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GPT-OSS

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Dynamo supports disaggregated serving of gpt-oss-120b with TensorRT-LLM. This guide demonstrates how to deploy gpt-oss-120b using disaggregated prefill/decode serving on a single B200 node with 8 GPUs, running 1 prefill worker on 4 GPUs and 1 decode worker on 4 GPUs.

Overview

This deployment uses disaggregated serving in TensorRT-LLM where:

  • Prefill Worker: Processes input prompts efficiently using 4 GPUs with tensor parallelism
  • Decode Worker: Generates output tokens using 4 GPUs, optimized for token generation throughput
  • Frontend: Provides OpenAI-compatible API endpoint with round-robin routing

The disaggregated approach optimizes for both low-latency (maximizing tokens per second per user) and high-throughput (maximizing total tokens per GPU per second) use cases by separating the compute-intensive prefill phase from the memory-bound decode phase.

Prerequisites

  • 1x NVIDIA B200 node with 8 GPUs (this guide focuses on single-node B200 deployment)
  • CUDA Toolkit 12.8 or later
  • Docker with NVIDIA Container Toolkit installed
  • Fast SSD storage for model weights (~240GB required)
  • HuggingFace account and access token
  • HuggingFace CLI

Ensure that the etcd and nats services are running with the following command:

$docker compose -f deploy/docker-compose.yml up

Instructions

1. Download the Model

$export MODEL_PATH=<LOCAL_MODEL_DIRECTORY>
$export HF_TOKEN=<INSERT_TOKEN_HERE>
$
$pip install -U "huggingface_hub[cli]"
$
$huggingface-cli download openai/gpt-oss-120b --exclude "original/*" --exclude "metal/*" --local-dir $MODEL_PATH

2. Run the Container

Set the container image:

$export DYNAMO_CONTAINER_IMAGE=nvcr.io/nvidia/ai-dynamo/tensorrtllm-runtime:my-tag

Launch the Dynamo TensorRT-LLM container with the necessary configurations:

$docker run \
>    --gpus all \
>    -it \
>    --rm \
>    --network host \
>    --volume $MODEL_PATH:/model \
>    --volume $PWD:/workspace \
>    --shm-size=10G \
>    --ulimit memlock=-1 \
>    --ulimit stack=67108864 \
>    --ulimit nofile=65536:65536 \
>    --cap-add CAP_SYS_PTRACE \
>    --ipc host \
>    -e HF_TOKEN=$HF_TOKEN \
>    -e TRTLLM_ENABLE_PDL=1 \
>    -e TRT_LLM_DISABLE_LOAD_WEIGHTS_IN_PARALLEL=True \
>    $DYNAMO_CONTAINER_IMAGE

This command:

  • Automatically removes the container when stopped (--rm)
  • Allows container to interact with host’s IPC resources for optimal performance (--ipc=host)
  • Runs the container in interactive mode (-it)
  • Sets up shared memory and stack limits for optimal performance
  • Mounts your model directory into the container at /model
  • Mounts the current Dynamo workspace into the container at /workspace/dynamo
  • Enables PDL and disables parallel weight loading
  • Sets HuggingFace token as environment variable in the container

3. Understanding the Configuration

The deployment uses configuration files and command-line arguments to control behavior:

Configuration Files

Prefill Configuration (examples/backends/trtllm/engine_configs/gpt-oss-120b/prefill.yaml):

  • enable_attention_dp: false - Attention data parallelism disabled for prefill
  • enable_chunked_prefill: true - Enables efficient chunked prefill processing
  • moe_config.backend: CUTLASS - Uses optimized CUTLASS kernels for MoE layers
  • cache_transceiver_config.backend: ucx - Uses UCX for efficient KV cache transfer
  • cuda_graph_config.max_batch_size: 32 - Maximum batch size for CUDA graphs

Decode Configuration (examples/backends/trtllm/engine_configs/gpt-oss-120b/decode.yaml):

  • enable_attention_dp: true - Attention data parallelism enabled for decode
  • disable_overlap_scheduler: false - Enables overlapping for decode efficiency
  • moe_config.backend: CUTLASS - Uses optimized CUTLASS kernels for MoE layers
  • cache_transceiver_config.backend: ucx - Uses UCX for efficient KV cache transfer
  • cuda_graph_config.max_batch_size: 128 - Maximum batch size for CUDA graphs

Command-Line Arguments

Both workers receive these key arguments:

  • --tensor-parallel-size 4 - Uses 4 GPUs for tensor parallelism
  • --expert-parallel-size 4 - Expert parallelism across 4 GPUs
  • --free-gpu-memory-fraction 0.9 - Allocates 90% of GPU memory

Prefill-specific arguments:

  • --max-num-tokens 20000 - Maximum tokens for prefill processing
  • --max-batch-size 32 - Maximum batch size for prefill

Decode-specific arguments:

  • --max-num-tokens 16384 - Maximum tokens for decode processing
  • --max-batch-size 128 - Maximum batch size for decode

4. Launch the Deployment

Note that GPT-OSS is a reasoning model with tool calling support. To ensure the response is being processed correctly, the worker should be launched with proper --dyn-reasoning-parser and --dyn-tool-call-parser.

You can use the provided launch script or run the components manually:

Option A: Using the Launch Script

$cd /workspace/examples/backends/trtllm
$./launch/gpt_oss_disagg.sh

Option B: Manual Launch

  1. Start frontend:
$# Start frontend with round-robin routing
$python3 -m dynamo.frontend --router-mode round-robin --http-port 8000 &
  1. Launch prefill worker:
$CUDA_VISIBLE_DEVICES=0,1,2,3 python3 -m dynamo.trtllm \
>  --model-path /model \
>  --served-model-name openai/gpt-oss-120b \
>  --extra-engine-args examples/backends/trtllm/engine_configs/gpt-oss-120b/prefill.yaml \
>  --dyn-reasoning-parser gpt_oss \
>  --dyn-tool-call-parser harmony \
>  --disaggregation-mode prefill \
>  --max-num-tokens 20000 \
>  --max-batch-size 32 \
>  --free-gpu-memory-fraction 0.9 \
>  --tensor-parallel-size 4 \
>  --expert-parallel-size 4 &
  1. Launch decode worker:
$CUDA_VISIBLE_DEVICES=4,5,6,7 python3 -m dynamo.trtllm \
>  --model-path /model \
>  --served-model-name openai/gpt-oss-120b \
>  --extra-engine-args examples/backends/trtllm/engine_configs/gpt-oss-120b/decode.yaml \
>  --dyn-reasoning-parser gpt_oss \
>  --dyn-tool-call-parser harmony \
>  --disaggregation-mode decode \
>  --max-num-tokens 16384 \
>  --free-gpu-memory-fraction 0.9 \
>  --tensor-parallel-size 4 \
>  --expert-parallel-size 4

6. Verify the Deployment is Ready

Poll the /health endpoint to verify that both the prefill and decode worker endpoints have started:

curl http://localhost:8000/health

Make sure that both of the endpoints are available before sending an inference request:

{
  "endpoints": [
    "dyn://dynamo.tensorrt_llm.generate",
    "dyn://dynamo.prefill.generate"
  ],
  "status": "healthy"
}

If only one worker endpoint is listed, the other may still be starting up. Monitor the worker logs to track startup progress.

7. Test the Deployment

Send a test request to verify the deployment:

$curl -X POST http://localhost:8000/v1/responses \
>  -H "Content-Type: application/json" \
>  -d '{
>    "model": "openai/gpt-oss-120b",
>    "input": "Explain the concept of disaggregated serving in LLM inference in 3 sentences.",
>    "max_output_tokens": 200,
>    "stream": false
>  }'

The server exposes a standard OpenAI-compatible API endpoint that accepts JSON requests. You can adjust parameters like max_tokens, temperature, and others according to your needs.

8. Reasoning and Tool Calling

Dynamo has supported reasoning and tool calling in OpenAI Chat Completion endpoint. A typical workflow for application built on top of Dynamo is that the application has a set of tools to aid the assistant provide accurate answer, and it is ususally multi-turn as it involves tool selection and generation based on the tool result.

In addition, the reasoning effort can be configured through chat_template_args. Increasing the reasoning effort makes the model more accurate but also slower. It supports three levels: low, medium, and high.

Below is an example of sending multi-round requests to complete a user query with reasoning and tool calling: Application setup (pseudocode)

1# The tool defined by the application
2def get_system_health():
3    for component in system.components:
4        if not component.health():
5            return False
6    return True
7
8# The JSON representation of the declaration in ChatCompletion tool style
9tool_choice = '{
10  "type": "function",
11  "function": {
12    "name": "get_system_health",
13    "description": "Returns the current health status of the LLM runtime—use before critical operations to verify the service is live.",
14    "parameters": {
15      "type": "object",
16      "properties": {}
17    }
18  }
19}'
20
21# On user query, perform below workflow.
22def user_query(app_request):
23    # first round
24    # create chat completion with prompt and tool choice
25    request = ...
26    response = send(request)
27
28    if response["finish_reason"] == "tool_calls":
29        # second round
30        function, params = parse_tool_call(response)
31        function_result = function(params)
32        # create request with prompt, assistant response, and function result
33        request = ...
34        response = send(request)
35    return app_response(response)

First request with tools

$curl localhost:8000/v1/chat/completions   -H "Content-Type: application/json"   -d '
>{
>  "model": "openai/gpt-oss-120b",
>  "messages": [
>    {
>      "role": "user",
>      "content": "Hey, quick check: is everything up and running?"
>    }
>  ],
>  "chat_template_args": {
>      "reasoning_effort": "low"
>  },
>  "tools": [
>    {
>      "type": "function",
>      "function": {
>        "name": "get_system_health",
>        "description": "Returns the current health status of the LLM runtime—use before critical operations to verify the service is live.",
>        "parameters": {
>          "type": "object",
>          "properties": {}
>        }
>      }
>    }
>  ],
>  "response_format": {
>    "type": "text"
>  },
>  "stream": false,
>  "max_tokens": 300
>}'

First response with tool choice

1{
2  "id": "chatcmpl-d1c12219-6298-4c83-a6e3-4e7cef16e1a9",
3  "choices": [
4    {
5      "index": 0,
6      "message": {
7        "tool_calls": [
8          {
9            "id": "call-1",
10            "type": "function",
11            "function": {
12              "name": "get_system_health",
13              "arguments": "{}"
14            }
15          }
16        ],
17        "role": "assistant",
18        "reasoning_content": "We need to check system health. Use function."
19      },
20      "finish_reason": "tool_calls"
21    }
22  ],
23  "created": 1758758741,
24  "model": "openai/gpt-oss-120b",
25  "object": "chat.completion",
26  "usage": null
27}

Second request with tool calling result

$curl localhost:8000/v1/chat/completions   -H "Content-Type: application/json"   -d '
>{
>  "model": "openai/gpt-oss-120b",
>  "messages": [
>    {
>      "role": "user",
>      "content": "Hey, quick check: is everything up and running?"
>    },
>    {
>      "role": "assistant",
>      "tool_calls": [
>        {
>          "id": "call-1",
>          "type": "function",
>          "function": {
>            "name": "get_system_health",
>            "arguments": "{}"
>          }
>        }
>      ]
>    },
>    {
>      "role": "tool",
>      "tool_call_id": "call-1",
>      "content": "{\"status\":\"ok\",\"uptime_seconds\":372045}"
>    }
>  ],
>  "chat_template_args": {
>      "reasoning_effort": "low"
>  },
>  "tools": [
>    {
>      "type": "function",
>      "function": {
>        "name": "get_system_health",
>        "description": "Returns the current health status of the LLM runtime—use before critical operations to verify the service is live.",
>        "parameters": {
>          "type": "object",
>          "properties": {}
>        }
>      }
>    }
>  ],
>  "response_format": {
>    "type": "text"
>  },
>  "stream": false,
>  "max_tokens": 300
>}'

Second response with final message

1{
2  "id": "chatcmpl-9ebfe64a-68b9-4c1d-9742-644cf770ad0e",
3  "choices": [
4    {
5      "index": 0,
6      "message": {
7        "content": "All systems are green—everything’s up and running smoothly! 🚀 Let me know if you need anything else.",
8        "role": "assistant",
9        "reasoning_content": "The user asks: \"Hey, quick check: is everything up and running?\" We have just checked system health, it's ok. Provide friendly response confirming everything's up."
10      },
11      "finish_reason": "stop"
12    }
13  ],
14  "created": 1758758853,
15  "model": "openai/gpt-oss-120b",
16  "object": "chat.completion",
17  "usage": null
18}

Benchmarking

Performance Testing with AIPerf

The Dynamo container includes AIPerf, NVIDIA’s tool for benchmarking generative AI models. This tool helps measure throughput, latency, and other performance metrics for your deployment.

Run the following benchmark from inside the container (after completing the deployment steps above):

$# Create a directory for benchmark results
$mkdir -p /tmp/benchmark-results
$
$# Run the benchmark - this command tests the deployment with high-concurrency synthetic workload
$aiperf profile \
>    --model openai/gpt-oss-120b \
>    --tokenizer /model \
>    --endpoint-type chat \
>    --endpoint /v1/chat/completions \
>    --streaming \
>    --url localhost:8000 \
>    --synthetic-input-tokens-mean 32000 \
>    --synthetic-input-tokens-stddev 0 \
>    --output-tokens-mean 256 \
>    --output-tokens-stddev 0 \
>    --extra-inputs max_tokens:256 \
>    --extra-inputs min_tokens:256 \
>    --extra-inputs ignore_eos:true \
>    --extra-inputs "{\"nvext\":{\"ignore_eos\":true}}" \
>    --concurrency 256 \
>    --request-count 6144 \
>    --warmup-request-count 1000 \
>    --num-dataset-entries 8000 \
>    --random-seed 100 \
>    --artifact-dir /tmp/benchmark-results \
>    -H 'Authorization: Bearer NOT USED' \
>    -H 'Accept: text/event-stream'

What This Benchmark Does

This command:

  • Tests chat completions with streaming responses against the disaggregated deployment
  • Simulates high load with 256 concurrent requests and 6144 total requests
  • Uses long context inputs (32K tokens) to test prefill performance
  • Generates consistent outputs (256 tokens) to measure decode throughput
  • Includes warmup period (1000 requests) to stabilize performance metrics
  • Saves detailed results to /tmp/benchmark-results for analysis

Key parameters you can adjust:

  • --concurrency: Number of simultaneous requests (impacts GPU utilization)
  • --synthetic-input-tokens-mean: Average input length (tests prefill capacity)
  • --output-tokens-mean: Average output length (tests decode throughput)
  • --request-count: Total number of requests for the benchmark

Installing AIPerf Outside the Container

If you prefer to run benchmarks from outside the container:

$# Install AIPerf
$pip install aiperf
$
$# Then run the same benchmark command, adjusting the tokenizer path if needed

Architecture Overview

The disaggregated architecture separates prefill and decode phases:

Key Features

  1. Disaggregated Serving: Separates compute-intensive prefill from memory-bound decode operations
  2. Optimized Resource Usage: Different parallelism strategies for prefill vs decode
  3. Scalable Architecture: Easy to adjust worker counts based on workload
  4. TensorRT-LLM Optimizations: Leverages TensorRT-LLM’s efficient kernels and memory management

Troubleshooting

Common Issues

  1. CUDA Out-of-Memory Errors

    • Reduce --max-num-tokens in the launch commands (currently 20000 for prefill, 16384 for decode)
    • Lower --free-gpu-memory-fraction from 0.9 to 0.8 or 0.7
    • Ensure model checkpoints are compatible with the expected format

  2. Workers Not Connecting

    • Ensure etcd and NATS services are running: docker ps | grep -E "(etcd|nats)"
    • Check network connectivity between containers
    • Verify CUDA_VISIBLE_DEVICES settings match your GPU configuration
    • Check that no other processes are using the assigned GPUs

  3. Performance Issues

    • Monitor GPU utilization with nvidia-smi while the deployment is running
    • Check worker logs for bottlenecks or errors
    • Ensure that batch sizes in manual commands match those in configuration files
    • Adjust chunked prefill settings based on your workload
    • For connection issues, ensure port 8000 is not being used by another application

  4. Container Startup Issues

    • Verify that the NVIDIA Container Toolkit is properly installed
    • Check Docker daemon is running with GPU support
    • Ensure sufficient disk space for model weights and container images

  5. Token Repetition / Generation Won’t Stop

    • When using reasoning_effort: high, the model may produce repeated tokens and fail to stop
    • Solution: Set top_p=1 in your request. These are the recommended sampling parameters from OpenAI
    • Example request with recommended parameters: 
      $curl localhost:8000/v1/chat/completions -H "Content-Type: application/json" -d '{
      >  "model": "openai/gpt-oss-120b",
      >  "messages": [{"role": "user", "content": "Hello"}],
      >  "chat_template_args": {
      >     "reasoning_effort": "high"
      >   },
      >  "top_p": 1,
      >  "max_tokens": 300
      >}'

Next Steps

  • Production Deployment: For multi-node deployments, see the Multi-node Guide
  • Advanced Configuration: Explore TensorRT-LLM engine building options for further optimization
  • Monitoring: Set up Prometheus and Grafana for production monitoring
  • Performance Benchmarking: Use AIPerf to measure and optimize your deployment performance