Enhancement - Fix preprocessing crash when num_workers=0 in Megatron GPT3 dataset generation#800
Open
Enhancement - Fix preprocessing crash when num_workers=0 in Megatron GPT3 dataset generation#800
Conversation
Contributor
There was a problem hiding this comment.
Pull request overview
This PR fixes a crash in the Megatron GPT-3 dataset preprocessing path when --num_workers=0 by ensuring the worker count passed to Megatron’s tools/preprocess_data.py is at least 1, while keeping num_workers unchanged for other consumers (e.g., PyTorch DataLoader).
Changes:
- Clamp
--workersforpreprocess_data.pytomax(1, self._args.num_workers)to avoidmultiprocessing.Pool(0)failures. - Add inline clarification comment documenting why preprocessing clamps workers while other components may allow
num_workers=0.
💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.
- Megatron's preprocess_data.py appends '_text_document' to the
--output-prefix when producing the .bin/.idx files. Derive the
output-prefix from --data_prefix (stripping a trailing
'_text_document' suffix when present) so that the generated files
match the existence checks for any custom data_prefix value, instead
of being hardcoded to '<data_home>/dataset'.
- Add unit test test_megatron_gpt_dataset_generate_command covering:
num_workers=0 clamps to '--workers 1' with default data_prefix
('dataset_text_document' -> '<data_home>/dataset'), num_workers=4
with custom 'custom_text_document' -> '<data_home>/custom', and a
data_prefix without the suffix used as-is.
Avoid '--workers 1' matching '--workers 10' etc.
polarG
force-pushed
the
dev/hongtaozhang/fix-num_workers-usage
branch
from
a475b85 to
ee2839e
Compare
Contributor
There was a problem hiding this comment.
Pull request overview
Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.
💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## main #800 +/- ##
==========================================
+ Coverage 85.69% 85.85% +0.15%
==========================================
Files 103 103
Lines 7890 7894 +4
==========================================
+ Hits 6761 6777 +16
+ Misses 1129 1117 -12
Flags with carried forward coverage won't be shown. Click here to find out more. ☔ View full report in Codecov by Sentry. 🚀 New features to boost your workflow:
|
added 2 commits
May 2, 2026 00:19
…et generate test
- Replace brittle whitespace substring assertions ('--workers 1 ', '--output-prefix ... ') with normalize_command()-based parsed CLI unit checks, so the test validates semantics rather than formatting.
- Use --code_base {self._tmp_dir} together with createMockFiles(['pretrain_gpt.py']) to avoid the unrealistic /root/Megatron-DeepSpeed path. The mocked run_command now creates the expected .bin/.idx files via side_effect so _preprocess() succeeds end-to-end and is asserted to be True.
- Warn when num_workers is silently clamped from 0 to 1 for the preprocess subprocess so the user sees the override in the log. The DataLoader still receives the original num_workers value. - Guard the '_text_document' suffix-strip against the edge case where data_prefix == '_text_document' (which would otherwise produce a malformed --output-prefix '<data_home>/' with an empty basename). Fall back to the original data_prefix value in that case. - Extend test_megatron_gpt_dataset_generate_command with a 4th case asserting the empty-basename fallback.
Contributor
There was a problem hiding this comment.
Pull request overview
Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.
💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.
Comment on lines
+656
to
+666
| # when producing the .bin/.idx files. Derive the output-prefix from | ||
| # data_prefix (stripping the '_text_document' suffix when present) so that | ||
| # the generated files match the existence checks above for any custom | ||
| # data_prefix value. | ||
| output_prefix_basename = self._args.data_prefix | ||
| if output_prefix_basename.endswith('_text_document'): | ||
| stripped = output_prefix_basename[:-len('_text_document')] | ||
| # Guard against data_prefix == '_text_document' which would | ||
| # leave an empty basename and produce a malformed --output-prefix | ||
| # ending in '/'. Fall back to the original value in that case. | ||
| output_prefix_basename = stripped or output_prefix_basename |
Comment on lines
+243
to
+260
| # Case 3: data_prefix without the '_text_document' suffix is used as-is. | ||
| _run_case( | ||
| extra_params='--num_workers 2 --data_prefix mydata', | ||
| expected_workers=2, | ||
| expected_prefix_basename='mydata', | ||
| expected_data_prefix='mydata', | ||
| ) | ||
|
|
||
| # Case 4: edge case - data_prefix == '_text_document' should NOT strip down | ||
| # to an empty basename (which would produce '--output-prefix <data_home>/'). | ||
| # Fall back to using '_text_document' as the basename. | ||
| _run_case( | ||
| extra_params='--num_workers 1 --data_prefix _text_document', | ||
| expected_workers=1, | ||
| expected_prefix_basename='_text_document', | ||
| expected_data_prefix='_text_document', | ||
| ) | ||
|
|
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Description
preprocess_data.py uses multiprocessing.Pool(workers), which requires workers >= 1. When num_workers is set to 0 (valid for DataLoader, where it means "load in main process"), the preprocessing step crashes.
This change clamps the worker count to max(1, self._args.num_workers) before passing it to preprocess_data.py, while leaving the original num_workers value unchanged for other uses like DataLoader.