ywkang
6d24b9306f
Verified against sim_engine data path (memory_store, data_executor): - GQA reuse does NOT need a broadcast op. The baseline's h_q==h_kv limit is its head-packing reshape hack, not a missing primitive. Correct fix: per-KV-head loop with G folded into matmul M dim (byte-conserving reshape) — runs today, timing correct (m=G*T_q), data mode runs. - ADR-0061 broadcast demoted from 'the blocker' to optional convenience. - Surfaced tl.trans = reshape-not-transpose (memory_store reshapes; data_executor np.matmul on reshaped operands) -> numeric parity is bounded; verification is structural/timing/determinism-first (matches SPEC perf-model purpose). Optional tl.transpose deferred. - Reordered DDD phase plan (P1 GQA needs no new feature; P3 scratch_scope is the key scale feature); added open decisions 10.10 (transpose) and 10.11 (GQA-via-M-fold finding). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
204 lines
8.2 KiB
Markdown
204 lines
8.2 KiB
Markdown
# ADR-0061: `tl.broadcast` / `tl.repeat` — data-faithful GQA head reuse
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## Status
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Proposed
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> Supporting ADR for **ADR-0060** (AHBM GQA Fused Attention).
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>
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> **⚠ Re-evaluated during design iteration — this is now OPTIONAL, not a
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> GQA blocker.** The original framing (below) held that GQA `h_q > h_kv`
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> is blocked by the MemoryStore byte-conservation check on a broadcast
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> view. That is true *only of the baseline's head-packing hack*
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> (`_view(K, (h_q·d, S_kv))`). The correct GQA design (ADR-0060 §5.2)
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> processes **one KV head at a time** and folds the `G` group rows into
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> the matmul **M** dimension — using only byte-conserving reshapes — so it
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> needs **no broadcast op at all**, and `np.matmul` already broadcasts a
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> shared operand in Phase-2 data mode. This ADR therefore drops to a
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> *convenience* primitive: it cleans up additive-mask construction across
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> the `G·T_q` rows and serves general kernels. **Lowest priority of the
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> three supporting ADRs.** The rest of this document is retained as the
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> design for that convenience op.
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## Context
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### The GQA reuse lever
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Grouped-Query Attention amortises K/V loads: `G = H_q / H_kv` query heads
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share one KV head. For Llama3-70B `G = 8`. In a kernel this means a KV
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tile of shape `[S_kv, d]` (for one KV head) must be matmul'd against the
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`G` query rows of its group. Loading K/V **once** and reusing it across
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the group is the decode-time efficiency win (ADR-0060 §5.2).
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### Why it does not work today
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The existing mesh kernels reshape tensors with a metadata-only helper
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`_view` (`src/kernbench/benches/_attention_mesh_kv.py:25-36`,
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`_attention_mesh_mlo.py:25-36`):
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```python
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def _view(handle, new_shape):
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return TensorHandle(..., shape=new_shape, nbytes=handle.nbytes, ...)
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# ^^^ new shape ^^^ OLD nbytes (unchanged)
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```
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This rewrites `shape` but keeps the original `nbytes`. It is purely
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symbolic. It survives **Phase 1 (timing)** because Phase 1 never reads
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tensor bytes. It fails **Phase 2 (`enable_data=True`)** because the
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data executor materialises every op through `MemoryStore.read`, which
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enforces byte conservation
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(`src/kernbench/sim_engine/memory_store.py:67-73`):
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```python
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if shape is not None and arr.shape != shape:
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if arr.nbytes != np.prod(shape) * arr.dtype.itemsize:
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raise ValueError(f"Shape mismatch: stored {arr.shape} ({arr.nbytes}B) "
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f"vs requested {shape} ({...}B)")
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arr = arr.reshape(shape)
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```
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A reshape conserves bytes; a **broadcast does not** (`[S_kv, 1, d]` →
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`[S_kv, G, d]` multiplies the element count by `G`). So the moment a
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kernel tries to view a `h_kv`-headed K as if it had `h_q` heads, the
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stored array has `1/G` of the requested bytes and `read` raises.
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The current test suite documents this as a deliberate limitation
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(`tests/attention/test_milestone_gqa_llama70b.py:137-142`):
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> "v1 uses `h_q == h_kv == 1` to avoid … GQA broadcast view (which is
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> symbolic and does not survive MemoryStore's nbytes check under
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> simulator data execution). Real GQA (`h_q > h_kv`) is deferred."
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There is **no** broadcast / repeat / expand primitive in the `tl` API
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today (`src/kernbench/triton_emu/tl_context.py` exposes `trans`,
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`zeros`, `full`, `arange` as metadata-only ops; none replicate data).
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## Decision
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Add a **data-faithful broadcast op** to the `tl` API and the data
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executor. Unlike `_view`, it updates `nbytes` and, in Phase 2, actually
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replicates the underlying ndarray. The latency it charges models the
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on-chip register/SRAM fan-out, not an HBM re-load (the KV tile is read
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from HBM once; broadcast is an on-chip replication).
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### D1. `tl` surface
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```python
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def broadcast(self, x: TensorHandle, new_shape: tuple[int, ...]) -> TensorHandle:
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"""Replicate x along size-1 axes to new_shape (numpy broadcast rules).
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new_shape must be broadcast-compatible with x.shape: every axis is
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either equal to x's, or x's is 1 (the axis being expanded). nbytes of
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the result reflect new_shape (unlike the metadata-only _view).
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"""
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```
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`tl.repeat(x, axis, n)` is offered as a thin convenience wrapper over
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`broadcast` for the common "insert a group axis and expand it to G"
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pattern, but `broadcast` is the primitive.
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### D2. Command + op_log
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Emit a new `BroadcastCmd` (sibling of `MathCmd`):
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```python
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@dataclass(frozen=True)
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class BroadcastCmd:
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src: TensorHandle
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out: TensorHandle # out.nbytes reflects new_shape
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new_shape: tuple[int, ...]
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data_op: bool = True
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```
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op_log entry: `op_kind="math"`, `op_name="broadcast"` (it runs on the
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MATH/vector engine, like the other element-wise ops). The output handle
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is allocated from PE scratch via the existing `_make_compute_out`
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(`tl_context.py:148-156`), so `out.nbytes` is computed from `new_shape`
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and the byte-conservation check passes in Phase 2.
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### D3. Phase 2 data execution
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Add `_execute_broadcast` to the data executor
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(`src/kernbench/sim_engine/data_executor.py`):
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```python
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def _execute_broadcast(self, op):
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p = op.params
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src = self._resolve_read(p["src_space"], p["src_addr"],
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p["src_shape"], p["dtype"], op.t_start)
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result = np.broadcast_to(src, p["new_shape"]).copy() # .copy() → writable, owns bytes
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self.store.write(p["dst_space"], p["dst_addr"], result)
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```
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`.copy()` is required: `np.broadcast_to` returns a zero-stride view, and
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MemoryStore stores references — a later consumer's nbytes check must see
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a real `prod(new_shape)`-element array.
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### D4. Latency model
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Broadcast latency = vector-engine cost of writing `prod(new_shape)`
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elements (reuse `pe_math._compute_ns(num_elements)`,
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`src/kernbench/components/builtin/pe_math.py:46-51`). This models the
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on-chip replication into the register file / SRAM that real GQA hardware
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performs. It is **not** an HBM transaction — no `dma_read` is logged,
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consistent with "K/V tile loaded once" (ADR-0060 §5.2).
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## Alternatives
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### A1. Indexed per-head dot (no broadcast op)
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Loop the `G` query heads, each doing its own `tl.dot(Q_g, Kᵀ)` against
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the *same* (re-`ref`'d, not re-loaded) KV tile:
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```python
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for g in range(G):
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s_g = tl.dot(Q[g], K_T) # G separate GEMMs, KV reused via tl.ref
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```
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- Pro: no new primitive; works in Phase 2 today.
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- Con: `G×` more GEMM commands (op_log inflation), no batched-GEMM
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modelling, and the per-call overhead is charged `G` times. It also
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obscures the "single batched matmul over the group" the hardware does.
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Rejected as the primary path; acceptable as a fallback if the batched
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2D dot's shape handling proves awkward.
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### A2. Keep `_view`, relax the nbytes check
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Make `MemoryStore.read` tolerate broadcast (stride-0) shapes. Rejected:
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it silently breaks byte conservation for *all* readers and would mask
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genuine shape bugs elsewhere. The check is a correctness invariant
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(ADR-0052), not an obstacle to route around.
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### A3. Materialise the broadcast at deploy time
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Store K/V already expanded to `h_q` heads in HBM. Rejected: defeats the
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entire point of GQA (it `G×`'s KV-cache HBM footprint and bandwidth).
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## Consequences
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### Positive
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- Unblocks real GQA (`h_q > h_kv`) in Phase 2 — the headline blocker for
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ADR-0060 and the deferred milestone work.
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- A general, reusable primitive (any kernel needing numpy-style
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broadcast benefits, not just attention).
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- Byte conservation stays intact; no weakening of MemoryStore.
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### Negative
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- One new command type + one data-executor handler + one `tl` method.
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- Phase 2 `.copy()` allocates the expanded array in the store; for very
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large groups this is real memory. Bounded by tile size in the tiled
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design (ADR-0060), so acceptable.
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## Test Requirements
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1. **Phase 1 timing**: `tl.broadcast([S,1,d] → [S,G,d])` logs one
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`math/broadcast` op with `num_elements == S*G*d`; no `dma_read`.
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2. **Phase 2 data**: broadcasting a known array yields the numpy
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`broadcast_to(...).copy()` result; a downstream `tl.dot` consuming it
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passes the MemoryStore nbytes check (the exact case that fails today).
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3. **GQA end-to-end**: re-run a reduced `milestone-gqa-llama70b` panel
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with `h_q=8, h_kv=1` under `enable_data=True`; it must complete (today
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it raises `ValueError: Shape mismatch`).
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4. **Broadcast-incompatible shape** (e.g. axis size 3 → 8) raises a
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clear error at emit time, not deep in Phase 2.
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