docs: record --spec-default + Vulkan0 + PARALLEL=2 final state; 262k-ctx ROCm-vs-Vulkan bench

docs/qwen3.8-spec-matrix.md

@@ -4,7 +4,21 @@ This page records the speculative-decoding benchmark and the production
 tuning for the unsloth Qwen3.8-27B `UD-Q8_K_XL` service on Dankclaw
 (`frank-default-large`).
 
-## Result (applied, 2026-08-15: DSpark)
+## Result (applied, 2026-08-15: --spec-default)
+
+- Model: `Qwen3.8-27B-UD-Q8_K_XL.gguf` (29.29 GiB, bundled MTP).
+- Spec: llama.cpp **`--spec-default`** preset = **ngram-mod** (n_match 24,
+  n_min 48, n_max 64).  No draft model.  Chosen 2026-08-15 after DSpark
+  crashed under parallel load (see below).
+- **PARALLEL=2** — passes the 2x32k parallel stream test (both streams
+  complete 256/256 concurrently, no crash).  ngram-mod is draftless and
+  parallel-safe.
+- **Backend: Vulkan0** — at 262144 ctx (~200k prompt) Vulkan decodes
+  5.68 t/s vs ROCm 3.83 t/s (48% faster); ROCm only wins prefill (119 vs
+  106 t/s).  See the high-context bench table below.
+- Batch 8192 / ubatch 256, ctx 524288 (262144 per slot).
+
+## Result (superseded 2026-08-15: DSpark)
 
 - Model: `Qwen3.8-27B-UD-Q8_K_XL.gguf` (29.29 GiB, bundled MTP).
 - Spec: **DSpark sidecar draft** `magnitudedev/Qwen3.8-27B-DSpark-GGUF`
@@ -13,6 +27,7 @@ tuning for the unsloth Qwen3.8-27B `UD-Q8_K_XL` service on Dankclaw
 - **PARALLEL=1.**  Two concurrent 16k prose prompts crashed the ROCm backend
   with `ROCm error: an illegal memory access` during per-sequence block
   sampling.  See the DSpark parallel incident below.
+- Retired the same day in favor of `--spec-default`.
 - Batch 8192 / ubatch 256, ctx 524288 (262144 per slot).
 
 ## Result (superseded 2026-08-14: bundled MTP)
@@ -118,24 +133,42 @@ Two failure patterns appeared during early production tests:
    EOS on a wall of repeated text. This is sampling variance, not a
    corruption bug. Realistic prompts never stop early.
 
+## High-Context Backend Comparison (2026-08-15)
+
+Tool: `scripts/bench-high-ctx.sh <device> <ctx>`.  Both runs at ctx 262144
+with a ~200k-token realistic-prose prompt, `--spec-default`, batch 8192 /
+ubatch 256, single slot, on scratch port 7892.
+
+| Device | Prompt tokens | Prefill (t/s) | Generation (t/s) | Wall (single req) |
+| ------ | ------------- | ------------- | ---------------- | ----------------- |
+| ROCm0 | 202822 | **119.1** | 3.83 | **1770 s** |
+| Vulkan0 | 200235 | 106.3 | **5.68** | 1929 s |
+
+Verdict: at very high context, Vulkan0 decodes **48% faster** (5.68 vs
+3.83 t/s); ROCm0 prefills 12% faster.  Generation is the interactive cost
+(users stream tokens), so **Vulkan0** is the production backend.  Both
+`tg_3s` values were rock-steady across the run (3.83 and 5.67 t/s).
+High-context decode collapses on both backends versus ~25 t/s at 16k ctx;
+that is the long-context attention penalty, not a spec artifact.
+
 ## Production Configuration
 
 `scripts/frank-default-large.env`:
 
 ```ini
-SPEC_TYPE=draft-dspark
-DRAFT_FILE=Qwen3.8-27B-DSpark-Q8_0.gguf
-SPEC_DRAFT_N_MAX=7
-SPEC_DRAFT_P_MIN=
+SPEC_DEFAULT=1            # llama.cpp preset = ngram-mod (48/64/24)
+SPEC_TYPE=
+DRAFT_FILE=
 BATCH_SIZE=8192
 UBATCH_SIZE=256
 CTX_SIZE=524288
-PARALLEL=1   # DSpark multi-slot crashes on this box (2026-08-15)
+PARALLEL=2                # passes 2x32k parallel stream test (2026-08-15)
+DEVICE=Vulkan0            # 48% faster decode at 262k ctx
 ```
 
 Window: computed at 262144 per slot (the pi `models.json` contextWindow for
 `frank-default-large`).  The unit reports `n_ctx_slot = 262144`; CTX_SIZE is
-a pool ceiling and per-slot stays 262144 regardless of the single slot.
+a pool ceiling and per-slot stays 262144 regardless of the slot count.
 
 ## See Also