--- name: comfyui-launch-flags description: Pick the right ComfyUI startup flags for VRAM, attention, caching, and speed — the full decision matrix for OOM (--novram / --cache-none / --disable-smart-memory), shared-VRAM creep on Windows (--reserve-vram N), model-switching with big text encoders (--cache-none), high-VRAM throughput (--gpu-only / --highvram), and attention-backend selection (--use-sage-attention for speed, --use-pytorch-cross-attention as the highest-quality / Z-Image-safe fallback). Also the acceleration-stack + Blackwell/RTX 5000 (sm_120) notes. Use when a graph OOMs (especially long video like LTX 2 / WAN), when the GPU spills into shared VRAM and slows to a crawl, when switching between models eats all RAM, when Z-Image produces black/garbled output under Sage, or when deciding which attention backend to launch with. Flag names verified against upstream comfy/cli_args.py — see Sources. globs: - "**/*.json" - "**/packs/**" --- # ComfyUI launch/performance flags ## Overview ComfyUI's runtime behavior is controlled by CLI flags passed to `main.py` (e.g. `python main.py --reserve-vram 2 --use-sage-attention`). The three that matter most for making a graph *run* — rather than OOM or crawl — are the **VRAM strategy**, the **attention backend**, and the **cache mode**. This skill is the decision matrix for choosing them. > ⚠️ **Verification note (June 2026).** Every flag below was checked against > upstream [`comfy/cli_args.py`](https://github.com/comfyanonymous/ComfyUI/blob/master/comfy/cli_args.py). > ComfyUI adds/renames flags often — when in doubt run `python main.py --help` > in the target install and prefer that over this list. One common non-upstream > flag: **`--enable-triton-backend` is a SwarmUI backend flag, NOT a ComfyUI > `main.py` flag** — don't pass it to ComfyUI directly. > ℹ️ **How to apply today.** The MCP's `start_comfyui` currently *replays the > exact argv of the previous run* — it does not compose fresh flags. So set > these when you launch ComfyUI yourself (the `python main.py …` line, a > `run.bat`/shell alias, or the SwarmUI backend args box), then `start_comfyui` > will preserve them on restart. (Injecting flags through the tool is a tracked > follow-up.) --- ## Decide first: which flag do you need? ``` Symptom ▶ Flag(s) to try ───────────────────────────────────────────────────────────────────────────── CUDA out of memory, long video (LTX 2 / WAN) ▶ --novram (+ --cache-none) OOM, still want models resident when they fit ▶ --reserve-vram N then --disable-smart-memory GPU slows to a crawl, spills into "shared GPU ▶ --reserve-vram 2..4 memory" (Windows WDDM) mid-run RAM blows up switching between models, or a huge ▶ --cache-none text encoder (FLUX 2 / Mistral) won't unload Plenty of VRAM (48GB+), want max throughput ▶ --gpu-only or --highvram Want faster sampling on NVIDIA ▶ --use-sage-attention (see caveats) Z-Image produces BLACK / wrong output ▶ --use-pytorch-cross-attention (NOT sage) Sage gives black output on some models ▶ --use-pytorch-cross-attention (or fix dtype) ``` VRAM strategy and attention backend are each **mutually exclusive groups** — pass at most one from each. You can combine one VRAM flag + one attention flag + one cache flag (e.g. `--novram --use-sage-attention --cache-none`). --- ## VRAM strategy (mutually exclusive) | Flag | What it does | Use when | |------|--------------|----------| | `--gpu-only` | Keep everything (incl. text encoders) on GPU | 48GB+ card, single model, max speed | | `--highvram` | Keep models resident in VRAM after use | High-VRAM card, repeated runs of one model | | *(default)* | ComfyUI's smart offload | Most setups — try this first | | `--lowvram` | Offload text encoders / parts to CPU | Mid card OOMing on load | | `--novram` | Extreme offload — minimal VRAM footprint | OOM on long video / huge models; pair with `--cache-none` | | `--cpu` | Everything on CPU (very slow) | No usable CUDA GPU only | Modifiers (combine with the above): - **`--reserve-vram N`** — reserve N GB for the OS / other apps. The fix for the Windows failure mode where the GPU quietly starts using **shared** VRAM and throughput collapses. Typical `2`–`4`; bump to `10` for heavy video decode. - **`--disable-smart-memory`** — force aggressive offload to regular RAM instead of keeping models cached in VRAM. Reach for this when a run gets *stuck* or OOMs intermittently. Slightly slower, much more robust. - **`--async-offload`** — async weight offload streams (default on where supported); `--disable-async-offload` to turn off if it misbehaves. --- ## Attention backend (mutually exclusive) | Flag | Notes | |------|-------| | `--use-sage-attention` | Quantized SageAttention kernel, ~20–40% faster sampling. Needs the `sageattention` package installed and version-matched — see [`triton-sageattention`](../triton-sageattention/SKILL.md). | | `--use-flash-attention` | FlashAttention kernels. Needs `flash-attn` built for your torch/CUDA. | | `--use-pytorch-cross-attention` | PyTorch SDPA. **Highest quality, always available, no extra deps.** The safe default and the correct fallback. | | `--use-split-cross-attention` / `--use-quad-cross-attention` | Memory-optimized math attention for older/low-VRAM cards. | **Two gotchas worth memorizing:** 1. **Z-Image + Sage = broken.** Z-Image (Turbo/Base) does **not** sample correctly under `--use-sage-attention` — you get black or garbled output. Launch Z-Image with **`--use-pytorch-cross-attention`** instead. See [`z-image-txt2img`](../z-image-txt2img/SKILL.md). 2. **Sage black output on other models.** If a model outputs black *only* with Sage, either switch to `--use-pytorch-cross-attention`, or (SwarmUI) set Advanced Sampling → Preferred DType = Default (16-bit). Sage-on vs Sage-off also produces *slightly different* images — expect non-identical seeds. > When a graph hard-crashes with `No module named 'sageattention'` / > `triton: unavailable`, the fix is the sdpa / no-compile fallback in > [`triton-sageattention`](../triton-sageattention/SKILL.md), not this flag. --- ## Cache mode (mutually exclusive) | Flag | Effect | |------|--------| | *(default `--cache-ram`)* | Cache results under RAM pressure | | `--cache-classic` | Aggressive result caching | | `--cache-lru N` | Keep at most N node results (LRU) | | `--cache-none` | Cache nothing — re-executes every node; **lowest RAM/VRAM**. Essential when switching between dual models or when a giant text encoder (FLUX 2's Mistral) must fully unload. | --- ## Speed / precision - **`--fast`** — enables experimental, potentially quality-degrading optimizations. Accepts specific `PerformanceFeature` values: `fp16_accumulation`, `fp8_matrix_mult`, `cublas_ops`, `autotune`. Bare `--fast` turns them all on. Test output quality before committing to it. - **UNet/VAE/text-encoder dtype casts** exist too (`--fp8_e4m3fn-unet`, `--fp16-unet`, `--bf16-unet`, `--fp32-unet`, …) for forcing a compute precision; usually the model/loader picks the right one, so only reach for these to work around a specific dtype error. --- ## Recommended combos (recipes) ``` Long video OOM (LTX 2 / WAN, 24GB): --novram --cache-none (add --disable-smart-memory if it stalls) Windows shared-VRAM creep: --reserve-vram 3 FLUX 2 / huge text-encoder swaps: --cache-none High-VRAM throughput (48GB+): --gpu-only (or --highvram) Fast NVIDIA sampling (most models): --use-sage-attention Z-Image (any): --use-pytorch-cross-attention ``` Cross-refs: video OOM specifics in [`ltxv2-video`](../ltxv2-video/SKILL.md) / [`wan-t2v-video`](../wan-t2v-video/SKILL.md); per-model VRAM math in [`troubleshooting`](../troubleshooting/SKILL.md) and [`model-compatibility`](../model-compatibility/SKILL.md). --- ## Acceleration stack & GPU coverage (context) The attention/compile accelerators are **version-locked to your exact torch + CUDA + Python**. A mismatched wheel doesn't just fail to import — it can break the torch install. A known-good, mutually-compatible stack for late-2025 / 2026 NVIDIA (including **Blackwell / RTX 5000, `sm_120`**) looks like: | Component | Role | Notes | |-----------|------|-------| | Torch + CUDA | base | e.g. Torch 2.9.x on CUDA 12.8/13; use the wheel index matching your driver | | Triton | `torch.compile` / inductor | Windows: `triton-windows` (woct0rdho) | | SageAttention | `--use-sage-attention` | wheel matched to torch/CUDA/python | | FlashAttention | `--use-flash-attention` | built per torch/CUDA/python | | xFormers | memory-efficient attention | optional | | InsightFace | FaceID / IP-Adapter / ReActor | `onnxruntime-gpu` alongside | Operational facts worth carrying: - **No system-wide CUDA toolkit is required** to *run* ComfyUI — an up-to-date NVIDIA driver + prebuilt wheels are enough. A full CUDA/MSVC/cuDNN toolchain is only needed to *compile* kernels yourself. - **Broad arch coverage** when building wheels: `TORCH_CUDA_ARCH_LIST=7.5;8.0;8.6;8.9;9.0;10.0;12.0+PTX` spans RTX 20xx→50xx and datacenter (A100/H100/B200). `+PTX` lets newer archs JIT. - **DeepSpeed has no wheels for Python 3.13**; several accel wheels lag the newest Python — 3.10–3.12 is the safe range for the full stack. - **Clear the Triton cache** (`~/.triton` / `%USERPROFILE%\.triton` and temp) when you hit stale-kernel Triton errors after an upgrade. - Prefer **`uv pip install`** over pip for the venv — dramatically faster resolves/downloads. `install_comfyui` already supports this via `preferUv`. - **A single bad custom node can crash all of ComfyUI at startup.** Install/test acceleration and new node packs on a fresh/known-good install, not before a deadline. See [`troubleshooting`](../troubleshooting/SKILL.md). ## Quantization quick take - **FP8-*scaled*** (per-tensor scaled) is markedly higher quality than plain base FP8, ~half the size of BF16, and usually faster. - **Prefer FP8-scaled over GGUF when you have enough system RAM** — ComfyUI's block-swap streams from RAM, so BF16/FP8 can run on 24GB GPUs given ample RAM. Fall back to GGUF (Q8→Q4) only when RAM is the constraint. - **NVFP4 / NVFP8** are markedly faster on Blackwell (RTX 5000) at near-BF16 quality for supported models; LoRA support on NVFP4 is still partial. --- ## Sources - ComfyUI CLI args (authoritative): - ComfyUI startup flags docs: - Operational flag/stack guidance distilled from community ComfyUI auto-installer changelogs (SECourses) — flags cross-checked against upstream above; no third-party scripts, presets, or model files are reproduced here.