🧩Advanced Reinforcement Learning Documentation

Advanced documentation settings when using Unsloth with GRPO.

Detailed guides on doing GRPO with Unsloth for Batching, Generation & Training Parameters:

Training Parameters

• beta (float, default 0.0): KL coefficient.

  • 0.0 ⇒ no reference model loaded (lower memory, faster).

  • Higher beta constrains the policy to stay closer to the ref policy.

• num_iterations (int, default 1): PPO epochs per batch (μ in the algorithm). Replays data within each gradient accumulation step; e.g., 2 = two forward passes per accumulation step.

• epsilon (float, default 0.2): Clipping value for token-level log-prob ratios (typical ratio range ≈ [-1.2, 1.2] with default ε).

• delta (float, optional): Enables upper clipping bound for two-sided GRPO when set. If None, standard GRPO clipping is used. Recommended > 1 + ε when enabled (per INTELLECT-2 report).

• epsilon_high (float, optional): Upper-bound epsilon; defaults to epsilon if unset. DAPO recommends 0.28.

• importance_sampling_level (“token” | “sequence”, default "token"):

  • "token": raw per-token ratios (one weight per token).

  • "sequence": average per-token ratios to a single sequence-level ratio. GSPO shows sequence-level sampling often gives more stable training for sequence-level rewards.

• reward_weights (list[float], optional): One weight per reward. If None, all weights = 1.0.

• scale_rewards (str|bool, default "group"):

  • True or "group": scale by std within each group (unit variance in group).

  • "batch": scale by std across the entire batch (per PPO-Lite).

  • False or "none": no scaling. Dr. GRPO recommends not scaling to avoid difficulty bias from std scaling.

• loss_type (str, default "dapo"):

  • "grpo": normalizes over sequence length (length bias; not recommended).

  • "dr_grpo": normalizes by a global constant (introduced in Dr. GRPO; removes length bias). Constant ≈ max_completion_length.

  • "dapo" (default): normalizes by active tokens in the global accumulated batch (introduced in DAPO; removes length bias).

  • "bnpo": normalizes by active tokens in the local batch only (results can vary with local batch size; equals GRPO when per_device_train_batch_size == 1).

• mask_truncated_completions (bool, default False): When True, truncated completions are excluded from loss (recommended by DAPO for stability). Note: There are some KL issues with this flag, so we recommend to disable it.

  # If mask_truncated_completions is enabled, zero out truncated completions in completion_mask
  if self.mask_truncated_completions:
      truncated_completions = ~is_eos.any(dim=1)
      completion_mask = completion_mask * (~truncated_completions).unsqueeze(1).int()

  This can zero out all completion_mask entries when many completions are truncated, making n_mask_per_reward = 0 and causing KL to become NaN. See

• vllm_importance_sampling_correction (bool, default True): Applies Truncated Importance Sampling (TIS) to correct off-policy effects when generation (e.g., vLLM / fast_inference) differs from training backend. In Unsloth, this is auto-set to True if you’re using vLLM/fast_inference; otherwise False.

• vllm_importance_sampling_cap (float, default 2.0): Truncation parameter C for TIS; sets an upper bound on the importance sampling ratio to improve stability.

• dtype when choosing float16 or bfloat16, see FP16 vs BF16 for RL

RL on unsupported models:

You can also run RL with Unsloth on models that are not supported by vLLM, such as Qwen3.5. Simply set fast_inference=False when loading the model.

from unsloth import FastLanguageModel

model, tokenizer = FastLanguageModel.from_pretrained(
    model_name="unsloth/Qwen3.5-4B",
    fast_inference=False,
)

Generation Parameters

• temperature (float, defaults to 1.0): Temperature for sampling. The higher the temperature, the more random the completions. Make sure you use a relatively high (1.0) temperature to have diversity in generations which helps learning.

• top_p (float, optional, defaults to 1.0): Float that controls the cumulative probability of the top tokens to consider. Must be in (0, 1]. Set to 1.0 to consider all tokens.

• top_k (int, optional): Number of highest probability vocabulary tokens to keep for top-k-filtering. If None, top-k-filtering is disabled and all tokens are considered.

• min_p (float, optional): Minimum token probability, which will be scaled by the probability of the most likely token. It must be a value between 0.0 and 1.0. Typical values are in the 0.01-0.2 range.

• repetition_penalty (float, optional, defaults to 1.0): Float that penalizes new tokens based on whether they appear in the prompt and the generated text so far. Values > 1.0 encourage the model to use new tokens, while values < 1.0 encourage the model to repeat tokens.

• steps_per_generation: (int, optional): Number of steps per generation. If None, it defaults to gradient_accumulation_steps. Mutually exclusive with generation_batch_size.

It is a bit confusing to mess with this parameter, it is recommended to edit per_device_train_batch_size and gradient accumulation for the batch sizes

Batch & Throughput Parameters

Parameters that control batches

• train_batch_size: Number of samples per process per step. If this integer is less than num_generations, it will default to num_generations.

• steps_per_generation: Number of microbatches that contribute to one generation’s loss calculation (forward passes only). A new batch of data is generated every steps_per_generation steps; backpropagation timing depends on gradient_accumulation_steps.

• num_processes: Number of distributed training processes (e.g., GPUs / workers).

• gradient_accumulation_steps (aka gradient_accumulation): Number of microbatches to accumulate before applying backpropagation and optimizer update.

• Effective batch size:

  effective_batch_size = steps_per_generation * num_processes * train_batch_size

  Total samples contributing to gradients before an update (across all processes and steps).

• Optimizer steps per generation:

  optimizer_steps_per_generation = steps_per_generation / gradient_accumulation_steps

  Example: 4 / 2 = 2.

• num_generations: Number of generations produced per prompt (applied after computing effective_batch_size). The number of unique prompts in a generation cycle is:

  unique_prompts = effective_batch_size / num_generations

  Must be > 2 for GRPO to work.

GRPO Batch Examples

The tables below illustrate how batches flow through steps, when optimizer updates occur, and how new batches are generated.

Example 1

num_gpus = 1
per_device_train_batch_size = 3
gradient_accumulation_steps = 2
steps_per_generation = 4

effective_batch_size = 4 * 3 * 1 = 12
num_generations = 3

Generation cycle A

Step	Batch	Notes
0	[0,0,0]
1	[1,1,1]	→ optimizer update (accum = 2 reached)
2	[2,2,2]
3	[3,3,3]	optimizer update

Generation cycle B

Step	Batch	Notes
0	[4,4,4]
1	[5,5,5]	→ optimizer update (accum = 2 reached)
2	[6,6,6]
3	[7,7,7]	optimizer update

Example 2

num_gpus = 1
per_device_train_batch_size = 3
steps_per_generation = gradient_accumulation_steps = 4

effective_batch_size = 4 * 3 * 1 = 12
num_generations = 3

Generation cycle A

Step	Batch	Notes
0	[0,0,0]
1	[1,1,1]
2	[2,2,2]
3	[3,3,3]	optimizer update (accum = 4 reached)

Generation cycle B

Step	Batch	Notes
0	[4,4,4]
1	[5,5,5]
2	[6,6,6]
3	[7,7,7]	optimizer update (accum = 4 reached)

Example 3

num_gpus = 1
per_device_train_batch_size = 3
steps_per_generation = gradient_accumulation_steps = 4

effective_batch_size = 4 * 3 * 1 = 12
num_generations = 4
unique_prompts = effective_batch_size / num_generations = 3

Generation cycle A

Step	Batch	Notes
0	[0,0,0]
1	[0,1,1]
2	[1,1,3]
3	[3,3,3]	optimizer update (accum = 4 reached)

Generation cycle B

Step	Batch	Notes
0	[4,4,4]
1	[4,5,5]
2	[5,5,6]
3	[6,6,6]	optimizer update (accum = 4 reached)

Example 4

num_gpus = 1
per_device_train_batch_size = 6
steps_per_generation = gradient_accumulation_steps = 2

effective_batch_size = 2 * 6 * 1 = 12
num_generations = 3
unique_prompts = 4

Generation cycle A

Step	Batch	Notes
0	[0,0,0, 1,1,1]
1	[2,2,2, 3,3,3]	optimizer update (accum = 2 reached)

Generation cycle B

Step	Batch	Notes
0	[4,4,4, 5,5,5]
1	[6,6,6, 7,7,7]	optimizer update (accum = 2 reached)

Quick Formula Reference

effective_batch_size = steps_per_generation * num_processes * train_batch_size
optimizer_steps_per_generation = steps_per_generation / gradient_accumulation_steps
unique_prompts = effective_batch_size / num_generations   # must be > 2