multimodal-trtllm.md

---
# SPDX-FileCopyrightText: Copyright (c) 2025-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
# SPDX-License-Identifier: Apache-2.0
title: TensorRT-LLM Multimodal
---

This document provides a comprehensive guide for multimodal inference using TensorRT-LLM backend in Dynamo.

You can provide multimodal inputs in the following ways:
- By sending image URLs
- By providing paths to pre-computed embedding files

> **Note:** You should provide **either image URLs or embedding file paths** in a single request.

## Support Matrix

| Modality | Input Format | Aggregated | Disaggregated | Notes |
|----------|--------------|------------|---------------|-------|
| **Image** | HTTP/HTTPS URL | Yes | Yes | Full support for all image models |
| **Image** | Pre-computed Embeddings (.pt, .pth, .bin) | Yes | Yes | Direct embedding files |
| **Video** | HTTP/HTTPS URL | No | No | Not implemented |
| **Audio** | HTTP/HTTPS URL | No | No | Not implemented |

### Supported URL Formats

| Format | Example | Description |
|--------|---------|-------------|
| **HTTP/HTTPS** | `http://example.com/image.jpg` | Remote media files |
| **Pre-computed Embeddings** | `/path/to/embedding.pt` | Local embedding files (.pt, .pth, .bin) |

## Deployment Patterns

TRT-LLM supports aggregated and traditional disaggregated patterns. See [Architecture Patterns](README.md#architecture-patterns) for detailed explanations.

| Pattern | Supported | Launch Script | Notes |
|---------|-----------|---------------|-------|
| Aggregated | ✅ | `agg.sh` | Easiest setup, single worker |
| EP/D (Traditional Disaggregated) | ✅ | `disagg_multimodal.sh` | Prefill handles encoding, 2 workers |
| E/P/D (Full - Image URLs) | ✅ | `epd_multimodal_image_and_embeddings.sh` | Standalone encoder with `MultimodalEncoder`, 3 workers |
| E/P/D (Full - Pre-computed Embeddings) | ✅ | `epd_multimodal_image_and_embeddings.sh` | Standalone encoder with NIXL transfer, 3 workers |
| E/P/D (Large Models) | ✅ | `epd_disagg.sh` | For Llama-4 Scout/Maverick, multi-node |

### Component Flags

| Component | Flag | Purpose |
|-----------|------|---------|
| Worker | `--modality multimodal` | Complete pipeline (aggregated) |
| Prefill Worker | `--disaggregation-mode prefill` | Image processing + Prefill (multimodal tokenization happens here) |
| Decode Worker | `--disaggregation-mode decode` | Decode only |
| Encode Worker | `--disaggregation-mode encode` | Image encoding (E/P/D flow) |

## Aggregated Serving

Quick steps to launch Llama-4 Maverick BF16 in aggregated mode:

```bash
cd $DYNAMO_HOME

export AGG_ENGINE_ARGS=./examples/backends/trtllm/engine_configs/llama4/multimodal/agg.yaml
export SERVED_MODEL_NAME="meta-llama/Llama-4-Maverick-17B-128E-Instruct"
export MODEL_PATH="meta-llama/Llama-4-Maverick-17B-128E-Instruct"
./examples/backends/trtllm/launch/agg.sh
```

**Client:**
```bash
curl localhost:8000/v1/chat/completions -H "Content-Type: application/json" -d '{
    "model": "meta-llama/Llama-4-Maverick-17B-128E-Instruct",
    "messages": [
        {
            "role": "user",
            "content": [
                {
                    "type": "text",
                    "text": "Describe the image"
                },
                {
                    "type": "image_url",
                    "image_url": {
                        "url": "https://huggingface.co/datasets/huggingface/documentation-images/resolve/main/diffusers/inpaint.png"
                    }
                }
            ]
        }
    ],
    "stream": false,
    "max_tokens": 160
}'
```

## Disaggregated Serving

Example using `Qwen/Qwen2-VL-7B-Instruct`:

```bash
cd $DYNAMO_HOME

export MODEL_PATH="Qwen/Qwen2-VL-7B-Instruct"
export SERVED_MODEL_NAME="Qwen/Qwen2-VL-7B-Instruct"
export PREFILL_ENGINE_ARGS="examples/backends/trtllm/engine_configs/qwen2-vl-7b-instruct/prefill.yaml"
export DECODE_ENGINE_ARGS="examples/backends/trtllm/engine_configs/qwen2-vl-7b-instruct/decode.yaml"
export MODALITY="multimodal"

./examples/backends/trtllm/launch/disagg.sh
```

```bash
curl localhost:8000/v1/chat/completions -H "Content-Type: application/json" -d '{
    "model": "Qwen/Qwen2-VL-7B-Instruct",
    "messages": [
        {
            "role": "user",
            "content": [
                {
                    "type": "text",
                    "text": "Describe the image"
                },
                {
                    "type": "image_url",
                    "image_url": {
                        "url": "https://huggingface.co/datasets/huggingface/documentation-images/resolve/main/diffusers/inpaint.png"
                    }
                }
            ]
        }
    ],
    "stream": false,
    "max_tokens": 160
}'
```

For a large model like `meta-llama/Llama-4-Maverick-17B-128E-Instruct`, a multi-node setup is required for disaggregated serving (see [Multi-node Deployment](#multi-node-deployment-slurm) below), while aggregated serving can run on a single node. This is because the model with a disaggregated configuration is too large to fit on a single node's GPUs. For instance, running this model in disaggregated mode requires 2 nodes with 8xH200 GPUs or 4 nodes with 4xGB200 GPUs.

## Full E/P/D Flow (Image URLs)

For high-performance multimodal inference, Dynamo supports a standalone encoder with an **Encode-Prefill-Decode (E/P/D)** flow using TRT-LLM's `MultimodalEncoder`. This separates the vision encoding stage from prefill and decode, enabling better GPU utilization and scalability.

### Supported Input Formats

| Format | Example | Description |
|--------|---------|-------------|
| **HTTP/HTTPS URL** | `https://example.com/image.jpg` | Remote image files |
| **Base64 Data URL** | `data:image/jpeg;base64,...` | Inline base64-encoded images |

### How It Works

In the full E/P/D flow:

1. **Encode Worker**: Runs TRT-LLM's `MultimodalEncoder.generate()` to process image URLs through the vision encoder and projector
2. **Prefill Worker**: Receives `disaggregated_params` containing multimodal embedding handles, processes context and generates KV cache
3. **Decode Worker**: Performs streaming token generation using the KV cache

The encode worker uses TRT-LLM's `MultimodalEncoder` class (which inherits from `BaseLLM`) and only requires the model path and batch size - no KV cache configuration is needed since it only runs the vision encoder + projector.

### How to Launch

```bash
cd $DYNAMO_HOME

# Launch 3-worker E/P/D flow with image URL support
./examples/backends/trtllm/launch/epd_multimodal_image_and_embeddings.sh
```

### Example Request

```bash
curl localhost:8000/v1/chat/completions -H "Content-Type: application/json" -d '{
    "model": "llava-v1.6-mistral-7b-hf",
    "messages": [
        {
            "role": "user",
            "content": [
                {"type": "text", "text": "Describe the image"},
                {
                    "type": "image_url",
                    "image_url": {
                        "url": "https://huggingface.co/datasets/huggingface/documentation-images/resolve/main/diffusers/inpaint.png"
                    }
                }
            ]
        }
    ],
    "max_tokens": 160
}'
```

### E/P/D Architecture (Image URLs)

```mermaid
sequenceDiagram
    participant Client
    participant Frontend
    participant PrefillWorker as "Prefill Worker"
    participant EncodeWorker as "Encode Worker"
    participant DecodeWorker as "Decode Worker"

    Client->>Frontend: POST /v1/chat/completions (image URL)
    Frontend->>PrefillWorker: Route to prefill worker
    PrefillWorker->>EncodeWorker: Send request (image URL)
    Note over EncodeWorker: MultimodalEncoder.generate()<br/>runs vision encoder + projector
    EncodeWorker->>PrefillWorker: Return disaggregated_params<br/>(multimodal_embedding_handles)
    Note over PrefillWorker: Process context with embeddings<br/>Generate KV cache
    PrefillWorker->>Frontend: Return prefill response
    Frontend->>DecodeWorker: Route to decode worker
    DecodeWorker->>Frontend: Stream response chunks
    Frontend->>Client: Stream response
```

### Key Differences from EP/D (Traditional Disaggregated)

| Aspect | EP/D (Traditional) | E/P/D (Full) |
|--------|-------------------|--------------|
| **Encoding** | Prefill worker handles image encoding | Dedicated encode worker |
| **Prefill Load** | Higher (encoding + prefill) | Lower (prefill only) |
| **Use Case** | Simpler setup | Better scalability for vision-heavy workloads |
| **Launch Script** | `disagg_multimodal.sh` | `epd_multimodal_image_and_embeddings.sh` |

## Pre-computed Embeddings with E/P/D Flow

For high-performance multimodal inference, Dynamo supports pre-computed embeddings with an **Encode-Prefill-Decode (E/P/D)** flow using **NIXL (RDMA)** for zero-copy tensor transfer.

### Supported File Types

- `.pt` - PyTorch tensor files
- `.pth` - PyTorch checkpoint files
- `.bin` - Binary tensor files

### Embedding File Formats

TRT-LLM supports two formats for embedding files:

**1. Simple Tensor Format**

Direct tensor saved as `.pt` file containing only the embedding tensor:

```python
embedding_tensor = torch.rand(1, 576, 4096)  # [batch, seq_len, hidden_dim]
torch.save(embedding_tensor, "embedding.pt")
```

**2. Dictionary Format with Auxiliary Data**

Dictionary containing multiple keys, used by models like Llama-4 that require additional metadata:

```python
embedding_dict = {
    "mm_embeddings": torch.rand(1, 576, 4096),
    "special_tokens": [128256, 128257],
    "image_token_offsets": [[0, 576]],
    # ... other model-specific metadata
}
torch.save(embedding_dict, "llama4_embedding.pt")
```

- **Simple tensors**: Loaded directly and passed to `mm_embeddings` parameter
- **Dictionary format**: `mm_embeddings` key extracted as main tensor, other keys preserved as auxiliary data

### How to Launch

```bash
cd $DYNAMO_HOME/examples/backends/trtllm

# Launch 3-worker E/P/D flow with NIXL
./launch/epd_disagg.sh
```

> **Note:** This script is designed for 8-node H200 with `Llama-4-Scout-17B-16E-Instruct` model and assumes you have a model-specific embedding file ready.

### Configuration

```bash
# Encode endpoint for Prefill → Encode communication
export ENCODE_ENDPOINT="dyn://dynamo.tensorrt_llm_encode.generate"

# Security: Allowed directory for embedding files (default: /tmp)
export ALLOWED_LOCAL_MEDIA_PATH="/tmp"

# Security: Max file size to prevent DoS attacks (default: 50MB)
export MAX_FILE_SIZE_MB=50
```

### Example Request with Pre-computed Embeddings

```bash
curl localhost:8000/v1/chat/completions -H "Content-Type: application/json" -d '{
    "model": "meta-llama/Llama-4-Maverick-17B-128E-Instruct",
    "messages": [
        {
            "role": "user",
            "content": [
                {"type": "text", "text": "Describe the image"},
                {"type": "image_url", "image_url": {"url": "/path/to/embedding.pt"}}
            ]
        }
    ],
    "max_tokens": 160
}'
```

### E/P/D Architecture

The E/P/D flow implements a **3-worker architecture**:

- **Encode Worker**: Loads pre-computed embeddings, transfers via NIXL
- **Prefill Worker**: Receives embeddings, handles context processing and KV-cache generation
- **Decode Worker**: Performs streaming token generation

```mermaid
sequenceDiagram
    participant Client
    participant Frontend
    participant PrefillWorker as "Prefill Worker"
    participant EncodeWorker as "Encode Worker"
    participant DecodeWorker as "Decode Worker"
    participant NIXL as "NIXL (RDMA)"

    Client->>Frontend: POST /v1/chat/completions
    Frontend->>PrefillWorker: Route to prefill worker
    PrefillWorker->>EncodeWorker: Send request (embedding paths)
    EncodeWorker->>NIXL: Create readable operation
    EncodeWorker->>PrefillWorker: Send metadata + NIXL info
    PrefillWorker->>NIXL: Begin read operation
    NIXL-->>PrefillWorker: Zero-copy transfer complete
    PrefillWorker->>Frontend: Return prefill response
    Frontend->>DecodeWorker: Route to decode worker
    DecodeWorker->>Frontend: Stream response chunks
    Frontend->>Client: Stream response
```

## Embedding Cache

Dynamo supports embedding cache in both aggregated and disaggregated settings for TRT-LLM:

| Setting | Implementation | Launch Script | Status |
|---------|---------------|---------------|--------|
| **Disaggregated Encoder** | Dynamo-managed cache in the PD worker layer on top of TRT-LLM engine | `disagg_e_pd.sh` + `--multimodal-embedding-cache-capacity-gb` | Supported |
| **Aggregated** | N/A | N/A | Not yet supported |

The cache uses `MultimodalEmbeddingCacheManager` to maintain an LRU cache of encoder embeddings on CPU. When the same image is seen again, the cached embedding is reused instead of re-encoding.

### Disaggregated Encoder (Embedding Cache in Prefill Worker)

In the disaggregated setting, the Prefill Worker (P) owns a CPU-side LRU embedding cache (`EmbeddingCacheManager`). On each request P checks the cache first — on a hit, the Encode Worker is skipped entirely. On a miss, P routes to the Encode Worker (E), receives embeddings via NIXL, saves them to the cache, and then feeds the embeddings along with the request into the TRT-LLM Instance for prefill.

```mermaid
---
title: Embedding Cache — Disaggregated Encoder
---
flowchart LR
    req[Request] --> cpu_check{"CPU cache hit?<br/>(EmbeddingCacheManager)"}

    subgraph P ["Prefill Worker (P)"]
        cpu_check -. hit .-> use[Use cached embedding]
        use --> trtllm[TRT-LLM Instance]
    end

    cpu_check -- miss --> E["Encode Worker (E)"]
    E -- "embeddings via NIXL" --> save["Save to cache"]
    save --> trtllm
```

The `disagg_e_pd.sh` script launches a separate encode worker and a PD worker. Extra arguments are forwarded to the PD worker. Enable embedding cache by passing `--multimodal-embedding-cache-capacity-gb`:

```bash
cd $DYNAMO_HOME/examples/backends/trtllm
./launch/disagg_e_pd.sh --multimodal-embedding-cache-capacity-gb 10
```

## NIXL Usage

| Use Case | Script | NIXL Used? | Data Transfer |
|----------|--------|------------|---------------|
| Aggregated | `agg.sh` | No | All in one worker |
| EP/D (Traditional Disaggregated) | `disagg_multimodal.sh` | Optional | Prefill → Decode (KV cache via UCX or NIXL) |
| E/P/D (Image URLs) | `epd_multimodal_image_and_embeddings.sh` | No | Encoder → Prefill (handles via params), Prefill → Decode (KV cache) |
| E/P/D (Pre-computed Embeddings) | `epd_multimodal_image_and_embeddings.sh` | Yes | Encoder → Prefill (embeddings via NIXL RDMA) |
| E/P/D (Large Models) | `epd_disagg.sh` | Yes | Encoder → Prefill (embeddings via NIXL), Prefill → Decode (KV cache) |

> **Note:** NIXL for KV cache transfer is currently beta and only supported on AMD64 (x86_64) architecture.

## ModelInput Types and Registration

TRT-LLM workers register with Dynamo using:

| ModelInput Type | Preprocessing | Use Case |
|-----------------|---------------|----------|
| `ModelInput.Tokens` | Rust frontend may tokenize, but multimodal flows re-tokenize and build inputs in the Python worker; Rust token_ids are ignored | All TRT-LLM workers |

```python
# TRT-LLM Worker - Register with Tokens
await register_model(
    ModelInput.Tokens,      # Rust does minimal preprocessing
    model_type,             # ModelType.Chat or ModelType.Prefill
    generate_endpoint,
    model_name,
    ...
)
```

## Inter-Component Communication

| Transfer Stage | Message | NIXL Transfer |
|----------------|---------|---------------|
| **Frontend → Prefill** | Request with image URL or embedding path | No |
| **Prefill → Encode (Image URL)** | Request with image URL | No |
| **Encode → Prefill (Image URL)** | `ep_disaggregated_params` with `multimodal_embedding_handles`, processed prompt, and token IDs | No |
| **Prefill → Encode (Embedding Path)** | Request with embedding file path | No |
| **Encode → Prefill (Embedding Path)** | NIXL readable metadata + shape/dtype + auxiliary data | Yes (Embeddings tensor via RDMA) |
| **Prefill → Decode** | `disaggregated_params` with `_epd_metadata` (prompt, token IDs) | Configurable (KV cache: NIXL default, UCX optional) |

## Known Limitations

- **No video support** - No video encoder implementation
- **No audio support** - No audio encoder implementation
- **Multimodal preprocessing/tokenization happens in Python** - Rust may forward token_ids, but multimodal requests are parsed and re-tokenized in the Python worker
- **Multi-node H100 limitation** - Loading `meta-llama/Llama-4-Maverick-17B-128E-Instruct` with 8 nodes of H100 with TP=16 is not possible due to head count divisibility (`num_attention_heads: 40` not divisible by `tp_size: 16`)
- **llava-v1.6-mistral-7b-hf model crash** - Known issue with TRTLLM backend compatibility with `TensorRT LLM version: 1.2.0rc6.post1`. To use Llava model download revision `revision='52320fb52229` locally using HF.
- **Embeddings file crash** - Known issue with TRTLLM backend compatibility with `TensorRT LLM version: 1.2.0rc6.post1`. Embedding file parsing crashes in `attach_multimodal_embeddings(`. To be fixed in next TRTLLM upgrade.

## Supported Models

Multimodal models listed in [TensorRT-LLM supported models](https://github.com/NVIDIA/TensorRT-LLM/blob/main/docs/source/models/supported-models.md) are supported by Dynamo.

Common examples:
- **Llama 4 Vision models** (Maverick, Scout) - Recommended for large-scale deployments
- **LLaVA models** (e.g., `llava-hf/llava-v1.6-mistral-7b-hf`) - Default model for E/P/D examples
- **Qwen2-VL models** - Supported in traditional disaggregated mode
- Other vision-language models with TRT-LLM support

## Key Files

| File | Description |
|------|-------------|
| `components/src/dynamo/trtllm/main.py` | Worker initialization and setup |
| `components/src/dynamo/trtllm/engine.py` | TensorRTLLMEngine wrapper (LLM and MultimodalEncoder) |
| `components/src/dynamo/trtllm/constants.py` | DisaggregationMode enum (AGGREGATED, PREFILL, DECODE, ENCODE) |
| `components/src/dynamo/trtllm/encode_helper.py` | Encode worker request processing (embedding-path and full EPD flows) |
| `components/src/dynamo/trtllm/multimodal_processor.py` | Multimodal request processing |
| `components/src/dynamo/trtllm/request_handlers/handlers.py` | Request handlers (EncodeHandler, PrefillHandler, DecodeHandler) |
| `components/src/dynamo/trtllm/request_handlers/handler_base.py` | Base handler with disaggregated params encoding/decoding |
| `components/src/dynamo/trtllm/utils/disagg_utils.py` | DisaggregatedParamsCodec for network transfer |
| `components/src/dynamo/trtllm/utils/trtllm_utils.py` | Command-line argument parsing |