Implement configurable handling of excess tokens in datasets

- Added `excess_token_handling` option to the configuration, allowing users to choose between "drop" and "truncate" for handling tokens exceeding the maximum sequence length.
- Introduced `truncate_or_drop_long_seq` function to manage both single and batched samples based on the selected handling method.
- Updated relevant dataset processing functions to utilize the new handling option, ensuring backward compatibility with existing "drop" behavior.
- Enhanced logging to reflect truncation actions in dataset processing.

This change improves flexibility in managing sequence lengths during training and evaluation.

docs/config.qmd

@@ -332,6 +332,8 @@ dataset_shard_idx:
 # The maximum length of an input to train with, this should typically be less than 2048
 # as most models have a token/context limit of 2048
 sequence_len: 2048
+# How to handle tokens exceeding max sequence length - "drop" (default, removes sample) or "truncate" (cuts off excess tokens) 
+excess_token_handling: drop
 # Pad inputs so each step uses constant sized buffers
 # This will reduce memory fragmentation and may prevent OOMs, by re-using memory more efficiently
 pad_to_sequence_len:

src/axolotl/utils/data/pretraining.py

@@ -259,6 +259,8 @@ def encode_packed_pretraining(
         # FIXME using attention mask unpad/pad with trainer and packed pretraining is broken atm
         # workaround by using the position id logic for now in trainer
         drop_attention_mask=multipack_attn,
+        # pass through handling mode from config via ds_wrapper function
+        handling=getattr(ds_wrapper, "cfg", {}).get("excess_token_handling", "drop"),
     )
 
     sampler = MultipackBatchSampler(

src/axolotl/utils/data/rl.py

@@ -78,7 +78,7 @@ def map_dataset(cfg, data_set, ds_transform_fn, tokenizer, **map_kwargs):
 
 
 def drop_long_rl_seq(
-    sample, rl, tokenizer, sequence_len  # pylint: disable=invalid-name
+    sample, rl, tokenizer, sequence_len, handling="drop"  # pylint: disable=invalid-name
 ):
     if rl in ("dpo", "ipo", "orpo", "simpo"):
         if not (
@@ -96,9 +96,39 @@ def drop_long_rl_seq(
         len_chosen = len(tokenizer(chosen, add_special_tokens=False)["input_ids"])
         len_rejected = len(tokenizer(rejected, add_special_tokens=False)["input_ids"])
 
-        return (len_prompt + len_chosen) <= sequence_len and (
-            len_prompt + len_rejected
-        ) <= sequence_len
+        if handling == "drop":
+            return (len_prompt + len_chosen) <= sequence_len and (
+                len_prompt + len_rejected
+            ) <= sequence_len
+        else:  # truncate
+            # If both sequences fit, return sample unchanged
+            if (len_prompt + len_chosen) <= sequence_len and (
+                len_prompt + len_rejected
+            ) <= sequence_len:
+                return sample
+                
+            # For truncation, we need to truncate the chosen and rejected responses
+            # to fit within sequence_len, but preserve the prompt
+            
+            # Calculate maximum response length that can fit with the prompt
+            max_response_len = sequence_len - len_prompt
+            
+            if max_response_len <= 0:
+                # Prompt is already too long, we can't truncate effectively
+                return False if handling == "drop" else sample
+                
+            # Truncate the chosen and rejected responses if needed
+            if len_chosen > max_response_len:
+                # Tokenize, truncate, and decode
+                chosen_tokens = tokenizer(chosen, add_special_tokens=False)["input_ids"][:max_response_len]
+                sample["chosen"] = tokenizer.decode(chosen_tokens, skip_special_tokens=True)
+                
+            if len_rejected > max_response_len:
+                # Tokenize, truncate, and decode
+                rejected_tokens = tokenizer(rejected, add_special_tokens=False)["input_ids"][:max_response_len]
+                sample["rejected"] = tokenizer.decode(rejected_tokens, skip_special_tokens=True)
+                
+            return sample
 
     if rl == "kto":
         if not (sample.get("prompt") and sample.get("completion")):
@@ -112,10 +142,30 @@ def drop_long_rl_seq(
             tokenizer(completion, add_special_tokens=False)["input_ids"]
         )
 
-        return (len_prompt + len_completion) <= sequence_len
+        if handling == "drop":
+            return (len_prompt + len_completion) <= sequence_len
+        else:  # truncate
+            # If sequence fits, return sample unchanged
+            if (len_prompt + len_completion) <= sequence_len:
+                return sample
+                
+            # Calculate maximum completion length that can fit with the prompt
+            max_completion_len = sequence_len - len_prompt
+            
+            if max_completion_len <= 0:
+                # Prompt is already too long, we can't truncate effectively
+                return False if handling == "drop" else sample
+                
+            # Truncate the completion if needed
+            if len_completion > max_completion_len:
+                # Tokenize, truncate, and decode
+                completion_tokens = tokenizer(completion, add_special_tokens=False)["input_ids"][:max_completion_len]
+                sample["completion"] = tokenizer.decode(completion_tokens, skip_special_tokens=True)
+                
+            return sample
 
     if rl == "grpo":
-        return True
+        return True if handling == "drop" else sample
 
     raise ValueError("Unknown RL type")
 
@@ -169,20 +219,33 @@ def load_prepare_preference_datasets(cfg):
                     rl=_cfg.rl,
                     tokenizer=tokenizer,
                     sequence_len=cfg.sequence_len,
+                    handling=cfg.get("excess_token_handling", "drop"),
                 )
 
                 prior_len = len(split_datasets[i])
-                split_datasets[i] = split_datasets[i].filter(
-                    drop_long,
-                    num_proc=cfg.dataset_processes,
-                    load_from_cache_file=not cfg.is_preprocess,
-                    desc="Dropping Long Sequences",
-                )
-                dropped = prior_len - len(split_datasets[i])
-                if dropped:
-                    LOG.warning(
-                        f"Dropped {dropped} long samples from dataset index {i}"
+                
+                # Use filter for drop mode and map for truncate mode
+                handling = cfg.get("excess_token_handling", "drop")
+                if handling == "drop":
+                    split_datasets[i] = split_datasets[i].filter(
+                        drop_long,
+                        num_proc=cfg.dataset_processes,
+                        load_from_cache_file=not cfg.is_preprocess,
+                        desc="Dropping Long Sequences",
                     )
+                    dropped = prior_len - len(split_datasets[i])
+                    if dropped:
+                        LOG.warning(
+                            f"Dropped {dropped} long samples from dataset index {i}"
+                        )
+                else:
+                    split_datasets[i] = split_datasets[i].map(
+                        drop_long,
+                        num_proc=cfg.dataset_processes,
+                        load_from_cache_file=not cfg.is_preprocess,
+                        desc="Truncating Long Sequences",
+                    )
+                    LOG.info(f"Truncated long samples in dataset index {i} to {cfg.sequence_len} tokens")
 
         combined_datasets = concatenate_datasets(split_datasets)
         combined_datasets = combined_datasets.shuffle(seed=cfg.seed)

src/axolotl/utils/data/utils.py

@@ -13,7 +13,7 @@ from datasets import Dataset, IterableDataset
 
 from axolotl.utils.dict import DictDefault
 from axolotl.utils.samplers.utils import get_dataset_lengths
-from axolotl.utils.trainer import drop_long_seq
+from axolotl.utils.trainer import drop_long_seq, truncate_or_drop_long_seq
 
 LOG = logging.getLogger(__name__)
 
@@ -165,11 +165,24 @@ def drop_long_seq_in_dataset(dataset: Dataset, cfg: DictDefault):
         )
         return dataset
 
-    drop_long = functools.partial(
-        drop_long_seq,
-        sequence_len=cfg.sequence_len,
-        min_sequence_len=cfg.min_sample_len,
-    )
+    # Get the handling method from config, default to "drop" for backward compatibility
+    handling = cfg.get("excess_token_handling", "drop")
+    
+    if handling == "drop":
+        # Use the existing drop_long_seq function for backward compatibility
+        seq_handler = functools.partial(
+            drop_long_seq,
+            sequence_len=cfg.sequence_len,
+            min_sequence_len=cfg.min_sample_len,
+        )
+    else:  # handling == "truncate"
+        # Use the new function with truncate mode
+        seq_handler = functools.partial(
+            truncate_or_drop_long_seq,
+            sequence_len=cfg.sequence_len,
+            min_sequence_len=cfg.min_sample_len,
+            handling=handling,
+        )
 
     try:
         ds_lengths = get_dataset_lengths(dataset, from_arrow=True)
@@ -193,17 +206,31 @@ def drop_long_seq_in_dataset(dataset: Dataset, cfg: DictDefault):
 
     drop_long_kwargs = {}
     if filter_map_kwargs:
-        drop_long_kwargs["desc"] = "Dropping Long Sequences"
+        if handling == "drop":
+            drop_long_kwargs["desc"] = "Dropping Long Sequences"
+        else:
+            drop_long_kwargs["desc"] = "Truncating Long Sequences"
 
-    dataset = dataset.filter(
-        drop_long,
-        batched=True,
-        **filter_map_kwargs,
-        **drop_long_kwargs,
-    )
-    if prior_len:
-        dropped = prior_len - len(dataset)
-        if dropped:
-            LOG.warning(f"Dropped {dropped} long samples from dataset")
+    if handling == "drop":
+        # Use filter for drop mode
+        dataset = dataset.filter(
+            seq_handler,
+            batched=True,
+            **filter_map_kwargs,
+            **drop_long_kwargs,
+        )
+        if prior_len:
+            dropped = prior_len - len(dataset)
+            if dropped:
+                LOG.warning(f"Dropped {dropped} long samples from dataset")
+    else:
+        # Use map for truncate mode
+        dataset = dataset.map(
+            seq_handler,
+            batched=True,
+            **filter_map_kwargs,
+            **drop_long_kwargs,
+        )
+        LOG.info(f"Truncated long samples in dataset to {cfg.sequence_len} tokens")
 
     return dataset

src/axolotl/utils/schemas/config.py

@@ -186,6 +186,10 @@ class AxolotlInputConfig(
     unfrozen_parameters: list[str] | None = None
 
     sequence_len: int = Field(default=512)
+    excess_token_handling: Literal["drop", "truncate"] = Field(
+        default="drop",
+        json_schema_extra={"description": "how to handle tokens exceeding max sequence length - drop the sample or truncate"},
+    )
     min_sample_len: int | None = None
     max_prompt_len: int = Field(
         default=512,

src/axolotl/utils/trainer.py

@@ -235,6 +235,104 @@ def drop_long_seq(sample, sequence_len=2048, min_sequence_len=2):
     return results
 
 
+def truncate_or_drop_long_seq(sample, sequence_len=2048, min_sequence_len=2, handling="drop"):
+    """
+    Either drop or truncate samples whose sequence length is either too long (> sequence_len)
+    or too short (< min_sequence_len).
+
+    If handling is "drop":
+        - Samples that are too short or too long will be dropped
+    If handling is "truncate":
+        - Samples that are too short will still be dropped
+        - Samples that are too long will be truncated to sequence_len
+
+    Works for both single-example (list[int]) or batched (list[list[int]]).
+    Returns either a boolean/list of booleans (for drop mode) or the modified sample (for truncate mode).
+    """
+    min_sequence_len = min_sequence_len or 2
+
+    if handling == "drop":
+        return drop_long_seq(sample, sequence_len, min_sequence_len)
+
+    input_ids = sample["input_ids"]
+
+    # Edge case: if input_ids is empty
+    if not input_ids:
+        return False if handling == "drop" else sample
+
+    # Check if single example or batched by looking at the first element
+    if isinstance(input_ids[0], int):
+        # Single example (input_ids is a list of int)
+        length = len(input_ids)
+        
+        # Handle samples that are too short - always drop them
+        if length < min_sequence_len:
+            return False if handling == "drop" else sample
+        
+        # If truncation is enabled and the sample is too long, truncate it
+        if length > sequence_len and handling == "truncate":
+            sample["input_ids"] = input_ids[:sequence_len]
+            
+            # Also truncate attention_mask if present
+            if "attention_mask" in sample:
+                sample["attention_mask"] = sample["attention_mask"][:sequence_len]
+            
+            # Also truncate labels if present
+            if "labels" in sample:
+                sample["labels"] = sample["labels"][:sequence_len]
+                
+            # Also truncate position_ids if present
+            if "position_ids" in sample:
+                sample["position_ids"] = sample["position_ids"][:sequence_len]
+                
+            # Update length if present
+            if "length" in sample:
+                sample["length"] = sequence_len
+                
+            return sample
+        
+        # For drop mode or if the sample doesn't exceed max length
+        return min_sequence_len <= length <= sequence_len if handling == "drop" else sample
+
+    # Batched (input_ids is a list of lists)
+    if handling == "drop":
+        results = []
+        for seq in input_ids:
+            length = len(seq)
+            results.append(min_sequence_len <= length <= sequence_len)
+        return results
+    else:  # truncate
+        # Check each sequence in the batch
+        for i, seq in enumerate(input_ids):
+            length = len(seq)
+            
+            # Skip sequences that are too short
+            if length < min_sequence_len:
+                continue
+                
+            # Truncate sequences that are too long
+            if length > sequence_len:
+                input_ids[i] = seq[:sequence_len]
+                
+                # Also truncate attention_mask if present
+                if "attention_mask" in sample:
+                    sample["attention_mask"][i] = sample["attention_mask"][i][:sequence_len]
+                
+                # Also truncate labels if present
+                if "labels" in sample:
+                    sample["labels"][i] = sample["labels"][i][:sequence_len]
+                    
+                # Also truncate position_ids if present
+                if "position_ids" in sample:
+                    sample["position_ids"][i] = sample["position_ids"][i][:sequence_len]
+                    
+                # Update length if present
+                if "length" in sample:
+                    sample["length"][i] = sequence_len
+        
+        return sample
+
+
 def process_datasets_for_packing(cfg, train_dataset, eval_dataset):
     drop_attn_mask = cfg.model_config_type in ["mamba", "gemma3"]
     if drop_attn_mask:
@@ -370,15 +468,25 @@ def process_datasets_for_packing(cfg, train_dataset, eval_dataset):
 
 
 def process_pretraining_datasets_for_packing(
-    train_dataset, sequence_len, skip_position_ids=True, drop_attention_mask=False
+    train_dataset, sequence_len, skip_position_ids=True, drop_attention_mask=False, handling="drop"
 ):
-    drop_long = partial(drop_long_seq, sequence_len=sequence_len)
-
-    train_dataset = train_dataset.filter(
-        drop_long,
-        desc="Dropping Long Sequences",
-        load_from_cache_file=False,
-    )
+    drop_long_fn = partial(drop_long_seq, sequence_len=sequence_len)
+    
+    # Use filter for drop mode and map for truncate mode
+    if handling == "drop":
+        train_dataset = train_dataset.filter(
+            drop_long_fn,
+            desc="Dropping Long Sequences",
+            load_from_cache_file=False,
+        )
+    else:
+        truncate_fn = partial(truncate_or_drop_long_seq, sequence_len=sequence_len, handling=handling)
+        train_dataset = train_dataset.map(
+            truncate_fn,
+            desc="Truncating Long Sequences",
+            load_from_cache_file=False,
+        )
+        
     if not skip_position_ids:
         train_dataset = train_dataset.map(
             add_position_ids,