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Fuse transposition into first indexing for kv cache read #1456

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Fuse transposition into first indexing for kv cache read #1456

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Fuse transposition into first indexing for kv cache read
rsuderman May 16, 2025
4b3260a
Merge remote-tracking branch 'origin/main' into transpose_fuse_select
rsuderman May 16, 2025
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20 changes: 12 additions & 8 deletions sharktank/sharktank/layers/paged_attention.py
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Original file line number Diff line number Diff line change
Expand Up @@ -283,18 +283,22 @@ def read(

# Gather both partitions and split post gather. This is more
# computationally efficient without gather fusion:
subblock_table = page_table.flatten(start_dim=0, end_dim=1)
page_stride = self.pipeline_to_block_count[pipeline]

page_table = page_table.flatten(start_dim=0, end_dim=1)
transformer_block_index = torch.full(
(bs, block_seq_len), transformer_block_index, device=self.device
)
subblock_ids = page_ids * page_stride + transformer_block_index
selected = ops.index_select(subblock_table, 0, subblock_ids.flatten(0, 1))
subblock_ids = page_ids * self.transformer_block_count + transformer_block_index
selected = ops.index_select(page_table, 0, subblock_ids.flatten(0, 1))

selected = selected.transpose(0, 1)
key = selected[0]
value = selected[1]

key = key.unflatten(0, blocked_shape[:2])
value = value.unflatten(0, blocked_shape[:2])

selected = selected.unflatten(0, blocked_shape[:2])
key = selected[:, :, 0, :].flatten(1, 2)
value = selected[:, :, 1, :].flatten(1, 2)
key = key.flatten(1, 2)
value = value.flatten(1, 2)

return key, value

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