--- type: reference description: "This page records the speculative-decoding benchmark history and the" tags: - llm - speculative timestamp: 2026-09-03 --- > **SUPERSEDED 2026-08-17.** The Frank redo retired this topology. > See [frank-redo-2026-08-17.md](frank-redo-2026-08-17.md) and > [deployment.md](deployment.md) for the active configuration. # Qwen3.8-27B Speculative-Decoding Matrix This page records the speculative-decoding benchmark history and the production tuning for the Qwen3.8-27B service (`frank-dragoon`), now a pipeline-parallel head with a Dankserver worker. ## Production state (2026-08-17: Q6_K, q8_0/f16 KV, split 0.68) Current production, verified against the live process and journal (2026-08-17, load 02:06 + smoke tests). Topology detail: [[Pipeline Parallelism/Qwen3.8 PP Deployment]]. - Devices: `RPC0` (Dankserver RTX 3090) + `ROCm0` (Strix iGPU). The head passes no `--device` flag. Default selection puts RPC first and picks ROCm for the iGPU (HIP registers before Vulkan). The Vulkan0 side stays an option via `HEAD_BIN=.../build-vulkan-rpc`. - Model: `Qwen3.8-27B-Uncensored-Q6_K.gguf` (JonathanColetti, 22.43 GB, imatrix, 65 blocks, 866 tensors, NO bundled MTP head). The repo's standalone draft sidecar `Qwen3.8-27B-Uncensored-draft-Q8_0.gguf` (3.16 GB) is loaded via `--spec-draft-model`; it runs on ROCm0 only (1718 MiB, zero worker impact). - Spec: **MTP** (standalone draft) `--spec-draft-n-max 4` + the `--spec-default` ngram-mod fallback (48/64/24). Measured acceptance 0.83 then 0.75, mean len 4.0-4.3, per-position ~0.86/0.71/0.71/0.71 — stronger than the old bundled head (0.739/0.554/0.427/0.340). - KV: K **q8_0** + V **f16** (mismatched). 3136 B/token per FA layer = 784 MiB at 262144 (K 1088 + V 2048). RPC0 8624 MiB (11 FA), ROCm0 3920 MiB (5 FA); total 12544 = 16 x 784. **Flash Attention stays enabled with the mismatch** (`resolve_fused_ops: Flash Attention enabled`) via the GGML_CUDA_FA_ALL_QUANTS rebuild of both binaries at 9b05354ec — see the PP doc. - Split: `--tensor-split 0.68,0.32` = 45 of 65 blocks (0-44) on the 3090, 69% of the layer weight. The maximum: 46 blocks -> ~0.3 GiB margin; 48 blocks (f >= 0.7197) adds FA block 47 -> KV 9408 -> OOM by ~0.85 GiB. Boundary math: the PP doc. - Context: pool 262144, 1 slot, `n_ctx_slot = 262144`. The pi `models.json` declares 262144 (contextWindow = maxTokens, house convention). - Worker steady state: 23854/24576 MiB used on the 3090, ~273 MiB free (tight but stable; all allocations happen at load). - Batch 8192 / ubatch 256, `--direct-io`, `--fit off`, `--gpu-layers all`. ## How MTP works on Qwen3.8 (bundled-head era) The unsloth `UD-*` GGUFs keep the MTP (Multi-Token Prediction) head inside the model file. llama.cpp b10411+ auto-detects it and creates the draft context from the target model. Do not pass `--spec-draft-model` for those files. This is superseded: the production `Qwen3.8-27B-Uncensored-Q6_K.gguf` has NO bundled head, and the serve script passes the standalone draft sidecar with `--spec-draft-model` (see Production state above). The stock llama.cpp build must be b10411 or newer. The 2026-08-14 rebuild on Dankclaw is b10433 (`9b05354ec`) with `GGML_HIP` + `gfx1151` + Vulkan. ## History (superseded configurations, 2026-08-14 to 08-16) All superseded configs used `Qwen3.8-27B-UD-Q8_K_XL.gguf` (29.29 GiB, bundled MTP) before the Q6_K swap. The ngram-mod preset survives as the fallback layer under MTP. The Vulkan0 verdict from this era applies to the standalone configuration only; the PP head runs ROCm0. - **2026-08-14: bundled MTP** — `draft-mtp`, n_max 5, parallel 2, ctx 524288 (2 x 262144). ~13.3 t/s single, ~8 t/s per slot under 2 concurrent streams. No premature stop. - **2026-08-15: DSpark sidecar** — `magnitudedev/Qwen3.8-27B-DSpark-GGUF` (Q8_0, 1.36B, block_size 7), n_max 7, PARALLEL=1. Two concurrent 16k prose prompts crashed the ROCm backend with `ROCm error: an illegal memory access` in `common_speculative_impl_draft_dflash::draft` (status 6/ABRT, systemd restart, ~503 during reload). DSpark reuses the DFlash block-diffusion machinery, which shares one draft context across parallel slots; concurrent block drafting faults in HIP stream sync. This is the same multi-slot bug class that retired `draft-dflash` (premature EOS). The crash shows on 16k concurrent streams, not on a 67k pair — concurrency is the trigger, not size. Fix: PARALLEL=1 (requests queue safely). Retired the same day. - **2026-08-15 to 08-16: `--spec-default` (ngram-mod), Vulkan0** — n_match 24 / n_min 48 / n_max 64, draftless, PARALLEL=2 (2 x 32k parallel stream test passes). Chosen for parallel safety after DSpark; kept as the fallback under MTP in the current config. ## Benchmark Method The matrix ran 54 configurations: 3 spec types (`draft-mtp,ngram-mod`, `draft-mtp`, `ngram-mod`) x `--spec-draft-n-max` 2..10 x `--spec-draft-p-min` {off, 0.8}. Each config loaded `llama-server`, then ran 3 generations. The tool is `scripts/spec-matrix-bench.sh`; analysis is `scripts/analyze-spec-matrix.py`. ## Matrix Caveat: Benchmark Prompt The matrix used a repetitive 1100-token prompt (`scripts/bench_live.sh` style). MTP drafts that repeated sentence almost perfectly, which inflated the numbers for long drafts. The matrix reported `draft-mtp` n_max=10 at ~30 t/s. A realistic story prompt gave very different results: | n_max | median t/s (realistic) | | ----- | ----------------------- | | 4 | 12.5 | | 5 | 13.3 (peak) | | 6 | 10.6 | | 7 | 10.1 | | 10 | 8.6 | | none (baseline) | 7.2 | Long drafts collapse to ~18% acceptance on real writing. n_max=5 balances acceptance against draft cost. ## Premature Stop Investigation Two failure patterns appeared during early production tests: 1. Speed collapse. `ubatch >= 512` reduced MTP acceptance to ~25% and generation to ~9 t/s. `ubatch 128` with `batch >= 2048` triggered a premature instant-EOS under 2 concurrent slots. `ubatch 256` avoids both: ~30 t/s single, concurrent-safe. 2. Apparent premature-EOS. The repetitive benchmark prompt with `temp 1.0` sometimes stops after 1-2 tokens because the model legitimately emits an 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 `. 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). This verdict applies to the standalone configuration; the PP head runs ROCm0 (HIP registers before Vulkan). 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 The live tunables live in `scripts/frank-dragoon.env` (head) and `scripts/frank-pp-worker.env` (worker) in this repo. The Production state section at the top of this page records the current values: Q6_K model + standalone MTP draft, split 0.68/0.32, 262144 ctx, PARALLEL=1, q8_0/f16 KV. ## See Also - [[frank-dragoon-benchmark]] - [[ling-3.0-flash-trial]] - [[deployment]]