[bugfix]: use GCD to robustly configure sp and rp dimensions for any world_size #5698
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Summary of Changes
Hello @0russwest0, I'm Gemini Code Assist1! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!
This pull request introduces a critical bug fix to the device mesh configuration logic, ensuring that sequence and ring parallel dimensions are calculated robustly using the greatest common divisor. Additionally, it optimizes the attention mechanism by conditionally applying sequence parallelism operations, improving efficiency when such parallelism is not required.
Highlights
- Device Mesh Configuration: The device mesh initialization for sequence and ring parallelism (_init_device_mesh) has been refactored to use math.gcd (Greatest Common Divisor) for calculating sp_world_size. This ensures a more robust and correct configuration of parallel dimensions for any world_size, resolving potential issues with previous calculations.
- Sequence Parallelism Optimization: The forward method in UlyssesAttention now conditionally applies _SeqAllToAll operations for query, key, and value layers. These operations are only performed if self.sequence_parallel.sp_world_size is greater than 1, preventing unnecessary communication overhead when sequence parallelism is not active.
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Code Review
This pull request introduces a more robust method for configuring sequence parallel (sp) and ring parallel (rp) dimensions by using the greatest common divisor (GCD). This change simplifies the logic and correctly handles various world_size and num_heads configurations. The added checks to bypass sequence parallel communication when sp_world_size is 1 are also a good improvement. The changes look solid, and I have a couple of minor suggestions to improve code conciseness.
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