# ADR-0064: Per-op-type CPU issue cost model (command construct + dispatch)

## Status

Proposed

> Supporting ADR for **ADR-0060** (AHBM GQA Fused Attention). The hybrid
> decision there (GEMMs via `tl.composite`, softmax merge in the kernel)
> wins by **offloading tiling to PE_SCHEDULER so the CPU issues coarse
> descriptors and runs ahead, keeping the engines saturated**. That win is
> currently **invisible in the simulator** because per-op CPU issue cost is
> zero. This ADR makes the issue cost real and **op-type-differentiated**,
> so composite-vs-primitive trade-offs (and CPU saturation) are measurable.

## Context

### What exists today

- Every `tl.*` op calls `_emit_dispatch_overhead()` before emitting its
  command (`tl_context.py:123-125, 190, 227, 235, 612, …`), which emits
  `PeCpuOverheadCmd(cycles=dispatch_cycles)` **only if** `dispatch_cycles
  > 0`. So the issue cost is a **single uniform knob** applied identically
  to `tl.load`, `tl.dot`, a MATH op, and `tl.composite`.
- That knob is hardcoded to **0** in both live execution paths
  (`pe_cpu.py:101` greenlet runner, `:195` legacy replay). ⇒ issuing a
  command — *constructing the descriptor and pushing it to the scheduler
  queue* — currently costs **0 ns** on PE_CPU.
- `PeCpuOverheadCmd` is consumed as `yield env.timeout(cmd.cycles)` on
  PE_CPU (`kernel_runner.py:131-132`) and on PE_SCHEDULER
  (`pe_scheduler.py:97-100`); a manual `tl.cycles(n)` also exists
  (`tl_context.py:695`).

### Two findings that frame this ADR (from ADR-0060 review)

- **Q1 — composite issue cost.** Constructing + pushing a `tl.composite`
  is expected to cost on the order of **~40 ns** of CPU time (descriptor
  build + queue push). Today it is **0**. The hook exists; only the value
  (and its per-op-type differentiation) is missing.
- **Q2 — scheduler dispatch vs DMA latency.** `PE_SCHEDULER`'s composite
  dispatch is **non-blocking**: `_dispatch_composite` generates the tile
  plan and enqueues to the feeder, returning immediately
  (`pe_scheduler.py:104-121`). The actual **DMA latency is charged on
  PE_DMA** as each tile flows through (`drain_ns = compute_drain_ns(path,
  nbytes)`, `pe_dma.py:89`), **not** lumped into the scheduler dispatch ⇒
  **no double-counting**, and DMA stays on its modelled component
  (SPEC §0.1). The scheduler's own plan-generation currently costs **0**
  sim time.

### Why uniform-and-zero is wrong for the hybrid

The hybrid's whole argument is that **one** composite descriptor offloads
`N_tiles` worth of GEMM tiling, so the CPU issues `O(1)` coarse commands
instead of `O(N_tiles × ops/tile)` fine ones. With issue cost = 0 (and
uniform), the model cannot show:
- that the primitive path may **fail to saturate** the engines when the
  CPU cannot push fast enough (the core ADR-0060 §1 claim), nor
- that a composite **costs more to construct** than a single primitive but
  far less than the many primitives it replaces.

A single uniform `dispatch_cycles` cannot express this: `tl.load` ≪
`tl.composite` in real construct cost.

## Decision

### D1. Per-op-type issue-cost table on PE_CPU

Replace the single `dispatch_cycles` scalar with a **cost table keyed by
command type**, charged on PE_CPU at issue (before the command is
dispatched to the scheduler). The greenlet pays this time, which is
exactly what gates how fast the CPU can push work — the saturation lever.

Starting estimates (calibrate later — see review items):

| Issued op | CPU issue cost (construct + push) |
|---|---|
| `tl.composite` (GEMM/MATH descriptor) | ~40 ns (Q1 estimate) |
| `tl.load` / `tl.store` (DMA descriptor) | small (a few ns) |
| `tl.dot` / MATH / IPCQ send/recv | small (a few ns) |

The point is the **ratio**: a composite construct ≫ a primitive issue, but
≪ the sum of the primitive issues it replaces. Exact ns are configurable.

### D2. Execution latency stays on the engines (Q2 — no change)

DMA/GEMM/MATH execution latency remains charged on PE_DMA / PE_GEMM /
PE_MATH as today. The issue cost (D1) is **CPU-side only** and additive; it
does **not** touch `drain_ns`, so there is no double-count. This preserves
SPEC §0.1 (latency on modelled components).

### D3. Scheduler plan-generation cost — start at 0, revisit

`PE_SCHEDULER`'s tile-plan generation costs 0 sim time today. Keep that for
now (the dominant lever is CPU issue cost, D1); expose it later via the
existing `overhead_ns` node attr if a scheduler-side cost proves material.
Marked as a review item, not decided here.

### D4. Configurable values; goldens regenerate

The cost table is configurable (per-topology / node attrs, default to the
D1 estimates). Turning issue cost non-zero changes **every** bench's
latency, so golden latencies are **regenerated** once — a model-fidelity
improvement, same posture as ADR-0062's global lazy-load regression.

## Alternatives

### A1. Keep a single uniform `dispatch_cycles > 0`

Rejected: op construct costs differ by an order of magnitude (`tl.load` vs
`tl.composite`); a uniform value either over-charges primitives or
under-charges composites, and in both cases misrepresents the
composite-vs-primitive trade-off the hybrid depends on.

### A2. Charge the issue cost on PE_SCHEDULER instead of PE_CPU

Rejected: the saturation question is *"can the CPU push descriptors fast
enough to keep the engines busy?"* — that is a **PE_CPU** issue-bandwidth
property. Charging it on the scheduler would not model CPU back-pressure.

### A3. Model DMA program/setup time as a separate fixed per-descriptor cost

Real HW pays a DMA descriptor program cost distinct from transfer time.
Deferred: initially fold the descriptor-program cost into the **issuing
op's** D1 cost (a `tl.load` / composite that triggers DMA). Split it out to
a PE_DMA fixed setup only if calibration shows it matters. Review item.

## Consequences

### Positive
- The hybrid's CPU-offload / saturation win (ADR-0060 §1) becomes
  **measurable**, not just structural.
- Composite-vs-primitive and tiling-granularity trade-offs are visible in
  latency, enabling the eval narrative.
- More faithful to hardware (issue bandwidth is a real bottleneck).

### Negative
- **All** bench goldens shift → one-time regeneration (D4); CI golden
  fixtures update.
- Per-op-type values need calibration; the ~40 ns composite figure is an
  estimate, primitives are unspecified — results are only as good as the
  numbers (do not over-claim absolute latencies until calibrated).
- Adds a cost-table lookup on the issue path (negligible runtime).

## Open review items (decided autonomously; revise on review)

1. **Calibration source for the per-op-type values.** Composite ≈ 40 ns is
   a working estimate; primitive issue costs are placeholders. *Recommend:*
   pick values from a documented assumption (instruction-issue + queue
   push) and treat absolute latency as provisional until a real reference
   exists; keep the **ratios** defensible.
2. **Scheduler plan-gen cost (D3).** *Recommend:* keep 0 initially; expose
   via `overhead_ns` if a workload shows scheduler-bound behaviour.
3. **DMA program time (A3).** *Recommend:* fold into the issuing op's cost
   first; split to PE_DMA setup only if needed.
4. **Where the table lives.** *Recommend:* a small central cost module
   keyed by command type, overridable per topology — not scattered node
   attrs — so the values are reviewable in one place.
5. **Regression rollout.** *Recommend:* land lazy-load (ADR-0062) and this
   ADR's cost model in one golden-regeneration pass to avoid two churns.
6. **Interaction with the legacy replay path** (`pe_cpu.py:_execute_legacy`)
   — ensure both the greenlet and replay paths read the same cost table so
   results match. Verify.

## Test Requirements

1. **Composite charges once; primitives charge per-op.** A kernel issuing
   one `tl.composite` over `N_tiles` charges one composite issue cost on
   PE_CPU; the equivalent `N_tiles × ops` primitive kernel charges the
   per-op cost `N_tiles × ops` times. Assert the PE_CPU busy time differs
   accordingly.
2. **DMA latency unchanged (Q2).** For a fixed transfer, PE_DMA `drain_ns`
   is identical to the pre-ADR value — the issue cost is additive on
   PE_CPU, not folded into DMA (no double-count).
3. **Saturation is observable.** With non-zero per-op issue cost, a
   many-tile primitive sweep shows GEMM-engine idle (CPU-bound issue)
   whereas the composite sweep keeps it busy — the ADR-0060 §1 lever.
4. **Determinism:** identical inputs → identical op_log + latency
   (SPEC §0.1).
5. **Path parity:** greenlet and legacy-replay paths produce identical
   issue-cost accounting for the same kernel.
