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[llama-mm] Reduce copies in SDPA in MHA #6917

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Merged
larryliu0820 merged 1 commit into from
Nov 18, 2024
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[llama-mm] Reduce copies in SDPA in MHA #6917

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22 changes: 7 additions & 15 deletions extension/llm/modules/attention.py
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Original file line number Diff line number Diff line change
Expand Up @@ -352,26 +352,18 @@ def forward(
# View + expand + reshape bring num_kv_heads to num_heads for k and v
# to match q.

# k: [bsz, seq_len, n_kv, 1, h_d]
# v: [bsz, seq_len, n_kv, 1, h_d]
k = k.view(bsz, -1, self.num_kv_heads, 1, self.head_dim)
v = v.view(bsz, -1, self.num_kv_heads, 1, self.head_dim)

# Expand the key and value tensors to have the same shape
# as the query tensor by copying values across the relevant dim
if self.num_heads != self.num_kv_heads:
k = k.expand(bsz, -1, self.num_kv_heads, self.q_per_kv, self.head_dim)
v = v.expand(bsz, -1, self.num_kv_heads, self.q_per_kv, self.head_dim)

# [bsz, s, n_h, h_d]
k = k.reshape(bsz, -1, self.num_heads, self.head_dim)
v = v.reshape(bsz, -1, self.num_heads, self.head_dim)

# [bsz, n_h, s, h_d]
q = q.transpose(1, 2)
k = k.transpose(1, 2)
v = v.transpose(1, 2)

# Expand the key and value tensors to have the same shape
# as the query tensor by copying values across the relevant dim
if self.num_heads != self.num_kv_heads:
expand_shape = (-1, -1, self.q_per_kv, -1, -1)
k = k.unsqueeze(2).expand(expand_shape).flatten(1, 2)
v = v.unsqueeze(2).expand(expand_shape).flatten(1, 2)

output = self._attention_fn(
q,
k,
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