trl/references/troubleshooting.md

Troubleshooting TRL Training Jobs

Common issues and solutions when training with TRL on Hugging Face Jobs.

Training Hangs at "Starting training..." Step

Problem: Job starts but hangs at the training step - never progresses, never times out, just sits there.

Root Cause: Using eval_strategy="steps" or eval_strategy="epoch" without providing an eval_dataset to the trainer.

Solution:

Option A: Provide eval_dataset (recommended)

# Create train/eval split
dataset_split = dataset.train_test_split(test_size=0.1, seed=42)

trainer = SFTTrainer(
    model="Qwen/Qwen2.5-0.5B",
    train_dataset=dataset_split["train"],
    eval_dataset=dataset_split["test"],  # ← MUST provide when eval_strategy is enabled
    args=SFTConfig(
        eval_strategy="steps",
        eval_steps=50,
        ...
    ),
)

Option B: Disable evaluation

trainer = SFTTrainer(
    model="Qwen/Qwen2.5-0.5B",
    train_dataset=dataset,
    # No eval_dataset
    args=SFTConfig(
        eval_strategy="no",  # ← Explicitly disable
        ...
    ),
)

Prevention:

• Always create train/eval split for better monitoring
• Use dataset.train_test_split(test_size=0.1, seed=42)
• Check example scripts: scripts/train_sft_example.py includes proper eval setup

Job Times Out

Problem: Job terminates before training completes, all progress lost.

Solutions:

• Increase timeout parameter (e.g., "timeout": "4h")
• Reduce num_train_epochs or use smaller dataset slice
• Use smaller model or enable LoRA/PEFT to speed up training
• Add 20-30% buffer to estimated time for loading/saving overhead

Prevention:

• Always start with a quick demo run to estimate timing
• Use scripts/estimate_cost.py to get time estimates
• Monitor first runs closely via Trackio or logs

Model Not Saved to Hub

Problem: Training completes but model doesn't appear on Hub - all work lost.

Check:

• push_to_hub=True in training config
• hub_model_id specified with username (e.g., "username/model-name")
• secrets={"HF_TOKEN": "$HF_TOKEN"} in job submission
• User has write access to target repo
• Token has write permissions (check at https://huggingface.co/settings/tokens)
• Training script calls trainer.push_to_hub() at the end

See: references/hub_saving.md for detailed Hub authentication troubleshooting

Out of Memory (OOM)

Problem: Job fails with CUDA out of memory error.

Solutions (in order of preference):

1. Reduce batch size: Lower per_device_train_batch_size (try 4 → 2 → 1)
2. Increase gradient accumulation: Raise gradient_accumulation_steps to maintain effective batch size
3. Enable LoRA/PEFT: Use peft_config=LoraConfig(r=16, lora_alpha=32) to train adapters only
4. Use larger GPU: Switch from t4-medium → a10g-large → a100-large
5. Enable gradient checkpointing: Set gradient_checkpointing=True in config (slower but saves memory)
6. Use smaller model: Try a smaller variant (e.g., 0.5B instead of 3B)

Memory guidelines:

• T4 (16GB): <1B models with LoRA
• A10G (24GB): 1-3B models with LoRA, <1B full fine-tune
• A100 (40GB/80GB): 7B+ models with LoRA, 3B full fine-tune

Dataset Format Error

Problem: Training fails with dataset format errors or missing fields.

Solutions:

1. Check format documentation:

   hf_doc_fetch("https://huggingface.co/docs/trl/dataset_formats")
   

2. Validate dataset before training:

   uv run https://huggingface.co/datasets/mcp-tools/skills/raw/main/dataset_inspector.py \
     --dataset <dataset-name> --split train
   

   Or via hf_jobs:

   hf_jobs("uv", {
       "script": "https://huggingface.co/datasets/mcp-tools/skills/raw/main/dataset_inspector.py",
       "script_args": ["--dataset", "dataset-name", "--split", "train"]
   })
   

3. Verify field names:

   • SFT: Needs "messages" field (conversational), OR "text" field, OR "prompt"/"completion"
   • DPO: Needs "chosen" and "rejected" fields
   • GRPO: Needs prompt-only format

4. Check dataset split:

   • Ensure split exists (e.g., split="train")
   • Preview dataset: load_dataset("name", split="train[:5]")

Import/Module Errors

Problem: Job fails with "ModuleNotFoundError" or import errors.

Solutions:

1. Add PEP 723 header with dependencies:

   # /// script
   # dependencies = [
   #     "trl>=0.12.0",
   #     "peft>=0.7.0",
   #     "transformers>=4.36.0",
   # ]
   # ///
   

2. Verify exact format:

   • Must have # /// delimiters (with space after #)
   • Dependencies must be valid PyPI package names
   • Check spelling and version constraints

3. Test locally first:

   uv run train.py  # Tests if dependencies are correct
   

Authentication Errors

Problem: Job fails with authentication or permission errors when pushing to Hub.

Solutions:

1. Verify authentication:

   mcp__huggingface__hf_whoami()  # Check who's authenticated
   

2. Check token permissions:

   • Go to https://huggingface.co/settings/tokens
   • Ensure token has "write" permission
   • Token must not be "read-only"

3. Verify token in job:

   "secrets": {"HF_TOKEN": "$HF_TOKEN"}  # Must be in job config
   

4. Check repo permissions:

   • User must have write access to target repo
   • If org repo, user must be member with write access
   • Repo must exist or user must have permission to create

Job Stuck or Not Starting

Problem: Job shows "pending" or "starting" for extended period.

Solutions:

• Check Jobs dashboard for status: https://huggingface.co/jobs
• Verify hardware availability (some GPU types may have queues)
• Try different hardware flavor if one is heavily utilized
• Check for account billing issues (Jobs requires paid plan)

Typical startup times:

• CPU jobs: 10-30 seconds
• GPU jobs: 30-90 seconds
• If >3 minutes: likely queued or stuck

Training Loss Not Decreasing

Problem: Training runs but loss stays flat or doesn't improve.

Solutions:

1. Check learning rate: May be too low (try 2e-5 to 5e-5) or too high (try 1e-6)
2. Verify dataset quality: Inspect examples to ensure they're reasonable
3. Check model size: Very small models may not have capacity for task
4. Increase training steps: May need more epochs or larger dataset
5. Verify dataset format: Wrong format may cause degraded training

Logs Not Appearing

Problem: Cannot see training logs or progress.

Solutions:

1. Wait 30-60 seconds: Initial logs can be delayed
2. Check logs via MCP tool:

   hf_jobs("logs", {"job_id": "your-job-id"})
   

3. Use Trackio for real-time monitoring: See references/trackio_guide.md
4. Verify job is actually running:

   hf_jobs("inspect", {"job_id": "your-job-id"})
   

Checkpoint/Resume Issues

Problem: Cannot resume from checkpoint or checkpoint not saved.

Solutions:

1. Enable checkpoint saving:

   SFTConfig(
       save_strategy="steps",
       save_steps=100,
       hub_strategy="every_save",  # Push each checkpoint
   )
   

2. Verify checkpoints pushed to Hub: Check model repo for checkpoint folders

3. Resume from checkpoint:

   trainer = SFTTrainer(
       model="username/model-name",  # Can be checkpoint path
       resume_from_checkpoint="username/model-name/checkpoint-1000",
   )
   

Getting Help

If issues persist:

1. Check TRL documentation:

   hf_doc_search("your issue", product="trl")
   

2. Check Jobs documentation:

   hf_doc_fetch("https://huggingface.co/docs/huggingface_hub/guides/jobs")
   

3. Review related guides:

   • references/hub_saving.md - Hub authentication issues
   • references/hardware_guide.md - Hardware selection and specs
   • references/training_patterns.md - Eval dataset requirements
   • SKILL.md "Working with Scripts" section - Script format and URL issues

4. Ask in HF forums: https://discuss.huggingface.co/