ComfyUI-JITBlockSwap

Block Swap (RAM Offload) node for native ComfyUI MODEL — run DiT models larger than VRAM (e.g. Wan 2.2 / Bernini-R 14B fp16, 28.6 GB on a 24 GB GPU) by keeping the first N transformer blocks in system RAM and streaming each block to the GPU only while its forward runs.

Node

advanced/model → Block Swap (RAM Offload) (BlockSwap)

input	default	meaning
model	—	MODEL from UNETLoader (place AFTER LoRA loaders)
blocks_to_swap	20	blocks kept in RAM (Wan 14B has 40). Raised automatically if the resident part still exceeds the VRAM weight budget
pin_memory	true	page-lock CPU copies for fast PCIe transfer

How it works

• Hooks ModelPatcher ON_LOAD: after ComfyUI's normal load pass it re-organizes the diffusion model — swap blocks get their weight patches (LoRA) baked in once, moved to (pinned) RAM; everything else is made fully GPU-resident, removing the slow per-layer LowVramPatch cast path.
• Each swapped block's forward is wrapped: params are repointed to a GPU copy just-in-time and repointed back to the CPU master afterwards (no device-to-host copy needed — weights never change during inference). Transfers are synchronized before compute: async submission of large pinned H2D copies interleaved with DiT kernels busy-loop hangs the GPU on Windows/WDDM (observed: 100% util at ~100 W). Cost is ~0 when compute-bound (measured 28-29 s/it either way on Wan 14B).
• ON_DETACH restores state; placement is re-applied on every load, so model switching (high/low noise) and LoRA changes are safe.

Notes / limitations

• Chain: UNETLoader → LoraLoaderModelOnly → BlockSwap → ModelSamplingSD3 → KSampler.
• Requires launching ComfyUI with --disable-dynamic-vram (legacy ModelPatcher). Without the flag the node does NOT error or stop the workflow — it passes the model through unchanged and prints [BlockSwap] dynamic VRAM patcher detected, skipping. to the console only (nothing is shown in the browser UI). The run then relies on ComfyUI's standard memory management: small jobs may still succeed, large models will OOM or fall back to slow partial loading. If block swap "doesn't seem to work", check the console for this line first.
• Works on any model whose diffusion model exposes .blocks or .transformer_blocks (Wan-family and LTX-2.3 22B AV tested; Flux double blocks are NOT covered).
• Do not chain two BlockSwap nodes on the same model.
• Approx. cost: one PCIe H2D transfer per swapped block per forward call (~25-30 ms per 660 MB fp16 block on PCIe 4.0 x16).

LTX-2.3 22B (fp8) support

Tested 2026-07-10 on RTX 4090 24 GB with ltx-2.3-22b-dev-fp8.safetensors (48 transformer_blocks): 360×360×49f I2V completed in 40.2 s with blocks_to_swap=12 (auto-raised to 14 to fit the weight budget), 17.9 GB resident vs a fully packed card without the node (47.4 s) — sample workflows in workflows/.

fp8 checkpoints store weights as comfy_kitchen QuantizedTensor wrapper subclasses, which need special handling implemented here:

• Wrapper subclasses cannot be moved with the .data repointing trick (their device is fixed at construction and the quantized payload lives in _qdata); swapped QuantizedTensor params are exchanged at the module attribute level instead.
• A sibling ModelPatcher clone (a chain without BlockSwap) calling unpatch_model → model.to() makes Module._apply rebuild the wrappers, which would free a still-pinned _qdata and poison the CUDA context (CUDA error: invalid argument much later). The node guards the base model's .to() to unpin first.

Known issue (open)

Large bf16 checkpoints (LTX-2.3 22B bf16, 43 GB, 30/48 blocks swapped) fail after the first sampling pass with CUDA error: invalid argument when the next node moves its own model to the GPU (observed with LTXVLatentUpsampler in a 2-pass workflow). Sampling itself completes at full speed; the CUDA context is left poisoned. Ruled out so far: runtime LoRA, pin_memory (fails with pinning disabled), comfy's own pin budget, ModelPatcher.partially_unload (fails without it running), mmap-backed masters (fails after materializing them to plain RAM), comfy-aimdo allocator (not active in legacy mode). The fp8 checkpoint (29 GB, 14 blocks) and Wan 14B fp16 flows are unaffected. Until fixed, do not combine BlockSwap with 40 GB-class bf16 checkpoints in multi-model workflows.

Caveat — fp8 + runtime LoRA: with --disable-dynamic-vram, ComfyUI's legacy loader merges LoRA into fp8 weights per-key on a VRAM-packed GPU and can crawl for hours before this node's ON_LOAD hook ever runs. BlockSwap cannot help there. Either run fp8+LoRA workflows with dynamic VRAM (default, where this node is a no-op), or pre-merge the LoRA (bf16 merge → requantize to fp8) and drop the LoRA loader from the chain. fp16/bf16 + LoRA are unaffected.