Adds `-iq3` to the ablation harness: a faithful torch model of llama.cpp's
deployed 3.06-bpw IQ3_XXS format — 256-entry 4-dim magnitude grid
(extracted from ggml-common.h, MIT), signs factored per 8 weights with
the odd-parity constraint priced in (a violating block flips its
smallest-magnitude sign), fp16 super-scale per 256 + 4-bit sub-scale per
32 searched over all 16 codes. Nearest-grid search runs as chunked
matmul-argmin (|g|^2 - 2 q.g) — a full cdist materializes tens of GB on
a 100M-param tensor and OOMed the first run.
Measured (OLMoE-1B-7B, n=200 x hellaswag/arc/mmlu; the first four rows
reproduce the published ablations exactly):
fp16 58.0%
int4 per-row 48.7% (-9.3pp, the shipped container's scheme)
int3-g64 50.5% (-7.5pp)
int3-g64-e8-rot 51.5% (-6.5pp, simulated rate-scaled ball)
int3-iq3 49.3% (-8.7pp)
int3-iq3-rot 51.5% (-6.5pp)
The deployable IQ3 codebook plus rotation exactly ties the simulated E8
ball — that settles #452's codebook decision toward the IQ3-style block
structure, with rotation mandatory (worth 2.2pp on this codebook).
The tool tokenized every context with add_special_tokens=False, so pointing it at a
GLM snapshot scored the model out-of-distribution — the same bug @bokiko found in
eval_glm.py (#108).
Measured on this box (GLM-5.2 int4, engine SCORE mode, same 1023 target tokens,
only the conditioning prefix changed):
corpus no prefix +[gMASK]<sop>
natural prose PPL 29.2 PPL 9.4
markdown + code PPL 131.0 PPL 24.5
It does not just depress scores, it distorts sensitivity to numerical changes: an
exact-vs-IDOT kernel A/B measured without the prefix reported a penalty that halved
AND flipped sign on one of two corpora once the prefix was restored (#153). A
quantization-ablation tool that scores OOD is therefore worse than useless — it
produces confident deltas that are artifacts.
The GLM tokenizer does not add the prefix itself (add_special_tokens=True is a no-op
there), so it must be prepended explicitly. Auto-detected from the vocab rather than
opt-in, so it cannot be lost by omission; --prefix overrides. Models with no such
prefix are unaffected: OLMoE (the tool's default) has no BOS at all, add_special_tokens
True/False give identical ids, so the numbers in #108 measured on OLMoE still stand.
Extends the ablation grammar to int{2,3,4,8}[-g<N>][-rot][-nohead]. -rot round-trips weights through an orthogonal Hadamard Q=diag(±1)·H/√n on the input dim, measuring the exact weight error of a deployed rotate-activations scheme. Engine-free tool (c/tools/quant_ablation.py only). Verified: syntax clean, scheme parser correct (int3/-rot/-nohead), no unsafe constructs. Findings feed the v2 int3-g64 direction (#81/#108).
Measuring "what does int4 cost?" by comparing colibri's score to a published
model-card number does not work: this harness scores 0-shot log-likelihood while
published numbers are few-shot/CoT, and that protocol gap can swamp the
quantization effect entirely (#108).
This removes the confound by construction: take an fp16 model, push its weights
through colibri's own quantizer (quantize -> dequantize, in place), and score both
with the SAME harness on the SAME questions. The only variable is the quantizer, so
the delta IS the quantization cost. Runs on OLMoE in minutes, so a scheme can be
ranked BEFORE committing to a multi-hour GLM conversion.
Quantizer math is replicated from tools/convert_fp8_to_int4.py (symmetric absmax,
per-row scales) and generalised with an optional group size, so grouped/finer schemes
can be compared directly against what ships today.
Measured on OLMoE-1B-7B, n=200/task (#108):
scheme hellaswag arc_c mmlu mean delta
fp16 77.0% 47.0% 47.0% 57.0% --
int4 (shipped) 74.0% 41.0% 31.5% 48.8% -8.2pp
int4-nohead 73.5% 40.5% 37.5% 50.5% -6.5pp
int4-g128 78.5% 45.5% 38.0% 54.0% -3.0pp
int4-g128-nohead 78.5% 46.5% 38.0% 54.3% -2.7pp
The per-row int4 container costs ~8pp, concentrated on the HARD task: MMLU falls to
31.5% against a 25% random baseline while easy HellaSwag barely moves -- per-row
scales eat the small logit margins that hard questions depend on (the same margin
erosion that flips near-tie tokens in #100). group=128 recovers ~63% of the loss.
Keeping lm_head/embed in fp16 is NOT the fix (+1.7pp alone, +0.3pp atop grouping).
Includes a coverage assert: transformers fuses MoE experts into 3D tensors, so a
ndim==2 filter silently skips every expert and leaves ~85% of the model in fp16 while
appearing to work. The tool fails loudly instead of reporting fiction.
Dev-only tool (torch + transformers); the engine's dependency-free path is untouched.
ZacharyZcR