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docs: E8 prefill pipeline result — flat, DDR-bandwidth-bound

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Implemented the prepA/collectC <-> rknn_matmul_run overlap as an M-chunk
software pipeline (correct, byte-identical). Flat at every chunk depth.
Timing the CPU-prep section with the NPU running vs idle showed it runs +43%
slower under overlap (1973 -> 2827 us): the "idle" cores contend for DDR with
the NPU streaming weights. rknn_matmul_run does not busy-poll (cores 96-100%
idle) but the bus is saturated. No-go; not merged.

Campaign through-line: decode, GEMM ceiling (30.5%), and prefill overlap are
all DDR-bandwidth-bound. Only levers left are fewer bytes (int4, broken) or
fewer passes (MTP, shipped).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EWpfhDgYNA21tETDP9ueBE
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mfritsche
2026-07-10 16:24:40 +02:00
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@@ -351,6 +351,40 @@ Standalone C client on `rknn_matmul_api.h` + `librknnrt` (int8, timing only `rkn
decode, but prefill is 20% serial CPU). K>8192 to one context is catastrophic → our manual
K-segmentation is correct and necessary.
## E8 — prefill CPU/NPU pipelining: NEGATIVE (DDR-bandwidth-bound, 2026-07-10)
Prefill is ~22% CPU-serial (prepA quantize + collectC dequant) around the NPU run
(Amdahl from core-scaling 18.28→38.13 = 2.09×). Idea: chunk M and overlap the CPU
prep of chunk c+1 with the NPU run of chunk c. Ceiling ~1.28× prefill (less e2e:
O(M²) attention isn't pipelined). Designed with the rknpu-specialist; implemented as
a 2-stage M-chunk software pipeline (double-buffered A/C, one context set, NPU run on
a `std::thread` worker so a concurrent OMP team can't serialize it).
**Correct but flat.** Greedy output byte-identical to the non-pipeline build. pp512
38.10 → 37.5–37.8 (all power-of-2 chunks); pp2048 (deep 8-chunk) 26.54 → 26.36. No
gain at any chunk depth. (`kChunkM=192` → 29.5 is self-inflicted: `next_power_of_two(192)=256`
context padding wastes 1.5× NPU rows. Power-of-2 chunks have `ctx_M==kChunkM`, no waste.)
**Why (measured, not inferred):** timed the CPU-prep section with the NPU running vs idle.
| CPU-prep section | mean |
|------------------|-----:|
| NPU idle (overlap off) | ~1973 µs |
| NPU running (overlap on) | ~2827 µs (**+43%**) |
`bind` cost ~100 µs either way (negligible — not the per-chunk bind tax). The overlap
*happens*, but the CPU-prep runs **43% slower while the NPU streams weights**, because
both contend for DDR. The inflation ≈ the time the overlap saves → net flat, by physics.
`rknn_matmul_run` does NOT busy-poll (A76 cores 96–100% idle during pure matmul) — the
cores are free, but the **bus is not**. Idle cores ≠ idle bandwidth.
**Verdict: no-go, reverted.** A 2-context-set rewrite would fix the bind tax but not the
bandwidth wall, so it can't help. Branch `perf-prefill-pipeline` kept local/unpushed as a
correct reference; not merged. This is the campaign's through-line closing: decode
(bandwidth), GEMM ceiling 30.5% (bandwidth), prefill overlap (bandwidth) — **no idle DDR
bandwidth on RK3588 to exploit.** Only levers left: fewer bytes (int4, broken) or fewer
passes (MTP, shipped).
## Campaign verdict — decode levers
Decode (~2.6→3.35 t/s) is bandwidth + CPU-tail bound; the NPU can't be the lever there.