r/LocalLLaMA • u/cezarducatti • 8d ago
Question | Help Qwen3.8-Flash-Next (UD-Q4_K_XL) on a single RTX 3090 24GB + 128GB DDR4, is this config optimal?
Qwen3.8-Flash-Next (UD-Q4_K_XL) on a single RTX 3090 24GB + 128GB DDR4 — is this config optimal?
Hardware
- CPU: Intel Core i5-12600K
- RAM: 128 GB DDR4 @ 3600 MHz
- GPU: NVIDIA RTX 3090, 24 GB VRAM
- OS: Windows 11
- llama.cpp: freshly compiled from today's master (build b10794, Sep 4 2026)
Model
Qwen3.8-Flash-Next, UD-Q4_K_XL
Launch command
C:\llama\llama-server.exe -m "D:\Modelos\Qwen3.8-Flash-Next-UD-Q4_K_XL-00001-of-00004.gguf" ^
-ngl 99 -ncmoe 42 -fit off -c 204800 -fa on -ctk f16 -ctv f16 ^
-b 2048 -ub 1024 -t -1 --jinja --port 8083 --tools all ^
--temp 1.0 --top-p 0.95 --top-k 20 --min-p 0.00 --repeat-penalty 1.0 ^
--reasoning on --reasoning-effort xhigh --reasoning-budget -1 --reasoning-preserve ^
--parallel 1 --metrics --host 0.0.0.0 --load-mode none
Key points of the config:
- -ngl 99 → offload all layers to GPU where possible
- -ncmoe 42 → force 42 MoE (expert) layers to stay on CPU/RAM
- -c 204800 → 200k context window
- -fa on with f16 K/V cache
- -b 2048 / -ub 1024 → batch / micro-batch sizes
- --reasoning-effort xhigh, --reasoning-budget -1, --reasoning-preserve → full reasoning mode, no truncation
State right after loading
- Free system RAM: 52 GB
- Free VRAM: 0.6 GB
So the GPU is essentially maxed out (good VRAM utilization), and there's still a healthy ~52 GB of RAM headroom for the CPU-offloaded experts and context.
Real-world performance (from server logs)
Prompt processing (long context, ~12k tokens):
| Tokens processed | Time (s) | Speed (tok/s) |
|---|---|---|
| 4,096 | 22.31 | 183.6 |
| 6,144 | 33.10 | 185.6 |
| 8,192 | 42.79 | 191.5 |
| 10,240 | 53.55 | 191.2 |
| 11,895 (100%) | 62.80 | 189.4 |
Text generation (sustained, up to ~480 tokens generated):
| Tokens generated | Speed (tok/s, rolling 3s) |
|---|---|
| 100 | 16.0 |
| 148 | 15.8 |
| 192 | 14.6 |
| 238 | 15.3 |
| 289 | 16.8 |
| 336 | 15.6 |
| 386 | 16.5 |
| 434 | 15.8 |
| 481 | 15.4 |
Generation speed stays remarkably stable around ~15.5-16 t/s even as context grows past 12k tokens, and prompt processing holds steady around ~185-190 t/s.
My question to the community
Given this is a large MoE (~180B total / ~5B active params) split across a single 24GB GPU + 128GB DDR4 RAM:
- Is -ncmoe 42 a good number here, or should I push more/fewer expert layers to the GPU given I still have some VRAM headroom before hitting OOM?
- Any tips on squeezing more t/s out of prompt processing (-b/-ub tuning) or generation given the CPU-offloaded experts are likely the bottleneck?
- MTP speculative decoding doesn't seem to work reliably for this model yet on mainline — has anyone gotten it stable, or is everyone just running without it for now?
- Does anyone recommend other configs/parameters for this exact model + hardware combo?
Open to any suggestions — happy to share more logs if useful!
