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f4f55b2c1c
Proposed prerequisites surfaced while evaluating the GQA fused-attention ADR against the actual kernbench tl/sim_engine implementation: - ADR-0061 tl.broadcast: data-faithful GQA head reuse (fixes the MemoryStore nbytes check that forces h_q==h_kv==1 today). - ADR-0062 tl.load_async: non-blocking HBM tile load for KV prefetch (KV-load-bound decode/long-context overlap). - ADR-0063 tl.scratch_scope: per-tile scratch recycling (removes the 1 MiB bump-allocator ceiling that caps context at S=16). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
163 lines
6.7 KiB
Markdown
163 lines
6.7 KiB
Markdown
# ADR-0063: `tl.scratch_scope` — per-tile scratch recycling for long context
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## Status
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Proposed
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> Supporting ADR for **ADR-0060** (AHBM GQA Fused Attention). Long-context
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> attention sweeps many K/V tiles; each tile's intermediates
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> (`scores`, `P`, `exp`, partial `O`, …) currently allocate fresh
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> scratch that is never freed within a kernel invocation. The 1 MiB
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> per-PE scratch budget is exhausted long before a realistic context
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> length.
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## Context
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### The bump allocator
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`TLContext` allocates every math/compute output handle from a per-PE
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scratch pool with a **linear bump cursor**
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(`src/kernbench/triton_emu/tl_context.py`; `_scratch_alloc`):
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```python
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def _scratch_alloc(self, nbytes):
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aligned = (nbytes + 15) & ~15
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addr = self._scratch_base + self._scratch_cursor
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self._scratch_cursor += aligned
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if self._scratch_cursor > self._scratch_size: # default 1 << 20 = 1 MiB
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raise RuntimeError("TLContext scratch overflow: ...")
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return addr
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```
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The docstring states the cursor **"resets on every kernel invocation"** —
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i.e. only at kernel entry, never *within* the kernel body. Every
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`tl.dot`, `tl.softmax`, `tl.exp`, `tl.sum`, `a - b`, `a * b`, … grabs a
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fresh slice and nothing is reclaimed until the kernel returns.
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### Why this bites the GQA kernel
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The current milestone bench keeps `S_q = S_kv_per_rank = 16` **explicitly
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because** of this limit
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(`tests/attention/test_milestone_gqa_llama70b.py:123-148`):
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> "S_q_prefill and S_kv_per_rank are deliberately small (16 each) so the
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> simulator's 1 MB per-PE TCM kernel scratch is not exhausted by the
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> bump-allocated handle outputs of softmax/exp/dot/sum chains over
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> n_ranks ring steps."
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A FlashAttention sweep over `n_tiles` tiles allocates O(`n_tiles` ×
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per-tile-intermediates). For any realistic context this overflows. The
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*math* of flash attention needs only **O(1)** live scratch — the running
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`(m, l, O)` plus the current tile's working set — because each tile's
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temporaries are dead once that tile is folded into the running state. The
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allocator just doesn't know they're dead.
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## Decision
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Add a **scratch scope** that lets a kernel mark a region of allocations
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as reclaimable, so per-tile temporaries are recycled while live running
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state (and in-flight prefetch buffers) are preserved.
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### D1. `tl` surface — context manager
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```python
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with tl.scratch_scope():
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s = tl.dot(q, k_t) # all handles allocated inside the `with`
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p = tl.softmax(s) # share a region that is rewound on exit
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o_j = tl.dot(p, v)
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# ... fold o_j into running (m,l,O) which live OUTSIDE the scope ...
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# on __exit__: cursor rewinds to its value at __enter__
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```
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Semantics:
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- `__enter__` records the current `_scratch_cursor` as a save-point.
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- `__exit__` restores the cursor to the save-point, freeing everything
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allocated inside.
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- Handles allocated **outside** the scope (running `m,l,O`, prefetch
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buffers held by `LoadFuture` from ADR-0062) keep their addresses —
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they were allocated before the save-point or in an enclosing scope.
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### D2. Safety contract
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A handle allocated inside a scope **must not** be read after the scope
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exits — its bytes may be overwritten by the next scope's allocations.
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The flash loop respects this naturally: the only values that survive a
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tile iteration are the running accumulators, which are allocated outside
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the per-tile scope and updated by ops *inside* it writing to outside
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addresses (the merge writes new running state — see D3).
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### D3. Interaction with running accumulators
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The online-softmax merge reads the old running `(m, l, O)` and the
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current tile's `(m_j, l_j, O_j)`, producing new running values. To keep
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the new running values outside the recycled region, the merge writes them
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to **stable scratch** allocated once before the loop (a small fixed
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"running-state" arena, distinct from the per-tile scope). Concretely the
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kernel keeps two arenas:
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- **persistent arena** (allocated once): `m, l, O` (and their
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double-buffer if needed for the merge).
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- **scoped arena** (rewound each tile): `scores, P, exp, O_j, scale_*`.
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This mirrors how real flash-attention SRAM budgeting works: a small
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persistent accumulator region + a recycled tile working set.
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### D4. Nesting
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Scopes nest (stack of save-points). Inner scope exit rewinds to the inner
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save-point; outer exit rewinds further. This supports an outer
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"per-query-block" scope around an inner "per-KV-tile" scope for prefill.
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## Alternatives
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### A1. Round-trip temporaries through HBM
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Store intermediates to HBM and reload to "free" TCM scratch. Rejected:
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turns a TCM-resident streaming kernel into an HBM-bandwidth-bound one —
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the opposite of the goal, and it pollutes op_log with spurious DMA.
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### A2. Tiled `tl.composite` (scheduler-managed scratch)
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`tl.composite` recycles per-tile scratch inside PE_SCHEDULER
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automatically. As in ADR-0062 A1, this is attractive but blocked on a
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flash-capable composite kind (two GEMMs + carried `(m,l,O)`), a much
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larger change. `scratch_scope` gives the same memory behaviour for the
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greenlet kernel with a tiny, general primitive. The two can coexist.
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### A3. Grow the scratch budget
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Bump `pe_tcm.kernel_scratch_mb`. Rejected as a fix: it only pushes the
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context-length ceiling out linearly while still leaking O(`n_tiles`)
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scratch, and it misrepresents the hardware (real TCM is small; the point
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of flash attention is O(1) working set). Useful only as a coarse knob,
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not a substitute for recycling.
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## Consequences
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### Positive
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- Removes the artificial `S = 16` validation-scale ceiling; enables
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realistic context lengths in both timing and data modes.
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- Faithfully models flash attention's O(1) working-set property.
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- Small, general primitive (any tiled kernel benefits).
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### Negative
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- A use-after-scope bug silently reads stale bytes. Mitigated by the D2
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contract, by keeping the scope discipline inside a shared attention
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helper, and (optionally) by a debug build that poisons rewound regions.
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- Must coordinate with ADR-0062: prefetch buffers are live across tile
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iterations, so they belong to the persistent arena, not the scoped one.
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## Test Requirements
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1. **Recycling**: a loop of `N` tiles inside `tl.scratch_scope()` keeps
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peak `_scratch_cursor` bounded by one tile's footprint, independent of
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`N` (today it grows linearly and overflows).
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2. **Correctness**: a flash sweep with scopes produces the same `O` as
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the same sweep without scopes at a small `N` that fits without
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recycling (Phase 2).
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3. **Long context**: a sweep at an `N` that would overflow 1 MiB without
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scopes completes (the exact failure the `S=16` cap avoids today).
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4. **Persistent-vs-scoped isolation**: running `(m,l,O)` allocated
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outside the scope retains correct values across `__exit__`.
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5. **Nesting**: nested scopes rewind to the correct save-points.
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