# Example: Qwen3-4B Model

[中文版](../../zh/models/qwen3-4B.md)

## Environment Setup

After pulling the `zhuzilin/slime:latest` image, initialize the image environment as follows:

```bash
cd /root/
git clone https://github.com/THUDM/slime.git
pip install -e .
```

Download the model and data:

```bash
# hf checkpoint
huggingface-cli download Qwen/Qwen3-4B --local-dir /root/Qwen3-4B

# train data
huggingface-cli download --repo-type dataset zhuzilin/dapo-math-17k \
  --local-dir /root/dapo-math-17k

# eval data
huggingface-cli download --repo-type dataset zhuzilin/aime-2024 \
  --local-dir /root/aime-2024
```

Convert the Hugging Face checkpoint into a format that Megatron can load:

```bash
# mcore checkpoint
cd /root/slime
PYTHONPATH=/root/Megatron-LM python tools/convert_hf_to_torch_dist.py \
    --hf-checkpoint /root/Qwen3-4B \
    --save /root/Qwen3-4B_torch_dist
```

## Run Training

Execute the training script:

```bash
cd /root/slime
bash scripts/run-qwen3-4B.sh
```

### Parameter Introduction

Here, we will briefly introduce the various components of the [run-qwen3-4B.sh](../../../scripts/run-qwen3-4B.sh) script:

#### MODEL\_ARGS

```bash
SCRIPT_DIR="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" &>/dev/null && pwd)"
source "${SCRIPT_DIR}/models/qwen3-4B.sh"
```

This reads the model's configuration from [scripts/models/qwen3-4B.sh](../../../scripts/models/qwen3-4B.sh). These are all Megatron parameters. When training with Megatron, it cannot read the model config from the checkpoint, so we need to configure it ourselves. We provide some examples in [scripts/models](../../../scripts/models/).

#### CKPT\_ARGS

```bash
CKPT_ARGS=(
   # HF checkpoint required by sglang; we also read the tokenizer from here
   --hf-checkpoint /root/Qwen3-4B
   # Checkpoint for the reference model
   --ref-load /root/Qwen3-4B_torch_dist
   # Load directory for the actor; if empty, it will be loaded from `ref_load`
   --load /root/Qwen3-4B_slime/
   --save /root/Qwen3-4B_slime/
   --save-interval 20
)
```

#### ROLLOUT\_ARGS

```bash
ROLLOUT_ARGS=(
   # Prompt dataset, each line is a JSON object
   --prompt-data /root/dapo-math-17k/dapo-math-17k.jsonl
   --input-key prompt
   --label-key label
   # If the `input_key` in the prompt contains an OpenAI message,
   # tokenizer.apply_chat_template(...) will be executed
   --apply-chat-template
   # Whether to shuffle the data
   --rollout-shuffle

   # Reward model type.
   # slime provides many types and --custom-rm-path for custom models
   --rm-type deepscaler

   # Total number of rollouts to train
   --num-rollout 3000
   # Number of prompts in one rollout
   --rollout-batch-size 32
   # Number of responses to sample per prompt
   # A rollout will have rollout_batch_size * n_samples_per_prompt samples
   --n-samples-per-prompt 8
   # Rollout sampling parameters
   --rollout-max-response-len 8192
   --rollout-temperature 0.8

   # Number of training steps corresponding to one rollout
   --num-steps-per-rollout 1
   # Whether to balance data during training, which might improve speed
   --balance-data
)
```

#### EVAL\_ARGS

During evaluation, most rollout parameters are inherited, but we provide some parameters that can override the rollout configuration to allow for different sampling strategies for training and evaluation.

```bash
EVAL_ARGS=(
   --eval-interval 5
   --eval-prompt-data /root/aime-2024/aime-2024.jsonl
   --n-samples-per-eval-prompt 16
   --eval-max-response-len 16384
   --eval-top-p 0.7
)
```

#### PERF\_ARGS

This is a set of Megatron's parallelism parameters. Only `--use-dynamic-batch-size` and `--max-tokens-per-gpu` are added by slime.

`max_tokens_per_gpu` specifies the maximum number of tokens each GPU can process. When `use_dynamic_batch_size` is enabled, it attempts to pack data of varying lengths within a batch as close to `max_tokens_per_gpu` as possible, thus forming a dynamic micro-batch size. If a single data item exceeds `max_tokens_per_gpu`, it forms its own batch without being truncated. When context parallelism (CP) is enabled, it allows the CP GPUs to share a total of `CP * max_tokens_per_gpu` tokens.

When `dynamic_batch_size` is enabled, the traditional `micro_batch_size` is ignored.

⚠️ slime always trains the model using data packing and strictly guarantees per-sample or per-token loss. This means enabling dynamic batch size will not affect the loss calculation. It is recommended to enable it.

```bash
PERF_ARGS=(
   --tensor-model-parallel-size 2
   --sequence-parallel
   --pipeline-model-parallel-size 1
   --context-parallel-size 1
   --expert-model-parallel-size 1
   --expert-tensor-parallel-size 1

   --recompute-granularity full
   --recompute-method uniform
   --recompute-num-layers 1

   # --micro-batch-size 1
   --use-dynamic-batch-size
   --max-tokens-per-gpu 9216
)
```

#### GRPO\_ARGS

Currently, slime only supports GRPO. Here are some GRPO-related parameters:

```bash
GRPO_ARGS=(
   --advantage-estimator grpo
   --use-kl-loss
   --kl-loss-coef 0.00
   --kl-loss-type low_var_kl
   # Currently unused
   --kl-coef 0.00
   --entropy-coef 0.00
   --eps-clip 0.2
   --eps-clip-high 0.28
)
```

#### OPTIMIZER\_ARGS

```bash
OPTIMIZER_ARGS=(
   --optimizer adam
   --lr 1e-6
   --lr-decay-style constant
   --weight-decay 0.1
   --adam-beta1 0.9
   --adam-beta2 0.98
)
```

#### SGLANG\_ARGS

These are the parameters required by sglang. Here, `--rollout-num-gpus-per-engine` basically corresponds to sglang's `tp_size`. Other sglang parameters are passed to slime by adding the `--sglang-` prefix.

```bash
SGLANG_ARGS=(
   --rollout-num-gpus-per-engine 2
   --sglang-mem-fraction-static 0.5
)
```

⚠️ slime uses `sgl-router` to schedule multiple sglang servers. `dp_size` is not supported when DP attention is disabled.

### Dynamic Sampling

slime supports more complex sampling schemes, such as the dynamic sampling in [DAPO](https://dapo-sia.github.io/). To enable dynamic sampling, you need to configure:

```bash
   --over-sampling-batch-size ${OVER_SAMPLING_BS} \
   --dynamic-sampling-filter-path \
     slime.rollout.filter_hub.dynamic_sampling_filters.check_reward_nonzero_std \
```

Here, `over_sampling_batch_size` needs to be greater than `rollout_batch_size`. For example, you can configure it as:

```bash
   --rollout-batch-size 32 \
   --n-samples-per-prompt 8 \
   --over-sampling-batch-size 64 \
```

In this case, the sampling process will directly sample 64 prompts, with 8 samples per prompt. Since slime performs asynchronous sampling internally, we will receive the 8 responses for each prompt sequentially. Upon receiving the responses, the function specified by `dynamic_sampling_filter_path` is used for filtering. If the samples pass the filter, these 8 data points are kept; otherwise, they are discarded. The function in the example checks if the rewards for the samples are not all identical (i.e., not all correct or all incorrect):

```python
def check_reward_nonzero_std(args, samples: list[Sample], **kwargs):
    rewards = [sample.reward for sample in samples]
    return torch.tensor(rewards, dtype=torch.float).std() > 0.0
```

When we have received 32 \* 8 data points, we will immediately stop the current sampling round and will not wait for the remaining data to be sampled. If more than 32 prompts' worth of data is discarded (leaving fewer than 32 prompts' worth), we will then sample another 64 prompts.

### Partial Rollout

During the process of dynamic sampling, a large number of requests are aborted prematurely. We can configure the `--partial-rollout` parameter to save these partially generated requests to a data buffer. In the next rollout, these requests can be retrieved to continue data generation, thereby further optimizing performance.

You can customize how data is retrieved from the buffer by configuring the `--buffer-filter-path`. The default function is:

```python
def pop_first(args, rollout_id, buffer: list[list[Sample]], num_samples: int) -> list[list[Sample]]:
    num_to_pop = min(len(buffer), num_samples)
    samples = buffer[:num_to_pop]
    del buffer[:num_to_pop]
    return samples
```

This means that each time, the data corresponding to the first `num_samples` prompts is retrieved, totaling `num_samples * n_samples_per_prompt` items.

⚠️ The `sample.metadata` of each partial rollout sample stores the rollout ID from its initial generation, which can be used for data filtering.

### BF16 Training with FP8 Inference

slime also supports BF16 training with FP8 inference. For the Qwen3-4B model, you just need to download the following model:

```bash
huggingface-cli download Qwen/Qwen3-4B-FP8 --local-dir /root/Qwen3-4B-FP8
```

And replace `--hf-checkpoint` with:

```bash
#--hf-checkpoint /root/Qwen3-4B
--hf-checkpoint /root/Qwen3-4B-FP8
```

This will trigger FP8 inference. Currently, we directly cast the BF16 weights to FP8. In the future, we will gradually add more sophisticated quantization schemes that have less impact on precision.

⚠️ The Megatron checkpoint for training still needs to be the one that was originally converted from the BF16 Hugging Face model.

### Decoupled Training and Inference

In the original script, the resource configuration is as follows:

```bash
ray job submit ... \
   -- python3 train.py \
   --actor-num-nodes 1 \
   --actor-num-gpus-per-node 8 \
   --colocate \
   ...
```

This enables co-located training and inference, where the training part uses 1 machine with 8 GPUs, and inference shares these 8 GPUs with training.

If you want to use the decoupled training and inference feature, you need to remove `--colocate` and configure `--rollout-num-gpus`. For example:

```bash
ray job submit ... \
   -- python3 train.py \
   --actor-num-nodes 1 \
   --actor-num-gpus-per-node 2 \
   --rollout-num-gpus 6 \
   ...
```

In this case, 2 GPUs will be allocated for training, and 6 GPUs will be allocated for inference.

⚠️  If the concurrency on each sglang server is too high, it may exceed sglang's default CUDA graph concurrency limit (the default maximum is 160), which will affect inference speed. You can adjust this in the following two ways:

1.  Use `--sglang-server-concurrency` to limit the maximum number of concurrent requests sent to a single sglang server. For example:

    ```bash
    --sglang-server-concurrency 160
    ```

2.  Use `--sglang-cuda-graph-bs` (which corresponds to sglang's native `--cuda-graph-bs` argument) to increase the number of CUDA graphs initialized by sglang. For example:

    ```bash
    --sglang-cuda-graph-bs 1 2 4 8 $(seq 16 8 256)
    ```
