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Summary of Changes

Hello @chengtao-lv, 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 refines the mustdrop token reduction mechanism by introducing greater configurability and control over token retention, especially for vision models. It enables dynamic specification of the pruning layer and allows for a fixed number of tokens to be retained during spatial merging, enhancing the precision and flexibility of model compression.

Highlights

  • Configurable Pruning Location: The MustDrop class now initializes self.pruning_loc from self.special_config, allowing the specific layer where token reduction is applied to be dynamically configured.
  • Fixed Token Retention in Pooling: The conditional_pooling function has been updated to accept a fix_r parameter. When fix_r is non-zero, it overrides the dynamically calculated r value, enabling a precise, predefined number of tokens to be retained during the pooling process.
  • Dynamic Token Retention Calculation: The spatial_merge_hook now calculates the fix_r value based on retained_tokens and vision_token_length provided in pruning_paras. This allows the system to programmatically determine and enforce the exact number of tokens to retain, particularly useful for vision models.
  • Hook Mechanism Update: The token reduction hook registration has been changed from a register_forward_pre_hook to a register_forward_hook. This means the hook now processes the output of the specified block after its forward pass. Additionally, the target block for the hook is now dynamically determined by self.pruning_loc, making the hook application more flexible.
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Code Review

The pull request updates the MustDrop token reduction module, making the pruning location configurable, introducing an option to set a fixed number of tokens to merge (fix_r), and changing the reduction mechanism to a forward_hook. The review identified a critical issue with the forward_hook modifying the module's output, a high severity issue with a potential ZeroDivisionError, and a medium severity issue with configuration parameter handling.

merge = conditional_pooling(hidden_states, spatial_threshold, window_size, fix_r)
hidden_states, size = merge_wavg(merge, hidden_states, None)
return (hidden_states,) + args[1:], kwargs
return (hidden_states,)

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critical

The spatial_merge_hook is registered as a forward_hook. Returning only (hidden_states,) replaces the block's entire output, potentially discarding other outputs like attention weights, which can cause errors in subsequent layers. Ensure the hook preserves other elements of the output tuple.

return (hidden_states,) + layer_outs[1:]

Comment on lines +197 to +198
fix_r = (pruning_paras['vision_token_length'] - retained_tokens) \
// (window_size[0] * window_size[1] - 1)

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high

The denominator window_size[0] * window_size[1] - 1 in the fix_r calculation can be zero if window_size is (1, 1), leading to a ZeroDivisionError. Add a check to prevent this.

if (window_size[0] * window_size[1] - 1) > 0:
    fix_r = (pruning_paras['vision_token_length'] - retained_tokens) // (window_size[0] * window_size[1] - 1)
else:
    fix_r = 0

self.register_reduction_modules()

def add_sparse_config(self):
self.pruning_loc = self.special_config['pruning_loc']

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medium

Directly accessing self.special_config['pruning_loc'] can raise a KeyError if the key is missing. Consider adding a check to ensure the key exists before accessing it to prevent potential crashes.

@zhangbilang zhangbilang merged commit 43e54ac into main Jul 14, 2025
2 checks passed
@zhangbilang zhangbilang deleted the vlm branch July 14, 2025 07:54
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3 participants