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fix get max_q_len in page prefill plan #1930

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fix get max_q_len in page prefill plan #1930

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2 changes: 1 addition & 1 deletion flashinfer/prefill.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1715,7 +1715,7 @@ def plan(
self._max_q_len = max_token_per_sequence
else:
qo_indptr_host = qo_indptr.to("cpu")
self._max_q_len = max(qo_indptr_host).item()
self._max_q_len = max(qo_indptr_host[1:] - qo_indptr_host[:-1]).item()
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high

This line correctly calculates the maximum query length. However, it can raise a RuntimeError if qo_indptr_host contains only one element (i.e., when batch_size is 0), because max() would be called on an empty tensor. This can be handled by checking if batch_size > 0.

self._max_q_len = (qo_indptr_host[1:] - qo_indptr_host[:-1]).max().item() if batch_size > 0 else 0

total_num_rows = int(qo_indptr_host[-1])

if max_sequence_kv is not None:
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