SFT: Truncate during dataset preparation, not collation#6155
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qgallouedec wants to merge 12 commits into
Open
SFT: Truncate during dataset preparation, not collation#6155qgallouedec wants to merge 12 commits into
qgallouedec wants to merge 12 commits into
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Move sequence truncation in
SFTTrainerfrom the data collator into_prepare_dataset.Why
Truncation is a pure per-example slice. Doing it once during (cached) dataset preparation is cleaner than recomputing it in the collator on every batch, and keeps all dataset shaping (tokenize → build labels → truncate → pack) in one place.
It will also allow to drop rows with no trainable rewards, see #6025
Changes
_prepare_datasetnow truncatesinput_idsandlabelstomax_length(respectingtruncation_mode), right after labels are built. Skipped when packing (packing already chunks tomax_length).build_labelsnow drops thecompletion_mask/assistant_maskscolumns: they're fully baked intolabels.DataCollatorForLanguageModelingno longer truncates: themax_lengthandtruncation_modearguments are removed.With
skip_prepare_dataset=True, preparation (and therefore truncation) is skipped. The dataset must already be truncated.Note
Medium Risk
Behavior change for
skip_prepare_dataset=Trueand custom collators that relied on collator truncation; default prepared-dataset paths should match prior semantics but truncation timing affects assistant-only loss edge cases.Overview
SFT sequence truncation now runs in cached dataset preparation instead of on every batch in
DataCollatorForLanguageModeling.max_lengthandtruncation_modeare removed from the collator;_prepare_datasetslicesinput_idsandlabelsafter label building (honoringkeep_start/keep_end), skipped when packing is enabled.Mask columns are dropped after labels are built (
assistant_masks/completion_maskremoved viabuild_labelsremove_columns), so training rows only carryinput_idsandlabels.skip_prepare_dataset=True, truncation no longer happens anywhere — inputs must already be withinmax_length. Collator tests for truncation and the test that assertedtruncation_modeis passed to the collator were removed; preparation-focused tests were added/updated (including #3927: all-masked labels after aggressive truncation).Reviewed by Cursor Bugbot for commit e6eec1f. Bugbot is set up for automated code reviews on this repo. Configure here.