8.1 (MX owns an ephemeral collective transfer group per operation; workers join, never create; the trainer launches only once MX reports READY) and the two-sided API layering doc (RefitClient.Trainer/.Generator over a pluggable Publisher/Loader and a ShardRedistribution Backend Sender/Receiver). Adopts NeMo RL's merged nccl_reshard_refit wire contract (PR #2971): HF-named parameters with global shapes, DTensor Shard/Replicate placements over rank meshes, per-PP-stage communicators with trainer ranks first, and the bulk/misc split. MX replaces the TCPStore rendezvous with control-plane-brokered rank assignment, admission, worker-generation fencing, and an observable READY state. Signed-off-by: Yixin Huang <yixinh@nvidia.com>
Only the trainer that MX ranks first in a lane generates and posts that lane's ncclUniqueId; the inbound arrow on the generator column implied generators publish it too. Signed-off-by: Yixin Huang <yixinh@nvidia.com>
The repository is public. Restate the requirements this path implements in plain terms instead of quoting internal design-doc sections, and remove the section and requirement identifiers that only resolve against those documents. The technical content is unchanged. Also commit PNG renders of the three figures. GitHub renders SVG inside a markdown file but strips it from pull-request and issue bodies, so the PNGs are what make the figures visible in a PR description. Signed-off-by: Yixin Huang <yixinh@nvidia.com>
docs/images/ is PNG and JPG throughout; the SVGs in the refit package live in a different directory. Ship the three figures as PNG only rather than carrying each one twice. Signed-off-by: Yixin Huang <yixinh@nvidia.com>
Bring the design doc in line with the other docs under docs/ and with the refit package README: - drop the numbered top-level and lettered sub-sections; no other document numbers a design this way - open with a title and an orientation paragraph that cross-links the sibling documents, as every other page under docs/ does - use the > [!IMPORTANT] callout for not-yet-real status, matching the refit README's use of it for the same purpose - expand acronyms on first use throughout, as the refit README does - replace section-mark cross-references with anchor links - close with Related documentation Also link the design from the refit README so it is reachable from the pull path it is a sibling of. Signed-off-by: Yixin Huang <yixinh@nvidia.com>
Linking a design that has no implementation from the pull path's README widens this change into a file it does not need to touch. The link belongs in the implementation change, once there is something to link to. Signed-off-by: Yixin Huang <yixinh@nvidia.com>
The plan derivation carried assumptions from one trainer and one inference engine into what is meant to be the shared core: a hardcoded bulk-parameter whitelist, a hardcoded device-mesh ordering, and shard dimensions inferred by matching parameter names. A trainer that lays its ranks out differently would have got wrong meshes with no error, only wrong bytes. Invert it. The Publisher declares each parameter's canonical name, global shape, mesh, placements, partition and bulk-eligibility; the shared core stops inferring any of it. The prior behaviour ships as an opt-in default derivation so the common case stays cheap. Generalize reshard lanes from trainer pipeline stages to disjoint source partitions, with pipeline stages as the common instance, and explain why partitioning beats one global communicator: a collective requires every member to enter every operation, so one communicator makes each parameter a fleet-wide barrier. Record the first Publisher's and Loader's restrictions as limitations with an owner, rather than leaving them implicit. Signed-off-by: Yixin Huang <yixinh@nvidia.com>
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August 19, 2026 17:11
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WalkthroughThe PR adds ChangesNCCL M2N refit design
Estimated code review effort: 2 (Simple) | ~15 minutes Merge Risk: 🟡 Moderate · up to This documentation-only PR adds a proposed NCCL M2N refit design without changing production behavior, but it leaves several concrete correctness and failure-handling contracts unspecified, including stale-epoch protection, complete weight coverage, transfer ordering, retries, and timeout recovery. The design is not ready to guide implementation until these issues are resolved or explicitly accepted by the owners. Poem
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In `@docs/NCCL_M2N_REFIT.md`:
- Around line 287-301: Update RefitCollectiveService transfer RPC definitions so
CreateCollectiveTransfer and ReportCollectiveTransfer carry an operation
identity and support idempotent handling. Define request-level atomic validation
for operation_ref, group_id, epoch, and the admitted worker_id generation,
reusing the existing request or membership symbols where applicable; ensure
retries do not create duplicates and stale reports are rejected.
- Around line 267-301: Define how CollectiveGroup resources are reclaimed
despite the state machine’s DeleteCollectiveGroup transition: specify an
automatic TTL janitor, including its expiry and fencing behavior, or add a
corresponding RefitCollectiveService deletion RPC with explicit fencing
semantics. Align the resource reuse rules with the chosen reclamation mechanism.
- Around line 351-376: Update the declared plan contract to require
Publisher-side validation that the union of bulk parameters and the ordered misc
list covers every canonical parameter exactly once, rejecting missing or
duplicate entries. Include the validated coverage result in the plan digest and
perform this validation before any wire operation.
- Around line 6-18: Replace the new Markdown design document with the
repository-approved documentation format, preserving its NCCL M2N refit
architecture and implementation decisions; alternatively, obtain and record an
explicit exception if Markdown is required.
- Around line 240-248: Replace the ASCII rank-assignment block with a Mermaid
flowchart or state diagram, preserving the reshard and broadcast lane world-size
formulas and trainer/generator rank_in_lane mappings exactly.
- Around line 280-285: Update the group bootstrap flow so NCCL unique IDs are
scoped to the current epoch: make PublishGroupBootstrap target (group_id, epoch,
lane_id), atomically clear prior epoch identifiers when advancing the epoch, and
reject publications or readiness checks whose epoch does not match the active
group epoch.
- Around line 324-329: The group identity used by the later key construction
must include the canonical plan/layout digest, not only model name, trainer
topology, and admitted generator set. Update the relevant group-key logic and
plan validation so a changed digest cannot reuse cached metadata and is rejected
before READY.
- Around line 442-450: Update the NCCL M2N sequencing documentation so the misc
broadcast is a single refit-level phase after all bulk reshard lanes complete,
rather than part of each publish_weights/update_weights call. Move its placement
to finish_weight_update, preserving the requirement that every bulk transfer has
drained before the broadcast runs.
- Around line 474-502: Extend the NCCL refit design to define bounded deadlines
and failure handling after READY for Communicator.init, nccl.m2n.reshard, packed
broadcast, and final stream synchronization. Specify how communicator abort or
revocation occurs, how failures are reported through the existing collective
status flow, and how aborted communicators are prevented from reuse; add
configuration entries for any new setup or transfer timeouts alongside
MX_NCCL_REFIT_GROUP_TIMEOUT_S.
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Eight fixes, most of them cases where the document contradicted itself or left a silent-failure path undefined: - scope the NCCL bootstrap identifier to the current epoch. A group outlives a membership, so a leftover identifier described a communicator with the wrong world size and every rank would block on it forever. - make the plan digest part of the group identity. Membership could stay constant while the plan changed, and generators would keep a cached plan for a model that no longer existed. - require the Publisher to prove that bulk and misc together name every parameter exactly once. A gap left a stale value in place and still reported success; an overlap applied a parameter twice. - move the misc broadcast to one refit-level phase in finish_weight_update. Running it per layer group could enter the all-ranks communicator while another group was still resharding, which is the deadlock the document warns about two sections earlier. - bound Communicator.init, the reshards, the broadcast and the final stream sync, and define communicator abort. Only group formation had a deadline, so a failure after READY could block peers indefinitely. - fence CreateCollectiveTransfer on an idempotency key and ReportCollectiveTransfer on (operation, group, epoch, worker generation). - define automatic group reclamation; the state machine referenced a DeleteCollectiveGroup RPC that the service never exposed. - replace the spatially-formatted rank-assignment block with a table. Signed-off-by: Yixin Huang <yixinh@nvidia.com>
The design said the plan digest was part of the group's identity; the implementation carries it on the group and bumps the epoch when it changes. Both close the stale-plan hole, but folding the digest into the group id strands the previous group as an orphan on every plan change, while the epoch already means 'same participants, invalidated caches' -- which is exactly what a plan change is. Describe the epoch behaviour, and say why identity deliberately excludes the digest. Signed-off-by: Yixin Huang <yixinh@nvidia.com>
This was referenced Aug 19, 2026
Implementing the NeMo RL integration surfaced a lighter adoption path than the document described. A framework that already performs an equivalent reshard consumes exactly one thing from its process group -- the NCCL communicator -- so MX can supply that alone and leave the framework's refit loop untouched. Worth stating explicitly because it is the better deal wherever it applies: the framework's two transports then share every line below the bootstrap, so a comparison between them isolates the rendezvous instead of confounding it with two implementations of the same transfer. Signed-off-by: Yixin Huang <yixinh@nvidia.com>
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NCCL M2N collective refit — design
Design only. No implementation code in this PR.
ModelExpress has one RL refit data plane today: the receiver-driven NIXL pull in
modelexpress/refit/reshard/. This adds a design for a second one — a sender-driven NCCLcollective push built on
nccl.m2n.reshard— as a sibling of that path rather than amode of it.
The design itself is in
docs/NCCL_M2N_REFIT.md.Implementation follows this design in order: #661 lands the server control plane, #665 the client control plane, and #666 the data plane that actually moves weights.
The NeMo RL integration that selects this transport is NVIDIA-NeMo/RL#3720.
Separation from NIXL is structural, not a flag
Nothing on this path imports NIXL, and nothing on the NIXL path imports this:
modelexpress_common/proto/refit_collective.proto, new serviceRefitCollectiveServicemodelexpress_server/src/refit/modelexpress_rl/collective/packageRegisterWorkerand theWeightVersionlifecycle from the mergedrefit.proto— none of the pull-path vocabulary (WeightVersionShard,VersionLease,RefitWorkerService)One deployment configures one backend. No runtime transport ranking, no NIXL fallback.
Figures
Components and messages — which side owns what, and the actual call on each edge.
Group formation — why the NCCL communicator setup is charged once per group epoch and not once per refit.
One warm refit — per-PP-stage lanes overlapping, and why the trailing broadcast cannot overlap them.
Prior art
The wire contract is adopted unchanged from NVIDIA-NeMo/RL#2971
(
nccl_reshard_refit, merged 2026-07-29), so an MX-brokered deployment and a NeMo-RL-nativedeployment move identical bytes. What MX adds is the rendezvous, admission, rank assignment,
fencing, and a readiness state — see the doc.
What's not here
No implementation. The doc proposes six slices (control plane, torch-free plan core,
rendezvous client, backend, two-sided clients, tests) so the shape can be agreed before any
code lands. Engine adapters stay out even then — they need real engines to validate.
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