# ADR-0027: Megatron-style Tensor Parallelism API

## Status

Accepted

## Context

### Goal

Support inter-SIP tensor parallelism (TP) via a **Megatron-LM style explicit
parallel layer** API. Declarative abstractions like DTensor are future work
in a separate ADR (0028).

Why Megatron-style was chosen:
- TP arises at specific layer boundaries of a model. Explicit primitives are
  natural to the mental model.
- The de-facto industry standard established by NVIDIA Megatron / DeepSpeed.
- DTensor is declarative, so its design space is larger → phased approach.

### TP primitive spec (Megatron-LM reference)

- **ColumnParallelLinear**: shards the weight's **column (out_features)** axis
  across TP ranks. Input is full-replicated, output is column-sharded. When a
  RowParallelLinear follows, no forward all-reduce is required.
- **RowParallelLinear**: shards the weight's **row (in_features)** axis across
  TP ranks. Input is already column-sharded (the output of ColumnParallel).
  Requires an **all-reduce** at the end of forward.
- **VocabParallelEmbedding**: shards the embedding along the vocab axis.
  all-reduce at the end of forward. (A stub in the initial scope; full
  implementation requires an all-gather kernel as a prerequisite.)
- **`copy_to_tp_region`**, **`reduce_from_tp_region`**, **`scatter_to_tp_region`**,
  **`gather_from_tp_region`** — basic primitives.

### Problems to solve

1. **Worker-wait generalization (D0)**: extend the defer/yield/drain pattern of
   `dist.all_reduce` to every `ctx.wait` path. **The biggest architectural
   decision of this ADR.**

2. **Launcher API normalization (D1)**: current benches use a hand-rolled
   greenlet loop. Absorb it into `torch.multiprocessing.spawn(fn, args, nprocs)`
   to preserve the real-PyTorch API surface + concentrate D0's scheduler drain
   in a single implementation site.

3. **Per-rank weight shard representation**: each worker owns its own slice of
   the weight tensor. Naturally expressed via ADR-0024's `set_device(rank)` +
   ADR-0026's intra-device DPPolicy.

4. **Forward-only scope**: KernBench currently has no backward (simulation
   purposes). This ADR prioritizes **forward only**. Training simulation is a
   separate ADR.

5. **Collective call site**: RowParallelLinear calls `all_reduce` at the end of
   forward. Naturally works with ADR-0024's multi-greenlet structure + D0
   generalization.

6. **TP group concept**: Megatron crosses DP × TP × PP groups. The initial
   scope simplifies to **TP group = all SIPs**. Mixed DP+TP is future work.

---

## Decision

### D0. Worker-wait generalization — `ctx.wait` defers to main when in worker context

**Restating the problem.** `kernel_runner.run` captures the `greenlet.getcurrent()`
at spawn time as the kernel greenlet's `_parent`
([kernel_runner.py:94](src/kernbench/triton_emu/kernel_runner.py#L94)).
If `env.run` runs in the main context, parent=main is safe. If `env.run` runs
in a worker context, parent=worker, and the moment the worker yields/finishes
the kernel greenlet becomes an orphan → `GreenletExit` → failure of ADR-0024
Phase B's `ring_default_ws`.

**Resolution.** When a worker greenlet calls `ctx.wait(h)`, instead of driving
`env.run` directly, **yield to the main scheduler**. main drives env.run and,
once the handle completes, control returns to the worker.

#### D0.1 `RuntimeContext` extension

```python
# context.py
@dataclass
class RuntimeContext:
    ...
    _pending_worker_waits: list[RequestHandle] = field(default_factory=list, init=False)
```

#### D0.2 `ctx.wait` worker fork

```python
def wait(self, handle, *, _meta=None):
    # Fast-path: already completed — skip enqueue + switch (consistent with
    # D0.4-(3) idempotency). Avoids needless worker→main→worker round-trip
    # and prevents redundant _pending_worker_waits growth.
    if handle in self._completed:
        completion, _trace = self.engine.get_completion(handle)
        return completion

    from greenlet import getcurrent
    g = getcurrent()
    if g.parent is not None and not g.parent.dead:
        # Worker greenlet: defer to main. Push handle, yield to parent.
        # Parent (scheduler loop) drains env.run, then switches back.
        self._pending_worker_waits.append(handle)
        g.parent.switch()
        # On resume: handle must have completed (main drained the list).
        # Fall through to the status-quo completion/trace assembly.

    # Main context (or single-driver): drive engine directly.
    wait_fn = getattr(self.engine, "wait", None)
    if wait_fn is not None:
        wait_fn(handle)
    completion, trace = self.engine.get_completion(handle)
    self._completed.add(handle)
    if _meta is not None and trace is not None:
        entry = dict(trace) if isinstance(trace, dict) else {"raw": trace}
        entry.update(_meta)
        self._traces.append(entry)
    return completion
```

#### D0.3 `ctx.wait` worker-context semantic contract (normative)

This ADR **explicitly changes** the semantics of `ctx.wait` in worker context.

- **Submit-vs-complete separation**: when called from a worker, `ctx.wait(h)`
  no longer guarantees "immediate completion" but instead guarantees
  "completion **after the next scheduler drain**". The point at which the
  worker returns from `wait()` = the point at which main has finished
  `engine.wait` for that handle. Main-context calls remain immediate-synchronous
  as before (status quo).
- **Resume invariant (normative)**: at the point a worker resumes from a
  worker-deferred `ctx.wait(h)` (when `g.parent.switch()` returns), **`h in
  ctx._completed` must be True**. If this invariant breaks, the worker
  proceeds in a stale state, so whichever of `_drain_pending` / the scheduler
  loop / `ctx.wait` is modified, this invariant must be preserved. T3.b
  directly asserts this invariant.
- **Observable change**: the pattern `h = ctx.submit(msg); ctx.wait(h);
  read(handle_result)` inside a worker still holds — but the semantic spec
  now includes the fact that a main-drain is automatically inserted between
  `wait()` and `read`.
- **Direct host-object reads see D0.5**: the contract for calling
  `tensor.numpy()` without `ctx.wait` is specified separately in D0.5.

#### D0.4 Main scheduler drain — protocol (normative)

(The internal implementation of D1's `multiprocessing.spawn`. Below is the
semantic definition.)

```python
while alive:
    for g in alive:              # (1) round-based worker switch
        g.switch()
    _drain_pending(ctx)           # (2) drain in main context
```

(The actual definition of `_drain_pending` is in D0.5 — an outer while-loop
that drains until both queues are empty.)

**Rules**:

1. **Round-based cooperative scheduling & yield obligation (worker contract)**.
   `g.switch()` does not return until the worker **voluntarily yields**
   (cooperative greenlet semantics). Therefore:
   - If a worker runs a pure-compute loop like `while True: do_compute()`
     without yielding, `g.switch()` never returns and **the scheduler loop
     itself hard-blocks** (other workers cannot get a switch turn, no drain
     occurs). This is not starvation but **scheduler non-progress (deadlock
     equivalent)**, and this ADR classifies it as **unsupported**.
   - Workers **must** call one of `ctx.wait(h)`, `dist.all_reduce`, or a
     host-read barrier (D0.5) within a finite number of steps. The `forward`
     of a TP layer includes a launch→wait pair at the end of every layer, so
     this condition is naturally met. CCL kernels also yield inside
     `dist.all_reduce`.
   - Implementations need not **detect** this (timeouts/steps-since-yield
     counters, etc.). It is a user contract; the symptom on violation is
     "simulation hang".
   - **Future extension**: if non-collective long compute paths become
     common, an explicit `torch.distributed.cooperative_yield()` primitive
     (no-op yield) could be introduced. Out of scope for this ADR. Not a
     breaking change — can be added if needed.
   - Within a round, every alive worker receives one `switch` turn. Even if
     a single worker calls wait multiple times within one round, the calls
     are enqueued sequentially within that turn and processed in a single
     scheduler drain batch (FIFO).

2. **Drain order = submission order (FIFO)**. `_pending_worker_waits` is
   strict FIFO via list append/pop(0). Drain occurs in submission order, not
   completion order, and SimPy's scheduler itself guarantees a causally
   correct completion order, so submission-order drain is safe. Do not
   confuse `completion order` with `drain order`.

   **Two-queue ordering (worker waits → collectives)**: `_drain_pending`
   drains the worker wait queue first, then the collective queue. Rationale
   for this ordering:
   - **The two queues are different dependency sources**: worker waits are
     handles produced by a worker's own `submit + wait` pair (tensor deploy,
     MmuMap, etc.). The collective queue holds kernel-launch handles that
     `dist.all_reduce` enqueues internally, which the worker never directly
     waits on (see the two-queue drain model in D0.5).
   - **Independent in correctness terms**: from the worker's perspective, a
     collective is "already submitted, then yielded". Its completion timing
     only needs to precede the worker's next action. There is no ordering
     dependency with the worker wait queue.
   - **Both finish within a single drain barrier**: per D0.5's
     loop-until-empty rule, a single barrier invocation drains worker →
     collective → (repeat if new ones appeared) in that order. By the time
     the worker resumes, both sides are drained.
   - **The alternative (collective first) is also valid**: this ADR fixes
     worker-first only for current implementation simplicity; semantically
     they are equivalent. Revisit if a performance-profile difference is
     observed.

3. **Duplicate enqueue — correctness via idempotent drain; dedup not
   guaranteed**. `ctx.wait(h)` returns immediately if `h in ctx._completed`.
   `_drain_pending` uses the same guard. Even if the same handle is appended
   to `_pending_worker_waits` multiple times, `engine.wait` is invoked only
   once (idempotent).
   - **Correctness**: relies on idempotent drain → safe.
   - **Memory/performance**: this ADR **does not guarantee dedup** of
     `_pending_worker_waits`. If the same handle is enqueued N times, the
     queue retains N elements and drain performs N pops + in-set guards.
     Unless a single worker abnormally repeats waits on the same handle, N
     stays at the order of 1 to a few.
   - **Implementation freedom**: implementations may optionally dedup (e.g.,
     hold a `set` as a side index, or check `h not in pending_set` before
     append). Classified as an optimization that does not change correctness.

4. **Exception propagation + sibling cleanup**.
   When a worker greenlet raises, `g.switch()` propagates the exception to
   main. The scheduler loop stops immediately and performs the following
   cleanup **explicitly**:

   ```python
   try:
       while True:
           alive = [g for g in gs if not g.dead]
           if not alive:
               break
           for g in alive:
               if not g.dead:
                   g.switch()
           _drain_pending(ctx)
   except Exception as outer:
       # (a) Force-terminate surviving sibling worker greenlets.
       for other in gs:
           if not other.dead:
               try:
                   other.throw(SystemExit)
               except Exception:
                   pass          # silent — already in exceptional state
       # (b) Reset backend barrier / pending state (in preparation for future epoch barrier).
       backend = getattr(ctx.distributed, "_backend", None)
       if backend is not None and hasattr(backend, "_barrier"):
           backend._barrier.reset()
       backend_pending = getattr(backend, "_pending_collective_handles", None)
       if backend_pending is not None:
           backend_pending.clear()
       ctx._pending_worker_waits.clear()
       # (c) Wrap the originating exception in SpawnException.
       raise SpawnException(errors) from outer
   ```

   Protocol:
   - **Sibling abort guarantee**: when one worker raises, `SystemExit` is
     thrown into all sibling greenlets — greenlets terminate immediately. No
     greenlet leaks.
   - **Explicit pending-queue clear**: both queues (worker-wait +
     collective-pending) are cleared. Prevents contamination on reuse.
   - **`SpawnException(errors)` wrapping**: `errors: dict[int, Exception]`
     contains the original exception per rank. Compatible with the failure
     pattern of real-PyTorch `torch.multiprocessing.spawn`.
     - **Scope restriction**: `errors` includes **only ranks that raised
       from their own code (root cause)**. Ranks terminated via
       `throw(SystemExit)` during sibling cleanup do not appear in `errors`
       (SystemExit is not caught by D1.2's entry wrapper `try/except
       Exception` — intentional design: sibling termination is a cleanup
       signal, not a failure). Made explicit so readers do not expect "all
       failed ranks" to appear.
   - **`ctx._traces` is the partial state up to the moment of exception**.
     Trace completeness is not guaranteed (some launches/all_reduces may
     terminate without leaving an entry).
   - **Allocator / MemoryStore** remain in their pre-exception state — reuse
     is non-goal; creating a fresh `RuntimeContext` is recommended.
   - **`join=False` / retry / partial recovery** are non-goals for this ADR.

   `SpawnException` is defined in `runtime_api/multiprocessing.py`:

   ```python
   class SpawnException(RuntimeError):
       def __init__(self, errors: dict[int, Exception]):
           self.errors = errors
           first = next(iter(errors.items()), None)
           msg = (f"spawn failed on ranks {sorted(errors.keys())}"
                  + (f": rank {first[0]} raised {first[1]!r}" if first else ""))
           super().__init__(msg)
   ```

5. **Single-driver compatibility**. In main-only execution where `g.parent is
   None` (legacy single-driver tests), D0.2's worker-fork condition is false
   → the existing immediate-synchronous path is preserved. `_drain_pending`
   is not invoked.

#### D0.5 Host-read barrier — decision (normative)

Inside a worker, **host-observable reads** such as `tensor.numpy()`,
`tensor.__getitem__`, and `tensor.data` are defined as **automatic drain
barriers**. Immediately before the call:

1. If `ctx._pending_worker_waits` or `backend._pending_collective_handles`
   are non-empty, yield to main via `g.parent.switch()` → main runs
   `_drain_pending` → worker resumes after completion.
2. If both queues are empty, read immediately.

**Barrier iteration protocol (normative — re-entrance)**: `_drain_pending`
drains via a while-loop **until both queues are completely empty**, not in a
single pass:

```python
def _drain_pending(ctx):
    while ctx._pending_worker_waits or (
        ctx.distributed._backend
        and ctx.distributed._backend._pending_collective_handles
    ):
        while ctx._pending_worker_waits:
            h = ctx._pending_worker_waits.pop(0)
            if h not in ctx._completed:
                ctx.engine.wait(h)
        backend = ctx.distributed._backend
        if backend is not None:
            while backend._pending_collective_handles:
                h, _sip_id, meta = backend._pending_collective_handles.pop(0)
                ctx.wait(h, _meta=meta)  # main context: safe; ctx.wait will
                                          # not push back to pending
```

**Main-context ctx.wait non-recursion invariant (normative)**: the
`ctx.wait(h, _meta=meta)` call inside `_drain_pending` runs in the main
greenlet context. Because D0.2's worker-fork condition (`g.parent is not
None and not g.parent.dead`) is False, it enters the immediate-synchronous
path → **never enqueues to `_pending_worker_waits`**. Thanks to this
invariant, the drain loop terminates without recursion / queue re-growth.
When implementing, it is important to maintain `g.parent is None` as the
single-main-greenlet guarantee.

**Why a loop**: `ctx.wait(h, _meta=meta)` is called in main context, so per
the D0.2 path it **drives the engine directly** (no additional enqueue — the
invariant above). In theory a single pass would suffice — but the protocol
is fixed at **loop-until-empty**. Reasons:

1. **Future-extension safety**: a future implementation might enqueue new
   pending items mid-drain (e.g., tree-reduce collectives with sub-handles).
   The loop protocol preserves correctness in that case.
2. **Readability**: the single sentence "the barrier drains until pending
   is empty" closes the semantics. No dependence on the non-trivial
   invariant that `ctx.wait` calls do not produce new enqueues.
3. **Barrier semantics are "all dependencies needed for this read are
   complete"**: in the current model all pending = all dependencies, so the
   two are identical. The user mental model is the former.

**Termination guarantee**: described under two regimes.

- **Current implementation**: when called in main context, `ctx.wait`
  drives the engine directly (D0.2) → does not enqueue new pending. Each
  iteration strictly shrinks pending size by `pop(0)` + `engine.wait`. The
  iteration count is bounded by **the initial pending size itself** →
  finite termination.
- **Future extension (the bound that justifies the loop protocol)**: if an
  implementation enqueues new pending mid-drain (e.g., tree-reduce
  sub-handles) is introduced, the initial-size bound breaks. However,
  SimPy causality guarantees that the dependency DAG of handles is finite,
  so **nested depth is finite**. The loop protocol automatically
  accommodates this case.

Both regimes guarantee that infinite loops are impossible. The
single-pass bound of the current implementation is a reference value for
aggressive optimization; the protocol is fixed at loop-until-empty.

**Why implicit drain at read is correct**:

- In the original open question, the choice was between (a) implicit drain
  and (b) explicit barrier. (b) is clear but burdens TP layer users with
  the 3-step pattern `out = fc1.forward(x); ctx.drain(); result =
  out.numpy()` on every read. (a) is a single rule that "guarantees the
  read sees the reflected value" — identical to CUDA's `cudaDeviceSynchronize
  before host copy` pattern, which is not a hidden rule but the **contract
  of a named entry point**.
- This ADR adopts (a) but **closes the entry-point list explicitly**:
  `Tensor.numpy()`, `Tensor.data` (numpy alias), `Tensor.__getitem__`,
  `Tensor.__repr__` (when data is included), and any other official
  host-read APIs are finalized via codebase search at the time of
  implementing this ADR. Any newly added host-read API must follow this
  contract (regression-guarded by tests).
- Even when calling `numpy` directly after only `ctx.submit` without
  `wait`, the drain barrier still operates (because the handle is in the
  pending queue). The invariant is restored at read time even if the user
  omits an explicit wait.

**`Tensor.copy_(source)` — write barrier specification**:

`copy_` is semantically "write to target", but internally it calls
`source.numpy()` to fetch source data on the host then writes to each
shard via `target._memory_store.write(...)`. Both directions are
barrier-handled:

1. **Source-side (read barrier)**: `source.numpy()` triggers the D0.5 read
   barrier (when source itself is a deployed tensor with pending).
2. **Target-side (write barrier — based on global pending)**: on `copy_`
   entry, if `ctx._pending_worker_waits` or
   `backend._pending_collective_handles` are non-empty, drain via
   `g.parent.switch()` before writing. **Not per-tensor / per-shard
   dependency tracking, but based on the global pending queue**.
   - Why global: KernBench's handle representation does not retain the
     reverse-mapping information "this handle writes to which shard of which
     target". A safe conservative rule: "drain if any global pending
     exists". As a result, **pending of an unrelated tensor can also block
     copy_** — drop-in invariant takes priority.
   - **Explicit tradeoff**: this rule can introduce unnecessary
     serialization between independent tensors. However, under the current
     single-queue execution model this cost is acceptable — guaranteeing
     cross-rank correctness and the "read sees latest" invariant via a
     simple rule takes precedence.
   - Practical impact: most pending of a single worker within a layer step
     is its own work — extra context switches from over-barrier often
     coincide with the end-of-round scheduler drain point, so no major
     issue.
   - Future refinement: per-tensor pending tracking could narrow this
     rule, but it is out of scope for this ADR.

**Non-barrier**:

- `tensor.shape`, `tensor.dtype`, `tensor.name`, and other
  **metadata-only** access does not drain. No data dependency.
- `tensor.pa`, `tensor.va`, and other raw address accessors also do not
  drain (address only, not content).

**Official barrier entry-points (closed set)**:

| API | Kind | Rationale |
|---|---|---|
| `Tensor.numpy()` | read | host-observable copy |
| `Tensor.data` | read | `numpy()` alias |
| `Tensor.__getitem__` | read | shard-aligned read |
| `Tensor.__repr__` (when data is included) | read | debugging/log |
| `Tensor.copy_(source)` | read + write | source read + target write |

This contract is verified directly in T5/T6.

#### D0.6 Why the worker function API is unchanged (informative)

- The inside of `torch.zeros(...)` is a `self.submit(msg)` + `self.wait(h)`
  pair. `wait` auto-defers to main per D0.2/D0.3 — appears synchronous from
  the outside but yields once.
- `tensor.numpy()` follows D0.5's host-read barrier → drain→read when
  pending exists, immediate read otherwise.
- `dist.all_reduce` continues to use the existing `_defer_wait=True` +
  `_pending_collective_handles` path. D0.4's drain processes both queues
  together.

#### D0.7 Invariants

- **The kernel greenlet's `_parent` is always main**: because env.run never
  runs in worker context. (Core assertion of T3.)
- **Cross-rank synchronization point**: drain occurs only after every
  worker has yielded → kernels of all ranks advance together within one
  round (a prerequisite for cross-rank IPCQ exchange).
- **Single-driver compatibility**: D0.4-(5).

### D1. `torch.multiprocessing.spawn(fn, args, nprocs)`

Real-PyTorch API parity + a single implementation site for D0's scheduler
loop.

#### D1.0 API parity only — not execution parity (normative)

The name `torch.multiprocessing.spawn` is restricted to **API signature
parity**. The actual execution model is a **cooperative greenlet scheduler**
(single Python process, single OS thread, round-robin drive per D0.4). The
following are **properties this ADR does NOT provide** — among the
guarantees of real-PyTorch `torch.multiprocessing.spawn`, explicitly
**non-goals**:

- Process isolation (independent OS process per rank).
- Independent address space (each rank with its own Python heap).
- Failure isolation (a hard crash in one rank not affecting others).
- OS-level scheduler fairness (preemptive time slicing between ranks).
- Inter-process primitives such as `mp.Queue`, `mp.Lock`.

Actual properties of this implementation:

- All ranks are greenlets inside the same Python process. Shared global
  state is visible as-is (intentional simulation convenience).
- Single-threaded under the GIL → not parallel execution. Only "logical
  concurrency" via SimPy event ordering is reproduced.
- Unhandled exception in any one worker → entire simulation aborts
  (D0.4-(4)).

**Caller's obligation**: when porting real-PyTorch multi-process samples to
KernBench, logic that relies on process isolation (e.g., `os.getpid`,
independent temp files, signal handling) must be removed. The namespace name
is preserved for code portability — semantics differ.

#### D1.1 Public surface

```python
# runtime_api/multiprocessing.py (new)
class _MultiprocessingNamespace:
    def __init__(self, ctx):
        self._ctx = ctx

    def spawn(self, fn, args: tuple, nprocs: int, join: bool = True) -> None:
        """Spawn `nprocs` worker greenlets, each calling fn(rank, *args).

        Mirrors torch.multiprocessing.spawn signature (minus `daemon`).
        Drives the D0 scheduler loop until all workers finish.
        """
        ...
```

#### D1.2 Implementation

```python
def spawn(self, fn, args, nprocs, join=True):
    from greenlet import greenlet
    ctx = self._ctx
    dist = ctx.distributed
    gs: list[greenlet] = []
    errors: dict[int, Exception] = {}
    for rank in range(nprocs):
        def _entry(r=rank):
            try:
                fn(r, *args)
            except Exception as e:
                errors[r] = e
                raise
        g = greenlet(_entry)
        dist._bind_rank(g, rank)
        gs.append(g)

    try:
        while True:
            alive = [g for g in gs if not g.dead]
            if not alive:
                break
            for g in alive:
                if not g.dead:
                    g.switch()
            _drain_pending(ctx)       # D0.5
    except Exception as outer:
        # Sibling cleanup per D0.4-(4)
        for other in gs:
            if not other.dead:
                try:
                    other.throw(SystemExit)
                except Exception:
                    pass
        backend = getattr(dist, "_backend", None)
        if backend is not None:
            if hasattr(backend, "_barrier"):
                backend._barrier.reset()
            if getattr(backend, "_pending_collective_handles", None) is not None:
                backend._pending_collective_handles.clear()
        ctx._pending_worker_waits.clear()
        raise SpawnException(errors) from outer
    # `join=True` semantics: we already wait for all workers.
```

#### D1.3 `torch` namespace attach

In `runtime_api/context.py` `__post_init__`:
```python
self.multiprocessing = _MultiprocessingNamespace(self)
```

→ in bench code: `torch.multiprocessing.spawn(worker, args=(ws,), nprocs=ws)`.

#### D1.4 Migration of existing benches

The hand-rolled loop in `benches/ccl_allreduce.py` collapses into a single
`torch.multiprocessing.spawn` line. Existing matrix regressions are
preserved. The currently xfail `ring_default_ws` is expected to flip to
PASS thanks to D0 (workers no longer orphan the kernel greenlet).

### D2. New package `kernbench.tp`

```
src/kernbench/tp/
    __init__.py          — public API re-exports
    parallel_state.py    — TP group management (currently a single global group)
    layers.py            — ColumnParallelLinear, RowParallelLinear, VocabParallelEmbedding
    primitives.py        — copy/reduce/scatter/gather_to/from_tp_region
    kernels.py           — gemm kernel launched by TP layers (reusable)
    mappings.py          — forward identity/all_reduce, backward stub
```

### D3. `parallel_state` — TP group

```python
# parallel_state.py
_TP_WORLD_SIZE = None

def initialize_model_parallel(tensor_model_parallel_size: int) -> None:
    """Initialize TP group. Must be called after dist.init_process_group."""
    global _TP_WORLD_SIZE
    from kernbench.runtime_api.distributed import get_dist  # or torch.distributed
    dist = get_dist()
    total = dist.get_world_size()
    if tensor_model_parallel_size != total:
        raise NotImplementedError(
            "Only TP == world_size supported in initial scope"
        )
    _TP_WORLD_SIZE = tensor_model_parallel_size

def get_tensor_model_parallel_world_size() -> int:
    return _TP_WORLD_SIZE

def get_tensor_model_parallel_rank() -> int:
    from kernbench.runtime_api.distributed import get_dist
    return get_dist().get_rank()         # ADR-0024 greenlet-local rank
```

Initial scope: TP size = world_size = topology SIP count. Pure TP model.

### D4-pre. TP shard ownership vs DPPolicy — role separation (normative)

In the weight/output representation of TP layers, two concepts are clearly
separated:

| Concept | Decided by | Scope |
|---|---|---|
| **TP shard ownership** (which rank owns which slice of the weight) | greenlet-local rank + `torch.ahbm.set_device(rank)` (ADR-0024 D2/D3) | **cross-rank, cross-SIP** |
| **Intra-rank placement** (how the owned slice is distributed across cube × PE inside the rank) | `DPPolicy(cube=..., pe=...)` (ADR-0026) | **inside one rank (within SIP boundary)** |

Thus when `ColumnParallelLinear` creates a weight of shape `(in_features,
out_features // ws)` and assigns `DPPolicy(cube="column_wise",
pe="column_wise")`:

- The slice owned by **rank r** = column-axis [r * k_local, (r+1) *
  k_local) of the weight — **set_device(r)** determines this (that rank
  resides on SIP r).
- **Inside that slice**, the cube × PE column-wise distribution — **DPPolicy**
  determines this.

The two axes are **independent**. If two ranks build their own slice with
the same DPPolicy, the slices themselves live on different SIPs but the
intra-SIP placement pattern is the same. Conversely, changing DPPolicy to
`cube="replicate", pe="replicate"` preserves TP shard ownership and only
changes intra-rank placement.

**Mistakes that blur this boundary** (forbidden by this ADR):

- The "SIP axis" reappearing in DPPolicy (removed in ADR-0026).
- TP layers expressing cross-rank sharding via `DPPolicy` alone without
  `set_device` → indistinguishable from a vertical split within a single
  rank.

The TP layers of this ADR always treat weight/output from the perspective
of "rank = SIP = owns one slice + DPPolicy intra-SIP distribution" only.

### D4. `ColumnParallelLinear`

**Important**: no new host-side `torch.matmul` abstraction is introduced.
The layer's forward calls the existing gemm kernel via `torch.launch("gemm",
gemm_kernel, ...)` — the pattern already used by KernBench benches
([benches/gemm_single_pe.py](benches/gemm_single_pe.py),
[benches/gpt3_qkv.py](benches/gpt3_qkv.py)).

```python
# layers.py
from kernbench.policy.placement.dp import DPPolicy
from kernbench.tp.kernels import _gemm_kernel
from kernbench.tp.parallel_state import (
    get_tensor_model_parallel_rank,
    get_tensor_model_parallel_world_size,
)

class ColumnParallelLinear:
    """Shards the K(out_features) axis of the weight across TP ranks.

    forward(x):
        x: (M, N) — full-replicated across ranks
        W_k: (N, K / world_size) — rank-local slice (placed on SIP r via set_device)
        y_k = x @ W_k → (M, K / world_size) — rank-local output

    Output is column-sharded. The input form expected by RowParallelLinear.
    """

    def __init__(self, in_features: int, out_features: int, bias: bool = False,
                 dtype: str = "f16", torch=None):
        ws = get_tensor_model_parallel_world_size()
        assert out_features % ws == 0
        self.in_features = in_features
        self.k_local = out_features // ws
        self._torch = torch
        # Each rank owns its own slice — placed on SIP r by set_device(rank).
        self.weight = torch.zeros(
            (in_features, self.k_local), dtype=dtype,
            dp=DPPolicy(cube="column_wise", pe="column_wise"),
            name="col_parallel_w",
        )
        self.bias = None
        if bias:
            self.bias = torch.zeros(
                (self.k_local,), dtype=dtype,
                dp=DPPolicy(cube="replicate", pe="replicate"),
                name="col_parallel_b",
            )

    def forward(self, x):
        # x is full-replicated (caller-guaranteed). Plain local gemm.
        M = x.shape[0]
        out = self._torch.empty(
            (M, self.k_local), dtype=x.dtype,
            dp=DPPolicy(cube="column_wise", pe="column_wise"),
            name="col_parallel_out",
        )
        self._torch.launch(
            "col_parallel_gemm", _gemm_kernel,
            x, self.weight, out, M, self.in_features, self.k_local,
        )
        # bias add as a separate kernel or as fused bias of a composite gemm.
        # Initial scope verifies bias=False sufficiently.
        return out
```

**Yield-safety contract (normative)**: `ColumnParallelLinear.forward`
includes one `torch.launch` call containing a kernel launch → internal
`ctx.wait` pair. This automatically satisfies the "worker yields within a
finite number of steps" condition of D0.4-(1) — TP layer users do not need
to insert yield patterns manually.

### D5. `RowParallelLinear`

```python
class RowParallelLinear:
    """Shards the N(in_features) axis of the weight across TP ranks.

    forward(x):
        x: (M, N / world_size) — rank-local slice (output of ColumnParallel)
        W_k: (N / world_size, K) — rank-local slice
        y_k = x @ W_k → (M, K) — partial sum on each rank
        y = all_reduce(y_k, op="sum") → (M, K) on every rank
    """

    def __init__(self, in_features: int, out_features: int, bias: bool = False,
                 dtype: str = "f16", torch=None):
        ws = get_tensor_model_parallel_world_size()
        assert in_features % ws == 0
        self.n_local = in_features // ws
        self.out_features = out_features
        self._torch = torch
        self.weight = torch.zeros(
            (self.n_local, out_features), dtype=dtype,
            dp=DPPolicy(cube="column_wise", pe="column_wise"),
            name="row_parallel_w",
        )
        # bias only on rank 0 (Megatron convention). Omitted in initial scope.
        self.bias = None

    def forward(self, x):
        M = x.shape[0]
        y_partial = self._torch.empty(
            (M, self.out_features), dtype=x.dtype,
            dp=DPPolicy(cube="column_wise", pe="column_wise"),
            name="row_parallel_partial",
        )
        self._torch.launch(
            "row_parallel_gemm", _gemm_kernel,
            x, self.weight, y_partial, M, self.n_local, self.out_features,
        )
        # Cross-rank reduce. ADR-0024's dist.all_reduce works correctly
        # under D0 + mp.spawn (kernel parent = main is preserved).
        self._torch.distributed.all_reduce(y_partial, op="sum")
        return y_partial
```

**Yield-safety contract (normative)**: `RowParallelLinear.forward` includes
launch → internal wait followed by `all_reduce` (defer + worker-yield
pattern), so **at least 2 yields per forward** are guaranteed. The
scheduler-progress condition of D0.4-(1) is automatically satisfied. All TP
layer forwards in this ADR maintain the invariant "yield-safe by containing
at least one wait or collective" — any future TP primitives (e.g.,
VocabParallelEmbedding) must keep the same contract.

### D6. Primitive functions

```python
# primitives.py
def copy_to_tp_region(x):
    """Forward: identity. Backward: all-reduce. (Implemented when training is added)."""
    return x

def reduce_from_tp_region(x, torch):
    """Forward: all-reduce. Backward: identity."""
    torch.distributed.all_reduce(x, op="sum")
    return x

def scatter_to_tp_region(x):
    raise NotImplementedError(
        "Phase 2: replaced by users creating already-sharded tensors"
    )

def gather_from_tp_region(x):
    raise NotImplementedError(
        "Phase 2: requires all-gather kernel as a prerequisite (future)"
    )
```

### D7. Sample bench — 2-layer MLP with TP

```python
# benches/tp_mlp.py (new)
from kernbench.policy.placement.dp import DPPolicy
import kernbench.tp as tp
import numpy as np


def worker(rank: int, world_size: int, torch):
    torch.ahbm.set_device(rank)
    tp.initialize_model_parallel(world_size)

    B, D_in, D_hidden, D_out = 1, 512, 2048, 512
    fc1 = tp.ColumnParallelLinear(D_in, D_hidden, torch=torch)
    fc2 = tp.RowParallelLinear(D_hidden, D_out, torch=torch)

    x = torch.zeros(
        (B, D_in), dtype="f16",
        dp=DPPolicy(cube="replicate", pe="replicate"),
        name="x",
    )
    # init x with some pattern (e.g., constant)
    x.copy_(torch.from_numpy(np.full((B, D_in), 0.1, dtype=np.float16)))

    h = fc1.forward(x)      # column-sharded (B, D_hidden / ws)
    y = fc2.forward(h)      # all-reduced (B, D_out) on every rank

    # Only rank 0 prints / verifies the result
    if rank == 0:
        result = y.numpy()
        # With zero-init weights, all values are 0 — within scope "completion itself" is the check
        print(f"  tp_mlp: shape={result.shape}, mean={float(result.mean()):.4f}")


def run(torch):
    torch.distributed.init_process_group(backend="ahbm")
    ws = torch.distributed.get_world_size()
    torch.multiprocessing.spawn(worker, args=(ws,), nprocs=ws)
```

### D8. Non-functional — training not supported

This ADR is **inference/forward only**. Backward / gradient / optimizer is
future work. Natural because KernBench is not a training system.

### D9. Initial-scope constraints

- TP size = world_size (no mixed DP+TP).
- `scatter_to_tp_region`, `gather_from_tp_region` are unimplemented.
- **Default weight value is zero**. Proper init schemes (Xavier, Kaiming,
  etc.) are future. Tests inject deterministic non-zero patterns via
  `tensor.copy_` to verify numerical correctness (T2/T6). I.e., operate as
  "production default = zero, verification = deterministic non-zero".
- Bias is omitted in the initial scope (Megatron's rank-0-only bias policy
  is future).
- Pipeline parallelism is out of scope.
- VocabParallelEmbedding requires a prerequisite all-gather → stub only.

### D10. Regression: `ring_default_ws` xfail removal — mandatory acceptance

Thanks to D0 (worker-wait generalization) + D0.5 (host-read barrier), every
worker-driven `ctx.wait` and host-read is routed through the main-drain path
→ the cause of the kernel-greenlet orphan in ADR-0024 Phase B disappears.
Flipping the existing matrix test's `ring_default_ws` strict-xfail case to
**PASS** after this ADR's implementation is included as a **mandatory
regression criterion**. Observable acceptance criteria are specified in
**T7** (no deadlock, no GreenletExit, numerical tolerance, etc.).

---

## Dependencies

- **ADR-0024** (launcher): rank = SIP, greenlet-local rank,
  `torch.ahbm.set_device(rank)`.
- **ADR-0026** (DPPolicy intra-device): per-rank slice representation of
  weight tensors.
- **ADR-0023 / ADR-0025** (IPCQ): foundation of `dist.all_reduce`
  implementation.

---

## Non-goals

- **Backward pass / training**: inference only. Training simulation is a
  separate ADR.
- **Mixed parallelism (DP + TP + PP)**: pure TP only at the start.
- **Weight init schemes**: simple zero / debug pattern.
- **Fused ops**: Megatron's fused matmul+bias+gelu is a kernel-level
  concern.
- **DTensor integration**: ADR-0028 future.
- **Host-side `torch.matmul` abstraction**: TP layers call the existing
  gemm kernel via `torch.launch(gemm_kernel, ...)`. No new matmul host-op
  is introduced.

---

## Open questions

- **Location of `initialize_model_parallel`**:
  `kernbench.tp.initialize_model_parallel` (current decision) vs
  real-PyTorch's `torch.distributed.init_device_mesh`. Kept in the TP-only
  module.
- **Weight init**: the ADR uses zero. A debug pattern (e.g., identity) may
  be needed for valid verification — add at Phase 1 test time if needed.
- **Bias placement policy**: Megatron places RowParallelLinear bias only on
  rank 0. Avoided in the initial scope via bias=False.
- **GEMM kernel location**: `kernbench.tp.kernels._gemm_kernel` vs
  importing from existing `benches/gemm_single_pe.py`. TP must not depend
  on benches, so duplicated inside tp. Migration to a shared
  `kernbench.kernels` package is possible later.

**Resolved (previously open in earlier revisions)**:
- ~~Drain timing on `tensor.numpy()` call~~ → **decided in D0.5**: the
  official host-read entry points (`numpy`, `data`, `__getitem__`,
  data-containing `__repr__`) are automatic drain barriers. Metadata-only
  accessors are not barriers.

---

## Consequences

### Positive

- **Easy porting of Megatron code**: API matches real training code.
- **TP benchmarking enabled**: research on scaling,
  communication-compute overlap, and other HW characteristics.
- **`ring_default_ws` xfail removal**: as a byproduct of D0, the ADR-0024
  Phase B blocker is resolved.
- **Scheduler-loop unification**: introducing D1 (`mp.spawn`) removes the
  hand-rolled loop. Subsequent collective/TP benches reuse the same
  pattern.
- **DPPolicy semantics clarified** (synergy with ADR-0026): TP layers as a
  best-practice example of using intra-device DPPolicy only.

### Negative

- Maintenance cost of a new module (`kernbench.tp`).
- Initial scope is limited (pure TP only, forward only).
- D0 generalization changes the semantics of `ctx.wait` — compatibility
  with single-driver tests must be explicitly verified (T7).

### Neutral

- A pure upper layer added on top of ADR-0024/0026. No impact on the
  hardware-simulation stack (apart from D0).