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perf: back the CPU similarity GEMM with OpenBLAS

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The CPU path was a scalar triple loop. It is the correctness oracle for the GPU
backends, but it is also what CI runs — there is no GPU on the N100 host — and
since AR-003 removed the per-frame face cap, a crowded frame now scores many
faces against a library-scale gallery. Scoring one face against 5000 embeddings
is 2.6 MFLOP; in scalar that does not hold up (AR-027).

S(g,f) viewed as row-major [n_faces x n_gallery] is exactly query * gallery^T,
so the loop nest collapses into a single cblas_sgemm.

OpenBLAS is optional in the build: found via pkg-config, and the scalar path
remains when it is absent so no hard dependency is added and the two can be
diffed when a similarity looks wrong. The configure step warns rather than
failing, since a developer without it should still get a working tree.

The test target links it too. Without that the suite compiles the scalar
fallback while the builder image ships CBLAS, so CI would be verifying a kernel
that is not the one running in production — the same class of mistake as testing
a path the gate never executes.

Recorded as required (not optional) in the DP-007 image, for the same reason.

Suite: 92 cases, 6136 assertions, with CBLAS compiled in.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

TRACES: AR-026, AR-027, DP-007 | SR-001
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dtourolle
2026-07-31 15:04:29 +02:00
co-authored by Claude Opus 5
parent 13bdc27566
commit cc1bed92d8
8 changed files with 325 additions and 17 deletions
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@@ -964,8 +964,21 @@ cannot silently change what a green build meant.
| HDF5 (C++) | Galleries are HDF5-native; also the dump format |
| FFmpeg dev libs — `libavformat`, `libavcodec`, `libavutil`, `libswscale`, **`libswresample`** | Decode. See the note below on swresample |
| Python 3 + numpy, h5py, scipy | Python-side tests, replay, traceability tooling |
| **OpenBLAS** | Backs the CPU similarity GEMM. Without it the fallback is a scalar loop, and the CPU path is exactly what this host runs — see below |
| Catch2, nlohmann/json | **Vendored into the image, not fetched.** Both are `FetchContent`-ed today (`CMakeLists.txt:220`, `tests/CMakeLists.txt:8`), which makes every CI run depend on GitHub reachability |
**OpenBLAS is not optional here, despite being optional in the build.** CI has no
GPU, so `SAE_GEMM_BACKEND=CPU` is the only path it exercises — and since AR-003
removed the per-frame face cap, a crowded frame scores many faces against a
library-scale gallery. The scalar fallback is correct but scales badly, which
would make the CPU path the bottleneck in the one place it cannot be avoided
(AR-027). The build warns when it is missing rather than failing, so a developer
without it still gets a working tree; the image must not be that case.
The test target links it too. Otherwise the suite compiles the scalar fallback
while the image ships CBLAS, and CI would verify a kernel that is not the one
running in production.
**Deliberately excluded:** CUDA, TensorRT, ROCm — no GPU to use them. Also the
ONNX Runtime *GPU* providers; only the CPU provider is relevant, and only for T3
smoke tests.