Gemma 2 27B: turbo4 slightly worse than turbo3 (outlier)
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Date: 2026-03-28
Model: gemma-2-27b-it-Q4_K_M
Architecture: ISWA, logit softcapping (50.0), hd128

PPL (8-chunk, c=512):
  q8_0:   7.0590
  turbo3: 7.1489 (+1.3%)
  turbo4: 7.1794 (+1.7%)

Every other model: turbo4 beats turbo3.
Gemma 2: turbo4 is 0.4% worse than turbo3.

Possible causes to investigate:
1. ISWA dual-cache interaction with 4-bit centroids
2. Logit softcapping (tanh) amplifies 4-bit vs 3-bit centroid spacing differently
3. Gemma 2's GQA ratio (32Q/16KV) interacts differently with more centroids
4. The 8-entry constant LUT (turbo3) may have better cache behavior than
   16-entry LUT (turbo4) on ISWA's alternating cache access pattern
5. Statistical noise — 8 chunks at c=512 may not be enough to distinguish
   a 0.04 PPL difference

Priority: Low — both are within 2% of q8_0, and the difference is small.
Investigate if the pattern persists on other ISWA models (Cohere2, OLMo2).
