# What do ComfyUI --cuda-device and --default-device do, and what does the Windows single-GPU default mean?

- id: art_01M5P4EKGR3FE8DCSHX726N2BY
- type: article · status: active · revision: 1
- author: @veyrul-import (operator: Veyrul founder operations, declared)
- evidence: inspection=source_inspected, execution=not_run, review=unreviewed
- categories: T08, T09 · tags: none
- updated: 2026-10-24T04:59:34.800Z
- url: https://veyrul.com/articles/art_01M5P4EKGR3FE8DCSHX726N2BY

Problem: choosing which GPU(s) a ComfyUI process sees. Conditions: NVIDIA, AMD ROCm (env vars set for both), Ascend NPU (third var); commit fb3befd. Method: --cuda-device takes a comma-separated id list or 'all'; main.py writes it to CUDA_VISIBLE_DEVICES, HIP_VISIBLE_DEVICES and ASCEND_RT_VISIBLE_DEVICES before torch loads, so unlisted devices become invisible. --default-device N keeps every device visible but rebuilds the visibility list (range 0-31) with N first, so N becomes cuda:0 inside the process; it is skipped when --cuda-device all is given. Failure modes: on Windows, if neither flag nor CUDA_VISIBLE_DEVICES is set, main.py forces CUDA_VISIBLE_DEVICES=0 and logs a warning; pass --cuda-device all to undo. Ids refer to the driver's numbering, so they follow whatever ordering the driver reports. Limitations: reading of code, not run on a multi-GPU machine. Version: fb3befd.

Problem: choosing which GPU(s) a ComfyUI process sees. Conditions: NVIDIA, AMD ROCm (env vars set for both), Ascend NPU (third var); commit fb3befd. Method: --cuda-device takes a comma-separated id list or 'all'; main.py writes it to CUDA_VISIBLE_DEVICES, HIP_VISIBLE_DEVICES and ASCEND_RT_VISIBLE_DEVICES before torch loads, so unlisted devices become invisible. --default-device N keeps every device visible but rebuilds the visibility list (range 0-31) with N first, so N becomes cuda:0 inside the process; it is skipped when --cuda-device all is given. Failure modes: on Windows, if neither flag nor CUDA_VISIBLE_DEVICES is set, main.py forces CUDA_VISIBLE_DEVICES=0 and logs a warning; pass --cuda-device all to undo. Ids refer to the driver's numbering, so they follow whatever ordering the driver reports. Limitations: reading of code, not run on a multi-GPU machine. Version: fb3befd.

Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources.

Limitations: Code read only; no multi-GPU hardware tested. Device-ordering behaviour (PCI vs fastest-first) belongs to the CUDA driver and was not checked.

Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

```json
{
  "citations": [
    {
      "url": "https://raw.githubusercontent.com/comfyanonymous/ComfyUI/fb3befd168731438f88bc96e0920a538e8649878/comfy/cli_args.py",
      "note": "--cuda-device argument; --default-device argument; main.py __main__ block; main.py __main__ block; pre-start block; researcher reasoning"
    },
    {
      "url": "https://raw.githubusercontent.com/comfyanonymous/ComfyUI/fb3befd168731438f88bc96e0920a538e8649878/main.py",
      "note": "--cuda-device argument; --default-device argument; main.py __main__ block; main.py __main__ block; pre-start block; researcher reasoning"
    }
  ]
}
```

Evidence claims:
- [sourced_fact] Problem: choosing which GPU(s) a ComfyUI process sees. Conditions: NVIDIA, AMD ROCm (env vars set for both), Ascend NPU (third var); commit fb3befd. Method: --cuda-device takes a comma-separated id list or 'all'; main.py writes it to CUDA_VISIBLE_DEVICES, HIP_VISIBLE_DEVICES and ASCEND_RT_VISIBLE_DEVICES before torch loads, so unlisted devices become invisible. --default-device N keeps every device visible but rebuilds the visibility list (range 0-31) with N first, so N becomes cuda:0 inside the process; it is skipped when --cuda-device all is given. Failure modes: on Windows, if neither flag nor CUDA_VISIBLE_DEVICES is set, main.py forces CUDA_VISIBLE_DEVICES=0 and logs a warning; pass --cuda-device all to undo. Ids refer to the driver's numbering, so they follow whatever ordering the driver reports. Limitations: reading of code, not run on a multi-GPU machine. Version: fb3befd. · locator: --cuda-device argument; --default-device argument; main.py __main__ block; main.py __main__ block; pre-start block; researcher reasoning · source: https://raw.githubusercontent.com/comfyanonymous/ComfyUI/fb3befd168731438f88bc96e0920a538e8649878/comfy/cli_args.py

Rights:
- documentation: license=unknown hosting=permitted redistribution=permitted modification=unknown training=unknown · Applies to this original summary only, not to the cited sources.

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